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Lifelines and earthquake hazards along the Interstate 5 Urban Corridor: Cottage Grove to Woodburn, Oregon E.A. Barnett , C.S. Weaver , K.L. Meagher Z. Wang , I.P. Madin , M. Wang , R.A. Haugerud , R.E. Wells , D.B.Ballantyne , M. Darienzo , and the Southern Willamette Valley Working Group 1 1 1 2 3 3 1 4 5 6 7 1) U.S. Geological Survey, Dept. of Earth & Space Sciences, Box 351310, University of Washington, Seattle, Washington, 98195. 2) Oregon Department of Geology and Mineral Industries, Portland, OR, now at Kentucky Geological Survey, Lexington, KY. 3) Oregon Department of Geology and Mineral Industries, Portland, OR. 4) U.S. Geological Survey, Menlo Park, CA. 5)ABS Consulting, Seattle, WA. 6) Oregon Emergency Management, Salem, OR. 7) Representatives from Benton County; Eugene Water and Electric Board; Lane Council of Governments; Linn County; Marion County; Kinder Morgan Energy Partners; Oregon Geospatial Data Clearinghouse; Oregon Public Electronics Network; Oregon Department of Transportation, Region 2; Polk County; University of Oregon; Williams Pipeline Company. Figure 1. Schematic diagram showing the regional plate tectonic setting of the Pacific Northwest. Oregon is sliced to show the location of the three seismic source zones: subduction, intraplate, and crustal. (Modified from Black and others, 2000) GEOLOGY AND EARTHQUAKE HAZARDS Despite the lack of recent, large, damaging earthquakes, earth scientists now understand that earthquake hazards in the Willamette Valley are greater than previously known. This may seem at odds with the experience of long-time residents who can recall only the large earthquakes further north in Olympia in 1949 and Seattle in 1965 versus the relative quiet in Oregon. The recent Nisqually earthquake on February 28, 2001, only seems to further highlight the Puget Sound area as the region more exposed to earthquake hazards. However, two fault zones have drawn the attention of earth scientists with respect to Oregon. In the early 1990s, scientists reached a broad consensus that geologic evidence supports the history of great subduction zone earthquakes, of magnitude 8 to 9, repeatedly striking along the Oregon coast and shaking the western interior of the state. Consequently, the understanding that these great earthquakes occur on average every 500-600 years is one reason that the awareness of earthquake hazards in the Willamette Valley has increased. In addition, earth scientists are beginning to develop an understanding of shallow faults near the earth's surface that may further influence earthquake hazard assessments for this part of the I-5 Urban Corridor. Pacific Northwest earthquakes occur in three source zones: along the Cascadia subduction plate boundary, within the subducting plate (called the intraplate or Benioff zone), and within the crust of the overlying North American plate. Earthquakes from all three zones threaten theWillamettteValley. SUBDUCTION ZONE The forces responsible for producing earthquakes in western Oregon are generated by the Juan de Fuca oceanic plate moving northeastward with respect to the North American continental plate at an average rate of about 4 centimeters (1.5 inches) per year along the Pacific Northwest coast (indicated by the arrow in Figure 1). At the region of contact between the two plates, the Juan de Fuca plate slides (or subducts) beneath the North American continent and sinks slowly into the earth's mantle, producing the Cascade volcanoes and earthquakes. The zone of the shallow, east-dipping subducting plate is called the Cascadia megathrust fault. During subduction, the eastward motion of the Juan de Fuca plate is absorbed by compression of the overriding North American plate, generally resulting in little slip on the Cascadia fault. However, geological evidence provided by buried soil layers, dead trees, and deep-sea deposits indicates to geologists that the upper portion of the shallowly dipping Cascadia fault ruptures offshore and releases this compression in great earthquakes of magnitude 8 to 9 about every 500-600 years. The last such earthquake occurred on January 26, 1700. When the Cascadia subdution zone ruptures, it will likely cause: 1) Severe ground motions along the coast, with shaking in excess of 0.6 to 0.8 g peak horizontal acceleration in many locations. (The unit 1g is the acceleration of gravity and is used as a measurement of the severity of earthquake ground motion). The central Willamette Valley can expect ground motions of about 0.2g in the areas of less hazardous geology (green regions on map). Shaking levels will be greater westward toward the coast; 2) Strong shaking that may last for two to four minutes as the earthquake propagates along the fault and may include long-period seismic waves that can affect very tall structures and high bridges; 3) Tsunamis generated by sudden uplift of the sea floor above the Cascadia fault. Geologists infer the history of earthquakes in the subduction zone by observing effects of past tsunamis such as marine sediments deposited inland and ancient drowned forests; 4) Shaking effects that may significantly damage the regional lifelines in all of Cascadia's major population centers, from Vancouver, B.C., to Eugene. INTRAPLATE ZONE As the Juan de Fuca oceanic plate subducts beneath North America, it becomes denser than the surrounding mantle rocks and breaks apart under its own weight creating earthquakes within the Juan de Fuca plate. Beneath Puget Sound, the Juan de Fuca plate reaches a depth of 40-60 km and begins to bend even more steeply downward, forming a “knee” (see cross-section in Figure 1). The knee is the location where the largest intraplate zone earthquakes occur, such as the 1949 and 2001 events beneath Olympia and the 1965 event beneath the Seattle-Tacoma International Airport. The lack of significant historic intraplate seismicity beneath western Oregon makes it difficult to assess the potential hazards from this source. The same mechanisms that cause the deep earthquakes beneath the Puget Sound may be active in Oregon. However, although there have been a few intraplate earthquakes beneath the Coast Range and Willamette Valley, there is only one notable event. In 1962 a magnitude 4.5 intraplate earthquake occurred northwest of Corvallis to the south of the map area. This is the most southerly known intraplate event of this size in Oregon. We do know that intraplate earthquakes have several distinctive characteristics. Because intraplate earthquakes large enough to cause damage occur at depths of 35 kilometers or more, high frequency ground-motion energy attenuates before it reaches the earth's surface. On rock, peak ground accelerations are expected to be about 0.2g to 0.3g for even the very largest events. We note that 0.2g shaking levels can cause substantial damage to poorly built structures and the shaking can be amplified in shallow, soft soils. Furthermore, intraplate earthquakes tend to be felt over much broader areas than crustal zone earthquakes of comparable magnitude. Finally, based on experience in the Puget Sound region, significant after-shocks are not expected for intraplate earthquakes beneath western Oregon. CRUSTALZONE The third earthquake source zone is the crust of the North American plate. Crustal zone earthquakes, typically of small magnitudes and usually not felt, are the most common earthquakes in western Oregon. At magnitude 5.7, the 1993 Scotts Mills earthquake (Map and Figure 2) is the largest known crustal zone earthquake in western Oregon occurring since a crustal event estimated to be magnitude 6.8 occurred in 1873 near the coast at the California-Oregon border. Most of the larger events plotted in Figure 2 are aftershocks of the Scotts Mills earthquake. There are many mapped faults in the Willamette Valley as shown in Figure 2. For most of these faults, not enough is known to estimate how often the faults might rupture and what magnitude earthquakes could result. Consequently, the hazards from shallow crustal earthquakes are poorly understood. Yeats and others (1996) noted that most of the mapped faults typically consist of short segments striking largely either northwest or northeast. It's not clear whether some of the faults we have highlighted in Figure 2, such as the Corvallis and Waldo Hills Frontal faults, might be part of a larger fault system or behave individually. The proximity of the Scotts Mills earthquake to the Mount Angel fault (Figure 2) has led some earth scientists to suggest that the fault is active, although the rate of surface-faulting events or the maximum size earthquakes to be expected has not been determined. There are also questions whether the Mount Angel fault might connect with the Gales Creek fault to the northwest thus providing a longer earthquake source area of the combined faults. Ground motions from crustal earthquakes of moderate size, magnitude 6 to 6.5, can produce strong shaking on rock exceeding 0.4g that can have major effects on buildings and lifelines. Therefore, better understanding of the mechanisms and possible activity of the crustal faults in western Oregon is important in lowering the uncertainty in earthquake hazard assessments. Geologic Setting National Seismic Hazard Mapping Project 124W 124W 122W 122W 120W 120W 118W 118W 116W 116W 44N 44N 46N 46N 48N 48N 124W 124W 122W 122W 120W 120W 118W 118W 116W 116W 44N 44N 46N 46N 48N 48N 124W 124W 122W 122W 120W 120W 118W 118W 116W 116W 44N 44N 46N 46N 48N 48N 20 20 20 30 40 60 60 80 124W 124W 122W 122W 120W 120W 118W 118W 116W 116W 44N 44N 46N 46N 48N 48N 0 2 4 6 8 10 12 14 16 18 20 30 40 50 60 80 120 160 200 350 Peak Acceleration (%g) with 2% Probability of Exceedance in 50 Years site: NEHRP B-C boundary %g U.S. Geological Survey Albers Conic Equal-Area Projection Standard Parallels:29.5 and 45.5 degrees 2001 Nisqually Washington Oregon Black, G. L., Wang, Z., Wiley, T. J., Wang,Y, and Keefer, D. K., 2000, Relative earthquake hazard map of the Eugene- Springfield metropolitan area, Lane County, Oregon, Oregon Department of Geology and Mineral Industries Interpretive Map Series IMS-14, 16 p., 1 map sheet, scale 1:48,000. Blakely, R.J., Wells, R.E., Tolan, T.L., Beeson, M.H., Trehu,A.M.,And Liberty, L.M., 2000, New aeromagnetic data reveal large strike-slip faults in the northern Willamette Valley, Oregon: Geological Society ofAmerica Bulletin, v. 112, p. 1225- 1233. Frankel,A., Mueller, C., Barnhard, T., Perkins, D., Leyendecker, E.V., Dickman, N., Hanson, S., and Hopper, M., 1996, National seismic hazard maps, June 1995 documentation, U.S. Geological Survey Open-File Report 96-532, 110p. Madin, I.P., and Mabey, M.A., 1996, Earthquake hazard maps for Oregon, Oregon Department of Geology and Mineral Industries Geological Map Series GMS-100. Madin, I.P., Priest, G.R., Mabey, M.A., Malone, S.,Yelin,T.S., and Meier, D., 1993, March 25, 1993, Scotts Mills earthquake Oregon's wakeup call, Oregon Geology, v. 55, no. 3, p. 51-57. Madin, I.P. and Wang, Z., 1999a, Relative earthquake hazard maps for selected urban areas in western Oregon: Dallas, Hood River, McMinnville-Dayton-Lafayette, Monmouth-Independence, Newberg-Dundee, Sandy, Sheridan-Willamina, St. Helens- Columbia City-Scapoose, Oregon Department of Geology and Mineral Industries Interpretive Map Series IMS-7, 24 p., 2 map sheets, scale 1:24,000. Madin, I.P. and Wang, Z., 1999b, Relative earthquake hazard maps for selected urban areas in western Oregon: Canby-Barlow- Aurora, Lebanon, Silverton-Mount Angel, Stayton-Sublimity-Aumsville, Sweet Home, Woodburn-Hubbard, Oregon Department of Geology and Mineral Industries Interpretive Map Series IMS-8, 22 p., 1 map sheet, scale 1:24,000. Madin, I.P. and Wang, Z., 2000, Relative earthquake hazard maps for selected urban areas in western Oregon:Ashland, Cottage Grove, Grants Pass, Roseburg, Sutherlin-Oakland, Oregon Department of Geology and Mineral Industries Interpretive Map Series IMS-9, 21 p., 1 map sheet, scale 1:24,000. O'Connor, J.E., Sarna-Wojcicki,A., Wozniak, K.C., Polette, D.J., and Fleck, R.J., 2001, Origin, extent, and thickness of Quaternary geologic units in the Willamette Valley, Oregon, in U.S. Geological Survey Professional Paper 1620. Oregon Department of Transportation, 2000, Transportation Key Facts 2000, Oregon State Transportation Commission, Salem, Oregon, 40 pp.Walker, G.W., and McLeod, N.S., 1991, Geologic map of Oregon, U.S. Geological Survey Special Geologic Map, 1:500,000. Wang,Y. and Leonard, W.J., 1996, Relative earthquake hazard maps of the Salem East and Salem West quadrangles, Marion and Polk Counties, Oregon, Oregon. REFERENCES Earthquake Distribution Probabilistic Ground Motion Map Earthquake Hazards Since the Cascadia subduction region stretches the length of the Pacific Northwest coastline, it is useful to consider the distribution of earthquakes across the entire plate boundary system and examine the regional picture formed by integrating all three earthquake source zones. Compared with earthquakes in the intraplate zone, crustal events are much more widespread, occurring over much of northwestern California and most of Washington. However, figure 3 shows that there are relatively few earthquakes in Oregon and that the Willamette Valley is particularly quiet. In spite of the fact that scientists know from field studies that subduction events are possible, there are no recent Cascadia zone earthquakes that have been located in Oregon. Thus, in the absence of recent significant seismic data, Figure 3 illustrates the importance of conducting more geological field studies and examining evidence of historical earthquakes in order to link recent earthquakes to faults and create a more complete understanding of the potential for future significant earthquake occurences in theWillametteValley. A useful representation of earthquake shaking hazards is a probabilistic hazard map, which the USGS has developed for the entire country (Frankel and others, 1996, and see http://geohazards.cr.usgs.gov/eq/index.html). These maps underpin seismic building codes and many highway construction standards. The probabilistic hazard map (Figure 4) shows the expected peak horizontal ground motions on a rock site with a 2% probability of being exceeded within a time frame of 50 years. Figure 4 includes all three potential earthquake sources for the Northwest: subduction zone, intraplate zone, and crustal faults. These maps rely on local geologic and seismic data. In this region the hazard is dominated by the subduction zone source, which also runs north-south. Moving eastward into the Willamette Valley, the contours remain north-south to the south, but from Linn County northward the contours turn northeastward. This change reflects increased rates of seismicity originating in the northern Oregon Cascade Range (Figure 3) and at Scotts Mills (Figures 2 & 3). The eastward bulge of higher expected ground motions in the Seattle area reflects the high rate of large-magnitude intraplate earthquakes that have occurred and can be expected in this region. The east-west oval contour of relatively higher hazard in central Puget Sound reflects current scientific understanding of the Seattle fault and illustrates how increasing the detailed geologic knowledge of an individual fault may change hazard assessment. For example, an area of higher hazard around the Seattle fault was included in later maps because field and seismic studies demonstrated that large (M 7.0) earthquakes have occurred on the Seattle fault in the past. Geologic studies examining faults are in progress in western Oregon to fine- tune the regional hazard assessments. When an earthquake occurs, some of the most important earthquake hazards, with respect to the lifelines, are shown on the map: ground shaking, liquefaction, and earthquake-induced landslides. We note that even though there is considerable uncertainty as to how often earthquakes may hit the Willamette Valley, it is possible to evaluate and improve estimates of potential earthquake damage. Ground shaking occurs in a wide area following an earthquake. Because of the complexity of the three source zones, it is useful to implement the probabilistic hazard map (see Figure 4) as an initial guide to determine areas of strong shaking. However, engineers know from experience that unconsolidated young deposits often amplify low to moderate ground motions, sometimes by a factor of two or more. Poorly consolidated soils are typically found in river and stream valleys and areas of artificial fill. The detailed maps prepared by DOGAMI discuss key factors affecting amplification (see for instance Madin and Wang, 2000). Areas of unconsolidated deposits shown on the map in beige should be viewed with caution as ground motions in these areas will likely be more intense than those predicted for rock sites. Significant sections of the lifelines in the Willamette Valley cross areas of unconsolidated materials that may amplify ground motions. Liquefaction is another hazard in areas of unconsolidated young deposits. Strong shaking may cause the pore pressure within water-saturated soil to dramatically increase causing the soil to lose its shear strength and its ability to support large loads, such as buildings and roads. For areas adjacent to a riverbank, or on a slope, ground can move laterally, carrying with it buried pipelines and foundations. Generally, many of the same areas subject to amplified ground shaking are also susceptible to liquefaction; thus the beige-shaded areas on our map carry additional hazard significance. Steep slopes may produce landslides during earthquakes.An important lesson for the Willamette Valley from the large earthquakes in Puget Sound is that not all landslides occur in the first few minutes following an earthquake, but can occur days later. In 1949, a large landslide near Tacoma slipped 3 days after the earthquake. Steep slopes along the edges of the Willamette Valley, often near saturated conditions because of high rainfall, are candidates for earthquake-induced failure, though we have not delineated these areas. As earth scientists improve their estimates of crustal fault behavior in the Willamette Valley, it is possible that surface rupture may become a concern. In the Seattle area, recent discovery of young faults breaking to the surface has enabled the identification of a zone in central Puget Sound where surface rupture may occur. Although for any given earthquake the chances of surface rupture may be small, it may be appropriate for lifeline system engineers to consider such a possibility. ABOUT THE MAP The base map was derived from standard USGS 30-meter digital elevation models (DEM's). Shorelines and streams are from USGS digital line graphs (DLG's) derived from standard 1:100,000-scale maps (see http://edc.usgs.gov/geodata). This map is based on material originally published in U.S. Geological Survey Open-File Report 99-387. There have been very few earthquakes located or detected in the Willamette Valley since a modern seismograph was installed in Corvallis in 1962. On the map we have plotted located earthquakes selected from the University of Washington seismic catalog ranging in magnitude from 2.0 to 5.7. Nearly all located earthquakes occurred in the crust of the North American plate. Most of the events are located in the northeastern portion of the map and were aftershocks of the 1993 Scotts Mills event. All aftershocks for this event were less than magnitude 3.5 (Madin and others, 1993). The only other notable earthquake in the map area is a deep earthquake that occurred in 1962 northwest of Corvallis, and was an intraplate type similar to the 2001 Nisqually earthquake. This magnitude 4.5 event is the largest known intraplate earthquake in Oregon from the California border north to the Columbia River. The geologic units shown on the map have been simplified into two basic units represented by the map colors of beige and green. The beige colors represent unconsolidated surface deposits, which are susceptible to liquefaction, ground amplification, and/or landslides triggered by a seismic event. Surface rocks and deposits considered to be seismically less subject to liquefaction, amplification, or landslides than the young deposits are shown in green colors. These units consist of bedrock and older well-consolidated deposits. Geologists working on this project reached a consensus on which mapped geologic units should be placed into each category. One way to refer to these units is to consider the beige areas as probably more hazardous relative to the green areas in terms of possible earthquake hazards. The geological information varies across the map area as a result of compiling several data sources reflecting different mapping scales from local to regional studies. The different geologic sources occasionally result in abrupt artificial boundaries due to data source boundaries. Please refer to references and the legend inset map for more details. The lowest resolution data are based on a statewide building code soils map developed using a 1:500,000 scale by Walker and McLeod (1991). Seismologists refer to the very near surface units as soils, which Wang and others (1998) divided into six types, of which types A-C fall into the green category, and D-F into the beige category. These data primarily cover the western and eastern edges of the lifeline map. The intermediate resolution data are taken from the 1:100,000 map of Quaternary time deposits in the Willamette Valley mapped by O'Connor and others (2001). Generally, hilly areas adjacent to the floor of the valley have been categorized as seismically less hazardous, green, whereas areas on the valley floor are all covered by less-consolidated deposits and are categorized as probably more hazardous ground, beige. The beige and green categories from both the 1:500,000 and 1:100,000 maps are determined solely on descriptive geologic information and do not incorporate engineering analyses. The highest resolution data are from 1:24,000 scale hazard maps produced by Oregon Department of Geology and Mineral Industries (DOGAMI) for many Oregon communities. IMS and GMS series maps plot relative earthquake hazards in four zones ranging from A, highest hazard to D, lowest hazard. Areas in zones A to C are categorized as beige on our map and zone D is green. DOGAMI map, O-01-05 that covers Benton County, used the soil classification of Wang and others (1998). Communities with completed relative earthquake hazard maps are listed in Table 3. There are many good sources for more information concerning earthquakes and the effects of earthquakes in the Willamette Valley. The Oregon Department of Geology and Mineral Industries has considerable expertise in evaluating and explaining earthquake issues (www.sarvis.dogami.state.or.us) and Oregon Emergency Management has information available regarding earthquake response and mitigation (www.osp.state.or.us/oem/). Both of these sites have many links to other resources. Current earthquake activity is available at www.ess.washington.edu. Complete details for national hazard maps can be found at the following USGS site: www.geohazards.cr.usgs.gov/eq/. A detailed study of the effects of various scenario earthquakes from each of the three source zones is available at www.eqe.com/impact. The Federal Emergency Management Agency maintains a long list of materials related to earthquake preparedness, mitigation, and response planning (www.fema.gov). The American Red Cross has details on personal preparedness (www.redcross.org). The Cascadia Regional Earthquake Workgroup (www.crew.org) is regional private-public partnership dedicated to increasing earthquake mitigation efforts in the Pacific Northwest. This map was made possible by the work of many individuals and organizations that contributed to the planning, shared data, and help critique and improve the final map and text. Members of the Southern Willamette Valley Working Group who participated in meetings or provided data include: George Anderson, Kevin Biersdorff, Jerry Hohman, and Roger Warner of the EugeneWater and Electric Board (EWEB); Steve Barnett, Oregon Geospatial Data Clearinghouse; Pete Brandstetter, City of Albany; Kevin Bryan and Janet Shearer, Oregon Department of Transportation; Oscar Cernas, Kinder Morgan Energy Partners; Bill Clingman, Lane Council of Governments; Rob Givler, Jim O'Conner, and Karen Wheeler, USGS; Jim Howell and Steve Lucker, Linn County; Kathy Kinkel, Craig Thompson, and Bruce Visser, Oregon Public Electronics Network; David Read, Williams Pipeline; Douglas Sackinger, Benton County; Paul Staub, DOGAMI. Bonneville Power Administration and StreamNet provided GIS files. The National Earthquake Hazard Program, Urban Hydrologic and Geologic Hazard Initiative, and National Cooperative Geologic Mapping Program funded work by USGS staff. Brian Sherrod and Woody Savage of the USGS made content review that improved the text. Earthquakes and geologic units on the map MORE INFORMATION ACKNOWLEDGEMENTS City or Urban Area DOGAMI Publication No. Year Canby-Barlow-Aurora IMS-8 1999 Cottage Grove IMS-9 2000 Dallas IMS-7 1999 Eugene-Springfield IMS-14 2000 Lebanon IMS-7 1999 McMinnville-Dayton- Lafayette IMS-7 1999 Monmouth-Independence IMS-7 1999 Newberg-Dundee IMS-7 1999 Salem GMS-105 1996 Sheridan-Willamina IMS-7 1999 Silverton-Mount Angel IMS-8 1999 Stayton-Sublimity- Aumsville IMS-8 1999 Sweet Home IMS-8 1999 Woodburn-Hubbard IMS-8 1999 Table 3: Communities on this map with completed relative earthquake hazard maps. The DOGAMI publication number is repeated in the full citation in the references LIFELINE SYSTEMS ON THE MAP One purpose of the map series is to schematically show how the major regional lifeline systems connect with population centers. Representing highways, railroads, electrical transmission lines, and petroleum and natural gas pipelines is relatively straightforward since these systems are regional. However, representing local water and wastewater systems is more difficult because there are many local systems in the Willamette Valley. With the assistance of local agencies, we have selected and schematically shown major systems for the five cities that have populations greater than 40,000 (Table 1). These cities represent about 50% of the population in the five central and southern Willamette Valley counties (Table 2). In all cases, the service area for water and wastewater utilities extends outside the city boundaries so that these five systems serve an estimated 65% of the population in the Willamette Valley. There are five large water suppliers in the Willamette Valley region: Salem, Albany, Corvallis, Eugene and Springfield. Watersheds in the Cascade Range supply Eugene, Albany, and Salem. The McKenzie River feeds the Eugene system, while Albany's system relies on the South Santiam River, and Salem's system uses the North Santiam River. Springfield is supplied by groundwater from a local aquifer and the system has emergency connections with Eugene. Corvallis relies on water from both the Willamette River and Rock Creek, located in the Coast Range. The map shows the rivers and reservoirs where surface water enters the water pipeline transmission systems, generally selected by pipe diameter, and shows the transmission systems connections to their terminal reservoirs or major distribution branches. There are four wastewater systems serving the five largest cities. One system serves the Eugene-Springfield area. Each system has a wastewater plant that discharges into the Willamette River. Major sewer lines, generally selected by pipe diameter, are shown on the map. The major electric power provider in the Pacific Northwest is the Bonneville Power Administration, which transmits the region's electricity from hydroelectric plants along the Columbia and Snake rivers to the I-5 Urban Corridor. BPA sells power to the major distributors in the region: Portland General Electric, Pacific Power, and Eugene Water and Electric Board. Each of these distributors has some generation capacity. Much of the power transmitted by BPA moves through 500 kV and 345 kV transmission lines shown on the map. The 230 kV and 115 kV lines shown on the map provide additional transmission capability. Williams Natural Gas Pipeline Company supplies natural gas to the Willamette Valley. Pipelines generally run parallel to I-5 and continue south of the map area to Grants Pass, Oregon. Unlike the Portland or Seattle areas, where gas can be supplied either from a north-south or east-west pipeline, there is no alternate to the gas supply line from the north available for the southernWillametteValley. The Willamette Valley is served by a steel, 8-inch diameter liquid fuel line operated by Kinder Morgan Energy Partners, which connects to a pipeline owned by BP-Amoco Pipeline Company in Portland. The BP-Amoco line transports liquid fuel in a pair of pipelines (16-inch and 20- inch) from refineries in northwestern Washington south to Renton near Seattle. One line continues from Renton to Portland and then south into the Willamette Valley, which receives much of its gasoline through this pipeline. The pipeline terminates in the Eugene area. Large volumes of traffic generally flow north-south through the I-5 Urban Corridor with between 25,000 vehicles per day near Cottage Grove to over 80,000 per day near Woodburn. In the Eugene urban area, both I-5 and I- 105 handle about 60,000 vehicles per day. In a post- earthquake emergency, routes parallel to I-5, such as Oregon 99W, may be important as initial corridors for relief efforts. Traffic counts on Oregon 99W generally are less than 15,000 vehicles per day between major population centers. Most of the I-5 bridges were constructed between the late 1950s and the mid-1970s. Truck traffic on I-5 is extremely important to the regional economy. In a study of 17 western states, including heavily populated Texas and California, the Eugene-to- Portland section of I-5 ranks second in tonnage and Portland-to-Eugene ranks fourth (Oregon Dept of Transportation, 2000). Furthermore, the Seattle-to-Portland section of I-5 ranks first in truck tonnage. There are three primary east-west highway routes and none have daily traffic counts exceeding 5000 vehicles per day into the area at the edges of the map. The Salem area is served by Oregon 22, which connects westward to the Oregon coast and eastward to US 20 and Bend via Santiam Pass. The Albany-Corvallis area is on US 20 and is connected with Newport on the coast and Bend to the east. Oregon 34 provides an alternate west-to-east link between Corvallis and the eastern Willamette valley, by-passing Albany. Oregon 126 connects the Eugene area to US 20 west of Santiam Pass. Route 126 also provides a link between Eugene and the Oregon coast. To the south, Oregon 58 links Eugene to the Crater Lake National Park area, Klamath Falls, and southeastern Oregon, crossing the Cascades at Willamette Pass. Again, traffic volumes flowing into the map area are small, below 5000 vehicles per day at Willamette Pass. However, about 1500 trucks use the pass daily, so the route is economically very important to the southernWillametteValley. The Union Pacific Railroad dominates freight traffic movement in the Willamette Valley. About half of Oregon's 63 million tons of rail freight moves through the Willamette Valley. The Eugene area is second to Portland in generating railroad shipping tonnage in Oregon. The Central Oregon and Pacific (CORP) and the Pacific and Western (P & W) are the two main short line railroads that service the map area. The CORP lines run west and south from Eugene, whereas the P & W lines run in a wide corridor approximately parallel to I-5. Six daily passenger trains, 3 northbound and 3 southbound, run from Eugene to Portland and on to Seattle. The Eugene Airport, Mahlon Sweet Field, is the fifth largest commercial airport in Oregon and Washington. The airport serves approximately 750,000 passengers and 55,000 landings annually and is the primary air cargo point for the southern Willamette Valley. On the map we have shown the Eugene Airport with a larger symbol size to distinguish it from smaller local airports. Water Wastewater Electrical Power Natural Gas Liquid Fuel Highways Railroads Airports Central/Southern Willamette Valley Cities Population (July 1, 2000) Eugene 138,615 Salem 137,785 Springfield 52,864 Corvallis 49,400 Albany 41,145 Major City Total 419,809 Table 1: Population of major cities in central and Southern Willamette Valley. Central/Southern Willamette Valley Counties Population (July 1, 2000) Benton 78,153 Lane 322,959 Linn 103,069 Marion 284,834 Polk 62,380 County Total 851,395 Table 2: Population of counties in central and southern Willamette Valley. Figure 5 . Water tank anchor damage in the Nisqually earthquake. The anchor is about 6” in length. (Ballantyne photo). LIFELINE VULNERABILITY TO EARTHQUAKES The vulnerability of a lifeline to earthquakes is related to the type and condition of lifeline structure and to the severity of the specific earthquake hazard. Lifeline building structures can be vulnerable to earthquake shaking, just as are some residential and commercial building structures. There are many special types of structures such as substation equipment, transmission towers, or pipelines that are found in lifeline systems. Damage to one of these system components may disrupt the capacity of the system to function as a whole. Buried pipelines carrying water, wastewater, natural gas, and liquid fuel can be vulnerable to surface faulting, liquefaction and lateral spreading, and landslides. Pipelines constructed of brittle materials are the most vulnerable because they are not able to bend and flex. Water and older gas distribution (low pressure) systems often have significant amounts of brittle cast iron pipe. Asbestos cement pipe found in many water systems is also brittle. Pipelines constructed of relatively ductile materials such as steel and ductile iron are more resistant to earthquake-induced failure. If liquefaction occurs, joint restraint is also important to prevent ruptures. Modern welded joints used on gas and liquid fuel lines, and “restrained” joints used for some water pipelines are preferred in areas subject to liquefaction. Pipelines buried in liquefiable soils can be susceptible to damage rates an order of magnitude larger than those in stable soils. Natural gas and liquid fuel pipelines constructed of steel with modern welded joints have performed well except in the most extreme conditions of large permanent ground displacements. Pipeline joints welded with older techniques are in some cases more brittle, and have failed. During an earthquake, it is common for many water pipelines on soft soils to fail, which can quickly drain the water system. Furthermore, after a failure, water is not available for fire suppression. This scenario occurred following the 1995 Kobe (Japan), 1994 Northridge (California), 1989 Loma Prieta (California), 1923 Tokyo (Japan) and 1906 San Francisco (California) earthquakes. In the worst earthquakes, such as Kobe, the water service was not fully restored for more than two months. Sewer pipelines are vulnerable to flotation if the ground around them liquefies. As these are often gravity- operated systems, a change in grade can impair system operation. In the 1965 Seattle earthquake, a 108-inch diameter sewer was damaged when it floated upwards approximately two feet. Many sewers floated in the 1989 Loma Prieta earthquake, particularly in Santa Cruz, and in the 1995 Kobe Earthquake. The Nisqually earthquake caused approximately 25 water pipeline failures, fewer than 10 natural gas distribution line failures, one sewer failure, and no natural gas transmission or liquid fuel line failures. Earthquakes can cause liquids, such as water and liquid fuels, to slosh in tanks and reservoirs. Sudden ground motion and subsequent movement of the base of a tank can load a tank wall beyond capacity. An unanchored tank may rock resulting in connecting piping to break. As sloshing continues, rocking may cause the tank to buckle or burst. Sloshing can also damage tank roofs and immersed components such as baffles and sludge rakes. In the Nisqually earthquake approximately 15 water tanks were damaged, none catastrophically (Figure 5). Tanks containing liquid fuel have been damaged, and their contents burned. Earthen reservoirs and dams can also be vulnerable to liquefaction and embankment failure. For example, the Lower Van Norman Dam was damaged by liquefaction in the 1971 San Fernando (California) earthquake although no catastrophic water release occurred. Regional power systems went out of service following the 1995 Kobe, 1994 Northridge, and 1989 Loma Prieta earthquakes. Such failures are often due to self-protecting features engineered into the system, and can often be restored within 24 to 72 hours. Many of the power failures in the Seattle area from the 2001 Nisqually earthquake were of this type. The most vulnerable components of electrical power systems typically are high voltage porcelain insulators. The higher the voltage, the larger and more vulnerable the insulator is to strong shaking. As a result, high-voltage substations, particularly 230 kV and above, can be vulnerable to earthquake ground motion. Live tank circuit breakers, commonly used in industry, have not performed well in earthquakes. Rigid busses connecting substation equipment can transfer dynamic loads from other equipment, and exacerbate insulator failures. If well anchored, lower voltage equipment functions well. Ground motions from the Nisqually earthquake were not strong enough to produce significant damage at most substations. Power poles and towers have performed well, except when they are founded on unstable soils where landslides or liquefaction can occur. In the 1993 Landers (California) earthquake, a fault ruptured through the base of a four-legged transmission tower. The tower was distorted, but it did not collapse. Ground shaking can cause low-voltage power lines to slap together causing short circuits. Higher voltage lines have greater separation, and thus are less prone to short circuits. Bridges are usually the most vulnerable components of highway systems. More robust bridge designs were developed in the 1970s and 1980s. Older bridges, built to lower design standards, may be more prone to failure. Bridge decks can slide off their seats if the seats are too narrow or the seats are not adequately restrained. Supporting columns can buckle if they are over-loaded and not designed with adequate ductility. Single-span bridges supported on abutments perform better. Bridge foundations in liquefiable soils can move, allowing the spans they support to slide off. The Nisqually earthquake caused significant damage to about a dozen bridges and highway structures (Figure 6), but none collapsed. A major intersection at the junction of Interstate 5 and Interstate 90 in downtown Seattle was closed for several weeks while inspections and repairs were made. Bridge damage caused closure of northbound lanes of Interstate 5 for 12 hours in Chehalis, and the Alaska Way viaduct in Seattle was closed intermittently for weeks to assess and repair earthquake damage. The Deschutes Parkway was closed for weeks in Olympia due to lateral spreading (Figure 7). Landslides caused closure of highways 101, 202 and 302 (Figure 8). Railway bridges in general performed well as a result of the very large loads they are designed to carry. Earthquakes in the U.S. and Japan have not tested the resistance of railroad bridges to liquefaction or lateral spreading but either mode of ground failure could cause loss of bridge approaches. In addition, a variety of hazards such as failed overpasses, building debris, and ground failures could affect railroad right of ways. Airport runways may be vulnerable to liquefaction. In the 1989 Loma Prieta Earthquake, 3000 feet at the end of the main runway of the Oakland Airport were taken out of service when liquefied sand erupted through runway joints. The Nisqually earthquake caused a similar failure at Boeing Field where most of the largest liquefaction zones correlated with old river channels. Airport control tower glass is vulnerable, as many tower structures are not adequately designed to transfer the roof load to the structure. Control towers at both the Seattle- Tacoma airport (Figure 9) and Boeing Field were damaged during the Nisqually earthquake. Pipelines: Water, Wastewater, Liquid Fuel, and Natural Gas Tanks and Reservoirs Electrical Power Facilities Highways Railways Airports Figure 8 . Road failure on Washington highway 302 caused by landsliding during the Nisqually earthquake (Ballantyne photo). Figure 6. Short-column damage at Holgate overcrossing of Interstate 5 in Seattle caused by Nisqually earthquake. (Photo courtesy of Mark Eberhard, University of Washington, Figure 9. Control tower failures at Seattle-Tacoma International Airport during the Nisqually earthquake. No one was seriously injured by the debris. (photo courtesy Carl Nelman, Boeing Company, ). www.maximus.ce.washington.edu Figure 7 . Road failure at Capital Lake in Olympia caused by lateral spreading. This roadway also failed in the 1949 and 1965 earthquakes. (Photo courtesy Steve Kramer, University of Washington, ) . www.maximus.ce.washington.edu The lifeline systems and geology shown on the accompanying map have been greatly simplified. Most systems are shown in a general way for graphical purposes and may not be accurate in detail. In several locations where one system overlies another, system elements have been adjusted so that they are more distinctly visible on the map. The surface geology has been greatly simplified for the purposes of providing regionally consistent geological characteristics throughout the entire study area (Cottage Grove, Oregon, to Vancouver, British Columbia). This map should not be used for any site-specific purpose.Any site-specific consideration requires more detailed geotechnical and geological data than are presented in this map. INTRODUCTION The Interstate 5 highway corridor, stretching from Mexico to Canada, is not only the economic artery of the Pacific Northwest, but is also home to the majority of Oregonians and Washingtonians. Accordingly, most regional utility and transportation systems, such as railroads and electrical transmission lines, have major components in the I- 5 corridor. The section of I-5 from Cottage Grove, Oregon, to Blaine, Washington, is rapidly urbanizing, with population growth and economic development centered around the cities of Eugene, Salem, Portland, Olympia, Tacoma, Seattle, Everett, and Bellingham. For the purposes of this map, we refer to this area as the I-5 Urban Corridor. Economic success in this urban corridor heavily depends on essential utility and transportation systems, called lifeline systems, such as highways, railroads, pipelines, ports, airports, communications, and electrical power. Consequently, natural disasters that disrupt these lifeline systems can cause economic losses. For example, a major winter windstorm may disrupt an electrical system causing loss of power at smaller distribution substations and widespread power outages due to falling trees breaking power lines. As a result, hundreds of thousands of residents and businesses may be without power for a day or longer. Larger scale natural disasters, such as earthquakes, can present more complex challenges because they tend to affect and disable many lifeline systems at once. For example, failures in the highway system after an earthquake may make restoration of critical electrical power substations or sewer treatment plants more difficult. Subsequently, determining priorities and strategies for recovery becomes increasingly difficult due to the potential simultaneous failures of several systems. As the 2001 Nisqually earthquake reminded us, the Puget Sound region is earthquake country. Large-magnitude, damaging earthquakes struck Olympia in 1949 and Seattle in 1965, and the 2001 Nisqually earthquake occurred very near the epicenter of the 1949 event. In addition to these large events, earthquakes are felt in the Puget Sound region about once a month. In contrast, the southern part of the I-5 Urban Corridor, the Eugene and Salem areas in particular, has experienced very few felt earthquakes this century. However, during the last decade earth scientists have uncovered convincing evidence suggesting that the entire Urban Corridor, from Eugene to Vancouver, B.C. is at risk from great off-shore subduction zone earthquakes, perhaps of magnitude 9. Understanding where major lifeline systems are located in relation to earthquake hazards and population centers is an important first step in developing mitigation strategies that can make the I-5 Urban Corridor more earthquake resistant and expedite economic recovery after an earthquake. Lifeline systems are complex webs that cross through many communities and areas of higher and lower earthquake hazards. The result of the geographic relationships between the lifelines and underlying geology is a complicated multi-layered network that can be difficult to visualize for planners, emergency response providers, elected officials, and other non-specialists. To meet the need for a simple and integrated graphical representation of lifeline systems and earthquake hazards, the United States Geological Survey, in cooperation with public agencies and private companies, has been developing a series of maps for the I-5 Urban Corridor. We have divided the I-5 Urban Corridor into four regions from Cottage Grove, Oregon to southern British Columbia. This map covers Cottage Grove to Woodburn, Oregon (from about I-5 mileposts 160 to 274). The intent is to provide an overview of the lifeline systems and the corresponding earthquake hazards for the citizens, engineers, planners, and decision-makers who live and work in this region. Please note that the map does not provide site- specific information for engineering or environmental purposes. The base of the I-5 Corridor maps is a shaded-relief background that provides a quick, qualitative depiction of slopes and river valleys. The regional geology is generalized and categorized as probably less hazardous (green) or probably more hazardous (beige) ground in the event of an earthquake. Simplified lifeline system elements superimposed on the geological base are shown for: major electric power transmission lines, water supply pipelines, major sewer pipelines and treatment plants, liquid fuel pipelines, natural gas pipelines, and major ports and airports. Each map also shows recent earthquakes of magnitude 2.0 and larger and historically important earthquakes estimated to be larger than magnitude 5. On this map from Cottage Grove to Woodburn, Oregon, the only seismic event known to be greater than magnitude 5 is the magnitude 5.7 Scotts Mills earthquake east of Salem in 1993. Lifelines in Earthquake country The lifelines and earthquake hazards map U.S. Department of the Interior U.S. Geological Survey Figure 2. Faults and earthquakes in the Willamette Valley and vicinity. Solid lines show crustal faults identified by Yeats and others (1996). Darker lines indicate faults discussed in text. Earthquakes are plotted in 3 magnitude ranges and two shades. Smallest symbols represent events between magnitude 2.0 and 3.4, medium symbols events between 3.5 and 4.9, and largest symbol size is over 5.0. Only the 1993 Scotts Mills earthquake (magnitude 5.7) is greater than magnitude 5. Light symbols are intraplate events, dark symbols crustal earthquakes. (After Blakely and others, 2000 ) Figure 3. Earthquakes in Cascadia. Known earthquakes greater than magnitude 6 since about 1870, magnitude 5 since 1950, and earthquakes of magnitude 2 and greater located by the modern seismographic networks and catalogued by the University of Washington (www.ess.washington.edu). Smallest circles are magnitude 2, intermediate circles are between magnitude 5 and 6, and the largest circles are greater than magnitude 6. Earthquakes are grouped into two broad zones. The red earthquakes occur in the intraplate zone, along with events that occurred within the shallow portion of the Juan de Fuca plate. Shallow crustal events are the green earthquakes. Triangles are volcanoes. Salem S pringfie ld Eugene Corvallis 1993 Scotts Mills Earthquake Mag 5.7 Lebanon Sweet Home Corvallis Faul t Mt Angel Fault Albany Willamette River Willamette River Waldo Hills Fault Cottage Grove 1963 Marys Peak 4.2 Woodburn 5 Figure 3. Earthquakes in Cascadia. Known earthquakes greater than magnitude 6 since about 1870, magnitude 5 since 1950, and earthquakes of magnitude 2 and greater located by the modern seismographic networks and catalogued by the University of Washington (www.ess.washington.edu). Earthquakes are grouped into two broad zones: red earthquakes occur in the intraplate zone, along with events that occurred within the shallow portion of the Juan de Fuca plate, and shallow crustal events are the green earthquakes. The 1700 AD Cascadia earthquake zone is shown in pink. The red triangles are the Cascade volcanoes. Smallest circles are magnitude 2, intermediate circles are between magnitude 5 and 6, and the largest circles are greater than magnitude 6. Mill Creek and Turner Faults 39 51 131 0 200 Kilometers 119 122 125 128 41 43 45 47 49 116 o o o o o o o o o o o o o Vancouver Island GORDA PLATE Juan de Fuca Ridge Blanco Fracture Zone PACIFIC PLATE JUAN DE FUCA PLATE Gorda Ridge IDAHO OREGON NEVADA WASHINGTON CALIFORNIA Mendocino Cape EXPLORER PLATE BRITISH COLUMBIA NORTH AMERICA PLATE C a s c a d i a e a r t h q u a k e z o ne 1700 AD 4 cm/yr C asc ad ia M e gat hr u s t F a u l t

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Page 1: Lifelines and earthquake hazards along the Interstate 5 Urban

Lifelines and earthquake hazards along the Interstate 5 Urban Corridor: Cottage Grove to Woodburn, OregonE.A. Barnett , C.S. Weaver , K.L. Meagher Z. Wang , I.P. Madin , M. Wang , R.A. Haugerud , R.E. Wells , D.B.Ballantyne , M. Darienzo , and the Southern Willamette Valley Working Group

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1) U.S. Geological Survey, Dept. of Earth & Space Sciences, Box 351310, University of Washington, Seattle, Washington, 98195. 2) Oregon Department of Geology and Mineral Industries, Portland, OR, now at KentuckyGeological Survey, Lexington, KY. 3) Oregon Department of Geology and Mineral Industries, Portland, OR. 4) U.S. Geological Survey, Menlo Park, CA. 5) ABS Consulting, Seattle, WA. 6) Oregon Emergency Management, Salem,OR. 7) Representatives from Benton County; Eugene Water and Electric Board; Lane Council of Governments; Linn County; Marion County; Kinder Morgan Energy Partners; Oregon Geospatial Data Clearinghouse; OregonPublic Electronics Network; Oregon Department of Transportation, Region 2; Polk County; University of Oregon; Williams Pipeline Company.

Figure 1. Schematic diagram showing the regional plate tectonic setting of the Pacific Northwest.Oregon is sliced to show the location of the three seismic source zones: subduction, intraplate, andcrustal. (Modified from Black and others, 2000)

GEOLOGY AND EARTHQUAKEHAZARDS

Despite the lack of recent, large, damagingearthquakes, earth scientists now understand thatearthquake hazards in the Willamette Valley are greater thanpreviously known. This may seem at odds with theexperience of long-time residents who can recall only thelarge earthquakes further north in Olympia in 1949 andSeattle in 1965 versus the relative quiet in Oregon. Therecent Nisqually earthquake on February 28, 2001, onlyseems to further highlight the Puget Sound area as the regionmore exposed to earthquake hazards. However, two faultzones have drawn the attention of earth scientists withrespect to Oregon. In the early 1990s, scientists reached abroad consensus that geologic evidence supports the historyof great subduction zone earthquakes, of magnitude 8 to 9,repeatedly striking along the Oregon coast and shaking thewestern interior of the state. Consequently, theunderstanding that these great earthquakes occur on averageevery 500-600 years is one reason that the awareness ofearthquake hazards in the Willamette Valley has increased.In addition, earth scientists are beginning to develop anunderstanding of shallow faults near the earth's surface thatmay further influence earthquake hazard assessments forthis part of the I-5 Urban Corridor.

Pacific Northwest earthquakes occur in threesource zones: along the Cascadia subduction plateboundary, within the subducting plate (called the intraplateor Benioff zone), and within the crust of the overlying NorthAmerican plate. Earthquakes from all three zones threatenthe Willamettte Valley.

SUBDUCTION ZONEThe forces responsible for producing earthquakes

in western Oregon are generated by the Juan de Fucaoceanic plate moving northeastward with respect to theNorthAmerican continental plate at an average rate of about4 centimeters (1.5 inches) per year along the PacificNorthwest coast (indicated by the arrow in Figure 1). At theregion of contact between the two plates, the Juan de Fucaplate slides (or subducts) beneath the North Americancontinent and sinks slowly into the earth's mantle, producingthe Cascade volcanoes and earthquakes. The zone of theshallow, east-dipping subducting plate is called theCascadia megathrust fault. During subduction, the eastwardmotion of the Juan de Fuca plate is absorbed by compressionof the overriding North American plate, generally resultingin little slip on the Cascadia fault. However, geologicalevidence provided by buried soil layers, dead trees, anddeep-sea deposits indicates to geologists that the upperportion of the shallowly dipping Cascadia fault rupturesoffshore and releases this compression in great earthquakesof magnitude 8 to 9 about every 500-600 years. The lastsuch earthquake occurred on January 26, 1700.

When the Cascadia subdution zone ruptures, it will likelycause:

1) Severe ground motions along the coast, withshaking in excess of 0.6 to 0.8 g peak horizontalacceleration in many locations. (The unit 1g is theacceleration of gravity and is used as ameasurement of the severity of earthquakeground motion). The central Willamette Valleycan expect ground motions of about 0.2g in theareas of less hazardous geology (green regions onmap). Shaking levels will be greater westwardtoward the coast;2) Strong shaking that may last for two to fourminutes as the earthquake propagates along thefault and may include long-period seismic wavesthat can affect very tall structures and highbridges;3) Tsunamis generated by sudden uplift of the seafloor above the Cascadia fault. Geologists inferthe history of earthquakes in the subduction zoneby observing effects of past tsunamis such asmarine sediments deposited inland and ancientdrowned forests;

4) Shaking effects that may significantly damagethe regional lifelines in all of Cascadia's majorpopulation centers, from Vancouver, B.C., toEugene.

INTRAPLATE ZONEAs the Juan de Fuca oceanic plate subducts beneath

North America, it becomes denser than the surroundingmantle rocks and breaks apart under its own weight creatingearthquakes within the Juan de Fuca plate. Beneath PugetSound, the Juan de Fuca plate reaches a depth of 40-60 kmand begins to bend even more steeply downward, forming a“knee” (see cross-section in Figure 1). The knee is thelocation where the largest intraplate zone earthquakes occur,such as the 1949 and 2001 events beneath Olympia and the1965 event beneath the Seattle-Tacoma InternationalAirport.

The lack of significant historic intraplate seismicitybeneath western Oregon makes it difficult to assess thepotential hazards from this source. The same mechanismsthat cause the deep earthquakes beneath the Puget Soundmay be active in Oregon. However, although there havebeen a few intraplate earthquakes beneath the Coast Rangeand Willamette Valley, there is only one notable event. In1962 a magnitude 4.5 intraplate earthquake occurrednorthwest of Corvallis to the south of the map area. This isthe most southerly known intraplate event of this size inOregon.

We do know that intraplate earthquakes haveseveral distinctive characteristics. Because intraplateearthquakes large enough to cause damage occur at depthsof 35 kilometers or more, high frequency ground-motionenergy attenuates before it reaches the earth's surface. Onrock, peak ground accelerations are expected to be about0.2g to 0.3g for even the very largest events. We note that0.2g shaking levels can cause substantial damage to poorlybuilt structures and the shaking can be amplified in shallow,soft soils. Furthermore, intraplate earthquakes tend to be feltover much broader areas than crustal zone earthquakes ofcomparable magnitude. Finally, based on experience in thePuget Sound region, significant after-shocks are notexpected for intraplate earthquakes beneath westernOregon.

CRUSTALZONEThe third earthquake source zone is the crust of the

North American plate. Crustal zone earthquakes, typicallyof small magnitudes and usually not felt, are the mostcommon earthquakes in western Oregon. At magnitude 5.7,the 1993 Scotts Mills earthquake (Map and Figure 2) is thelargest known crustal zone earthquake in western Oregonoccurring since a crustal event estimated to be magnitude6.8 occurred in 1873 near the coast at the California-Oregonborder. Most of the larger events plotted in Figure 2 areaftershocks of the Scotts Mills earthquake.

There are many mapped faults in the WillametteValley as shown in Figure 2. For most of these faults, notenough is known to estimate how often the faults mightrupture and what magnitude earthquakes could result.Consequently, the hazards from shallow crustal earthquakesare poorly understood. Yeats and others (1996) noted thatmost of the mapped faults typically consist of shortsegments striking largely either northwest or northeast. It'snot clear whether some of the faults we have highlighted inFigure 2, such as the Corvallis and Waldo Hills Frontalfaults, might be part of a larger fault system or behaveindividually. The proximity of the Scotts Mills earthquake tothe Mount Angel fault (Figure 2) has led some earthscientists to suggest that the fault is active, although the rateof surface-faulting events or the maximum size earthquakesto be expected has not been determined. There are alsoquestions whether the Mount Angel fault might connectwith the Gales Creek fault to the northwest thus providing alonger earthquake source area of the combined faults.Ground motions from crustal earthquakes of moderate size,magnitude 6 to 6.5, can produce strong shaking on rockexceeding 0.4g that can have major effects on buildings andlifelines. Therefore, better understanding of themechanisms and possible activity of the crustal faults inwestern Oregon is important in lowering the uncertainty inearthquake hazard assessments.

Geologic Setting

National Seismic Hazard Mapping Project

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Peak Acceleration (%g) with 2% Probability of Exceedance in 50 Years

site: NEHRP B-C boundary

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U.S. Geological Survey Albers Conic Equal-Area Projection

Standard Parallels: 29.5 and 45.5 degrees

2001 Nisqually

Washington

Oregon

Black, G. L., Wang, Z., Wiley, T. J., Wang, Y, and Keefer, D. K., 2000, Relative earthquake hazard map of the Eugene-Springfield metropolitan area, Lane County, Oregon, Oregon Department of Geology and Mineral IndustriesInterpretive Map Series IMS-14, 16 p., 1 map sheet, scale 1:48,000.

Blakely, R.J., Wells, R.E., Tolan, T.L., Beeson, M.H., Trehu, A.M., And Liberty, L.M., 2000, New aeromagnetic data reveallarge strike-slip faults in the northern Willamette Valley, Oregon: Geological Society of America Bulletin, v. 112, p. 1225-1233.

Frankel, A., Mueller, C., Barnhard, T., Perkins, D., Leyendecker, E.V., Dickman, N., Hanson, S., and Hopper, M., 1996,National seismic hazard maps, June 1995 documentation, U.S. Geological Survey Open-File Report 96-532, 110p.

Madin, I.P., and Mabey, M.A., 1996, Earthquake hazard maps for Oregon, Oregon Department of Geology and MineralIndustries Geological Map Series GMS-100.

Madin, I.P., Priest, G.R., Mabey, M.A., Malone, S., Yelin, T.S., and Meier, D., 1993, March 25, 1993, Scotts Mills earthquakeOregon's wakeup call, Oregon Geology, v. 55, no. 3, p. 51-57.

Madin, I.P. and Wang, Z., 1999a, Relative earthquake hazard maps for selected urban areas in western Oregon: Dallas, HoodRiver, McMinnville-Dayton-Lafayette, Monmouth-Independence, Newberg-Dundee, Sandy, Sheridan-Willamina, St. Helens-Columbia City-Scapoose, Oregon Department of Geology and Mineral Industries Interpretive Map Series IMS-7, 24 p., 2map sheets, scale 1:24,000.

Madin, I.P. and Wang, Z., 1999b, Relative earthquake hazard maps for selected urban areas in western Oregon: Canby-Barlow-Aurora, Lebanon, Silverton-Mount Angel, Stayton-Sublimity-Aumsville, Sweet Home, Woodburn-Hubbard, OregonDepartment of Geology and Mineral Industries Interpretive Map Series IMS-8, 22 p., 1 map sheet, scale 1:24,000.

Madin, I.P. and Wang, Z., 2000, Relative earthquake hazard maps for selected urban areas in western Oregon: Ashland, CottageGrove, Grants Pass, Roseburg, Sutherlin-Oakland, Oregon Department of Geology and Mineral Industries Interpretive MapSeries IMS-9, 21 p., 1 map sheet, scale 1:24,000.

O'Connor, J.E., Sarna-Wojcicki, A., Wozniak, K.C., Polette, D.J., and Fleck, R.J., 2001, Origin, extent, and thickness ofQuaternary geologic units in the Willamette Valley, Oregon, in U.S. Geological Survey Professional Paper 1620.

Oregon Department of Transportation, 2000, Transportation Key Facts 2000, Oregon State Transportation Commission, Salem,Oregon, 40 pp.Walker, G.W., and McLeod, N.S., 1991, Geologic map of Oregon, U.S. Geological Survey Special GeologicMap, 1:500,000.

Wang, Y. and Leonard, W.J., 1996, Relative earthquake hazard maps of the Salem East and Salem West quadrangles, Marionand Polk Counties, Oregon, Oregon.

REFERENCES

Earthquake Distribution

Probabilistic Ground Motion Map

Earthquake HazardsSince the Cascadia subduction region stretches the

length of the Pacific Northwest coastline, it is useful toconsider the distribution of earthquakes across the entireplate boundary system and examine the regional pictureformed by integrating all three earthquake source zones.Compared with earthquakes in the intraplate zone, crustalevents are much more widespread, occurring over much ofnorthwestern California and most of Washington.However, figure 3 shows that there are relatively fewearthquakes in Oregon and that the Willamette Valley isparticularly quiet. In spite of the fact that scientists knowfrom field studies that subduction events are possible, thereare no recent Cascadia zone earthquakes that have beenlocated in Oregon. Thus, in the absence of recentsignificant seismic data, Figure 3 illustrates the importanceof conducting more geological field studies and examiningevidence of historical earthquakes in order to link recentearthquakes to faults and create a more completeunderstanding of the potential for future significantearthquake occurences in the Willamette Valley.

A useful representation of earthquake shakinghazards is a probabilistic hazard map, which the USGS hasdeveloped for the entire country (Frankel and others, 1996,and see http://geohazards.cr.usgs.gov/eq/index.html).These maps underpin seismic building codes and manyhighway construction standards. The probabilistic hazardmap (Figure 4) shows the expected peak horizontal groundmotions on a rock site with a 2% probability of beingexceeded within a time frame of 50 years. Figure 4includes all three potential earthquake sources for theNorthwest: subduction zone, intraplate zone, and crustalfaults. These maps rely on local geologic and seismic data.In this region the hazard is dominated by the subductionzone source, which also runs north-south. Movingeastward into the Willamette Valley, the contours remainnorth-south to the south, but from Linn County northwardthe contours turn northeastward. This change reflectsincreased rates of seismicity originating in the northernOregon Cascade Range (Figure 3) and at Scotts Mills(Figures 2 & 3). The eastward bulge of higher expectedground motions in the Seattle area reflects the high rate oflarge-magnitude intraplate earthquakes that have occurredand can be expected in this region.

The east-west oval contour of relatively higherhazard in central Puget Sound reflects current scientificunderstanding of the Seattle fault and illustrates howincreasing the detailed geologic knowledge of anindividual fault may change hazard assessment. Forexample, an area of higher hazard around the Seattle faultwas included in later maps because field and seismicstudies demonstrated that large (M 7.0) earthquakes haveoccurred on the Seattle fault in the past. Geologic studiesexamining faults are in progress in western Oregon to fine-tune the regional hazard assessments.

When an earthquake occurs, some of the mostimportant earthquake hazards, with respect to the lifelines,are shown on the map: ground shaking, liquefaction, andearthquake-induced landslides. We note that even thoughthere is considerable uncertainty as to how oftenearthquakes may hit the Willamette Valley, it is possible toevaluate and improve estimates of potential earthquakedamage.

Ground shaking occurs in a wide area following anearthquake. Because of the complexity of the three sourcezones, it is useful to implement the probabilistic hazardmap (see Figure 4) as an initial guide to determine areas ofstrong shaking. However, engineers know from experiencethat unconsolidated young deposits often amplify low tomoderate ground motions, sometimes by a factor of two ormore. Poorly consolidated soils are typically found in riverand stream valleys and areas of artificial fill. The detailedmaps prepared by DOGAMI discuss key factors affectingamplification (see for instance Madin and Wang, 2000).Areas of unconsolidated deposits shown on the map inbeige should be viewed with caution as ground motions inthese areas will likely be more intense than those predictedfor rock sites. Significant sections of the lifelines in theWillamette Valley cross areas of unconsolidated materialsthat may amplify ground motions.

Liquefaction is another hazard in areas ofunconsolidated young deposits. Strong shaking may causethe pore pressure within water-saturated soil todramatically increase causing the soil to lose its shearstrength and its ability to support large loads, such asbuildings and roads. For areas adjacent to a riverbank, or ona slope, ground can move laterally, carrying with it buriedpipelines and foundations. Generally, many of the sameareas subject to amplified ground shaking are alsosusceptible to liquefaction; thus the beige-shaded areas onour map carry additional hazard significance.

Steep slopes may produce landslides duringearthquakes.An important lesson for the Willamette Valleyfrom the large earthquakes in Puget Sound is that not alllandslides occur in the first few minutes following anearthquake, but can occur days later. In 1949, a largelandslide near Tacoma slipped 3 days after the earthquake.Steep slopes along the edges of the Willamette Valley, oftennear saturated conditions because of high rainfall, arecandidates for earthquake-induced failure, though we havenot delineated these areas.

As earth scientists improve their estimates ofcrustal fault behavior in the Willamette Valley, it is possiblethat surface rupture may become a concern. In the Seattlearea, recent discovery of young faults breaking to thesurface has enabled the identification of a zone in centralPuget Sound where surface rupture may occur. Althoughfor any given earthquake the chances of surface rupturemay be small, it may be appropriate for lifeline systemengineers to consider such a possibility.

ABOUT THE MAP

The base map was derived from standard USGS 30-meterdigital elevation models (DEM's). Shorelines and streams arefrom USGS digital line graphs (DLG's) derived from standard1:100,000-scale maps (see http://edc.usgs.gov/geodata). Thismap is based on material originally published in U.S.Geological Survey Open-File Report 99-387.

There have been very few earthquakes located ordetected in the Willamette Valley since a modern seismographwas installed in Corvallis in 1962. On the map we have plottedlocated earthquakes selected from the University ofWashington seismic catalog ranging in magnitude from 2.0 to5.7. Nearly all located earthquakes occurred in the crust of theNorth American plate. Most of the events are located in thenortheastern portion of the map and were aftershocks of the1993 Scotts Mills event.All aftershocks for this event were lessthan magnitude 3.5 (Madin and others, 1993).

The only other notable earthquake in the map area is adeep earthquake that occurred in 1962 northwest of Corvallis,and was an intraplate type similar to the 2001 Nisquallyearthquake. This magnitude 4.5 event is the largest knownintraplate earthquake in Oregon from the California bordernorth to the Columbia River.

The geologic units shown on the map have beensimplified into two basic units represented by the map colors ofbeige and green. The beige colors represent unconsolidatedsurface deposits, which are susceptible to liquefaction, groundamplification, and/or landslides triggered by a seismic event.Surface rocks and deposits considered to be seismically lesssubject to liquefaction, amplification, or landslides than theyoung deposits are shown in green colors. These units consistof bedrock and older well-consolidated deposits. Geologistsworking on this project reached a consensus on which mappedgeologic units should be placed into each category. One way torefer to these units is to consider the beige areas as probablymore hazardous relative to the green areas in terms of possibleearthquake hazards. The geological information varies acrossthe map area as a result of compiling several data sourcesreflecting different mapping scales from local to regionalstudies. The different geologic sources occasionally result inabrupt artificial boundaries due to data source boundaries.Please refer to references and the legend inset map for moredetails.

The lowest resolution data are based on a statewidebuilding code soils map developed using a 1:500,000 scale byWalker and McLeod (1991). Seismologists refer to the verynear surface units as soils, which Wang and others (1998)divided into six types, of which types A-C fall into the greencategory, and D-F into the beige category. These data primarilycover the western and eastern edges of the lifeline map. Theintermediate resolution data are taken from the 1:100,000 mapof Quaternary time deposits in the Willamette Valley mappedby O'Connor and others (2001). Generally, hilly areas adjacentto the floor of the valley have been categorized as seismicallyless hazardous, green, whereas areas on the valley floor are allcovered by less-consolidated deposits and are categorized asprobably more hazardous ground, beige. The beige and greencategories from both the 1:500,000 and 1:100,000 maps aredetermined solely ondescriptive geologic information and do not incorporateengineering analyses.

The highest resolution data are from 1:24,000 scalehazard maps produced by Oregon Department of Geology andMineral Industries (DOGAMI) for many Oregon communities.IMS and GMS series maps plot relative earthquake hazards infour zones ranging from A, highest hazard to D, lowest hazard.Areas in zones A to C are categorized as beige on our map andzone D is green. DOGAMI map, O-01-05 that covers BentonCounty, used the soil classification of Wang and others (1998).Communities with completed relative earthquake hazard mapsare listed in Table 3.

There are many good sources for more informationconcerning earthquakes and the effects of earthquakes in theWillamette Valley. The Oregon Department of Geology andMineral Industries has considerable expertise in evaluating andexplaining earthquake issues (www.sarvis.dogami.state.or.us)and Oregon Emergency Management has informationavailable regarding earthquake response and mitigation(www.osp.state.or.us/oem/). Both of these sites have manylinks to other resources. Current earthquake activity isavailable at www.ess.washington.edu. Complete details fornational hazard maps can be found at the following USGS site:www.geohazards.cr.usgs.gov/eq/. A detailed study of theeffects of various scenario earthquakes from each of the threesource zones is available at www.eqe.com/impact. The FederalEmergency Management Agency maintains a long list ofmaterials related to earthquake preparedness, mitigation, andresponse planning (www.fema.gov). The American Red Crosshas details on personal preparedness (www.redcross.org). TheCascadia Regional Earthquake Workgroup (www.crew.org) isregional private-public partnership dedicated to increasingearthquake mitigation efforts in the Pacific Northwest.

This map was made possible by the work of manyindividuals and organizations that contributed to the planning,shared data, and help critique and improve the final map andtext. Members of the Southern Willamette Valley WorkingGroup who participated in meetings or provided data include:George Anderson, Kevin Biersdorff, Jerry Hohman, and RogerWarner of the Eugene Water and Electric Board (EWEB); SteveBarnett, Oregon Geospatial Data Clearinghouse; PeteBrandstetter, City of Albany; Kevin Bryan and Janet Shearer,Oregon Department of Transportation; Oscar Cernas, KinderMorgan Energy Partners; Bill Clingman, Lane Council ofGovernments; Rob Givler, Jim O'Conner, and Karen Wheeler,USGS; Jim Howell and Steve Lucker, Linn County; KathyKinkel, Craig Thompson, and Bruce Visser, Oregon PublicElectronics Network; David Read, Williams Pipeline; DouglasSackinger, Benton County; Paul Staub, DOGAMI. BonnevillePowerAdministration and StreamNet provided GIS files.

The National Earthquake Hazard Program, UrbanHydrologic and Geologic Hazard Initiative, and NationalCooperative Geologic Mapping Program funded work byUSGS staff. Brian Sherrod and Woody Savage of the USGSmade content review that improved the text.

Earthquakes and geologic units on the map

MORE INFORMATION

ACKNOWLEDGEMENTS

City or Urban Area DOGAMI

Publication

No.

Year

Canby-Barlow-Aurora IMS-8 1999

Cottage Grove IMS-9 2000

Dallas IMS-7 1999

Eugene-Springfield IMS-14 2000

Lebanon IMS-7 1999

McMinnville-Dayton-

Lafayette

IMS-7 1999

Monmouth-Independence IMS-7 1999

Newberg-Dundee IMS-7 1999

Salem GMS-105 1996

Sheridan-Willamina IMS-7 1999

Silverton-Mount Angel IMS-8 1999

Stayton-Sublimity-

Aumsville

IMS-8 1999

Sweet Home IMS-8 1999

Woodburn-Hubbard IMS-8 1999

Table 3: Communities on this map with completed relativeearthquake hazard maps. The DOGAMI publication numberis repeated in the full citation in the references

LIFELINE SYSTEMS ON THE MAP

One purpose of the map series is to schematicallyshow how the major regional lifeline systems connect withpopulation centers. Representing highways, railroads,electrical transmission lines, and petroleum and natural gaspipelines is relatively straightforward since these systemsare regional. However, representing local water andwastewater systems is more difficult because there aremany local systems in the Willamette Valley. With theassistance of local agencies, we have selected andschematically shown major systems for the five cities thathave populations greater than 40,000 (Table 1). These citiesrepresent about 50% of the population in the five central andsouthern Willamette Valley counties (Table 2). In all cases,the service area for water and wastewater utilities extendsoutside the city boundaries so that these five systems servean estimated 65% of the population in the WillametteValley.

There are five large water suppliers in theWillamette Valley region: Salem, Albany, Corvallis,Eugene and Springfield. Watersheds in the Cascade Rangesupply Eugene, Albany, and Salem. The McKenzie Riverfeeds the Eugene system, while Albany's system relies onthe South Santiam River, and Salem's system uses theNorth Santiam River. Springfield is supplied bygroundwater from a local aquifer and the system hasemergency connections with Eugene. Corvallis relies onwater from both the Willamette River and Rock Creek,located in the Coast Range.

The map shows the rivers and reservoirs wheresurface water enters the water pipeline transmissionsystems, generally selected by pipe diameter, and shows thetransmission systems connections to their terminalreservoirs or major distribution branches.

There are four wastewater systems serving the fivelargest cities. One system serves the Eugene-Springfieldarea. Each system has a wastewater plant that dischargesinto the Willamette River. Major sewer lines, generallyselected by pipe diameter, are shown on the map.

The major electric power provider in the PacificNorthwest is the Bonneville Power Administration, whichtransmits the region's electricity from hydroelectric plantsalong the Columbia and Snake rivers to the I-5 UrbanCorridor. BPA sells power to the major distributors in theregion: Portland General Electric, Pacific Power, andEugene Water and Electric Board. Each of these distributorshas some generation capacity. Much of the power

transmitted by BPA moves through 500 kV and 345 kVtransmission lines shown on the map. The 230 kV and 115kV lines shown on the map provide additional transmissioncapability.

Williams Natural Gas Pipeline Company suppliesnatural gas to the Willamette Valley. Pipelines generally runparallel to I-5 and continue south of the map area to GrantsPass, Oregon. Unlike the Portland or Seattle areas, wheregas can be supplied either from a north-south or east-westpipeline, there is no alternate to the gas supply line from thenorth available for the southern Willamette Valley.

The Willamette Valley is served by a steel, 8-inchdiameter liquid fuel line operated by Kinder Morgan EnergyPartners, which connects to a pipeline owned by BP-AmocoPipeline Company in Portland. The BP-Amoco linetransports liquid fuel in a pair of pipelines (16-inch and 20-inch) from refineries in northwestern Washington south toRenton near Seattle. One line continues from Renton toPortland and then south into the Willamette Valley, whichreceives much of its gasoline through this pipeline. Thepipeline terminates in the Eugene area.

Large volumes of traffic generally flow north-souththrough the I-5 Urban Corridor with between 25,000vehicles per day near Cottage Grove to over 80,000 per daynear Woodburn. In the Eugene urban area, both I-5 and I-105 handle about 60,000 vehicles per day. In a post-earthquake emergency, routes parallel to I-5, such asOregon 99W, may be important as initial corridors for reliefefforts. Traffic counts on Oregon 99W generally are lessthan 15,000 vehicles per day between major populationcenters. Most of the I-5 bridges were constructed betweenthe late 1950s and the mid-1970s.

Truck traffic on I-5 is extremely important to theregional economy. In a study of 17 western states, includingheavily populated Texas and California, the Eugene-to-Portland section of I-5 ranks second in tonnage andPortland-to-Eugene ranks fourth (Oregon Dept ofTransportation, 2000). Furthermore, the Seattle-to-Portlandsection of I-5 ranks first in truck tonnage.

There are three primary east-west highway routesand none have daily traffic counts exceeding 5000 vehiclesper day into the area at the edges of the map. The Salem areais served by Oregon 22, which connects westward to theOregon coast and eastward to US 20 and Bend via SantiamPass. The Albany-Corvallis area is on US 20 and isconnected with Newport on the coast and Bend to the east.Oregon 34 provides an alternate west-to-east link betweenCorvallis and the eastern Willamette valley, by-passingAlbany. Oregon 126 connects the Eugene area to US 20west of Santiam Pass. Route 126 also provides a linkbetween Eugene and the Oregon coast. To the south, Oregon58 links Eugene to the Crater Lake National Park area,Klamath Falls, and southeastern Oregon, crossing theCascades at Willamette Pass. Again, traffic volumesflowing into the map area are small, below 5000 vehiclesper day at Willamette Pass. However, about 1500 trucks usethe pass daily, so the route is economically very important tothe southern Willamette Valley.

The Union Pacific Railroad dominates freighttraffic movement in the Willamette Valley. About half ofOregon's 63 million tons of rail freight moves through theWillamette Valley. The Eugene area is second to Portland ingenerating railroad shipping tonnage in Oregon. TheCentral Oregon and Pacific (CORP) and the Pacific andWestern (P & W) are the two main short line railroads thatservice the map area. The CORP lines run west and southfrom Eugene, whereas the P& W lines run in a wide corridorapproximately parallel to I-5. Six daily passenger trains, 3northbound and 3 southbound, run from Eugene to Portlandand on to Seattle.

The Eugene Airport, Mahlon Sweet Field, is thefifth largest commercial airport in Oregon and Washington.The airport serves approximately 750,000 passengers and55,000 landings annually and is the primary air cargo pointfor the southern Willamette Valley. On the map we haveshown the Eugene Airport with a larger symbol size todistinguish it from smaller local airports.

Water

Wastewater

Electrical Power

Natural Gas

Liquid Fuel

Highways

Railroads

Airports

Central/Southern

Willamette

Valley Cities

Population

(July 1,

2000)

Eugene 138,615

Salem 137,785

Springfield 52,864

Corvallis 49,400

Albany 41,145

Major City Total 419,809

Table 1: Population of major cities in central andSouthern Willamette Valley.

Central/Southern

Willamette Valley

Counties

Population

(July 1,

2000)

Benton 78,153

Lane 322,959

Linn 103,069

Marion 284,834

Polk 62,380

County Total 851,395

Table 2: Population of counties in central and southernWillamette Valley.

Figure 5. Water tank anchor damage in the Nisquallyearthquake. The anchor is about 6” in length.(Ballantyne photo).

LIFELINE VULNERABILITY TOEARTHQUAKES

The vulnerability of a lifeline to earthquakes is relatedto the type and condition of lifeline structure and to the severityof the specific earthquake hazard. Lifeline building structurescan be vulnerable to earthquake shaking, just as are someresidential and commercial building structures. There are manyspecial types of structures such as substation equipment,transmission towers, or pipelines that are found in lifelinesystems. Damage to one of these system components maydisrupt the capacity of the system to function as a whole.

Buried pipelines carrying water, wastewater, naturalgas, and liquid fuel can be vulnerable to surface faulting,liquefaction and lateral spreading, and landslides. Pipelinesconstructed of brittle materials are the most vulnerable becausethey are not able to bend and flex. Water and older gasdistribution (low pressure) systems often have significantamounts of brittle cast iron pipe.Asbestos cement pipe found inmany water systems is also brittle. Pipelines constructed ofrelatively ductile materials such as steel and ductile iron aremore resistant to earthquake-induced failure. If liquefactionoccurs, joint restraint is also important to prevent ruptures.Modern welded joints used on gas and liquid fuel lines, and“restrained” joints used for some water pipelines are preferredin areas subject to liquefaction. Pipelines buried in liquefiablesoils can be susceptible to damage rates an order of magnitudelarger than those in stable soils.

Natural gas and liquid fuel pipelines constructed ofsteel with modern welded joints have performed well except inthe most extreme conditions of large permanent grounddisplacements. Pipeline joints welded with older techniquesare in some cases more brittle, and have failed.

During an earthquake, it is common for many waterpipelines on soft soils to fail, which can quickly drain the watersystem. Furthermore, after a failure, water is not available forfire suppression. This scenario occurred following the 1995Kobe (Japan), 1994 Northridge (California), 1989 Loma Prieta(California), 1923 Tokyo (Japan) and 1906 San Francisco(California) earthquakes. In the worst earthquakes, such asKobe, the water service was not fully restored for more thantwo months.

Sewer pipelines are vulnerable to flotation if theground around them liquefies. As these are often gravity-operated systems, a change in grade can impair systemoperation. In the 1965 Seattle earthquake, a 108-inch diametersewer was damaged when it floated upwards approximatelytwo feet. Many sewers floated in the 1989 Loma Prietaearthquake, particularly in Santa Cruz, and in the 1995 KobeEarthquake.

The Nisqually earthquake caused approximately 25water pipeline failures, fewer than 10 natural gas distributionline failures, one sewer failure, and no natural gas transmissionor liquid fuel line failures.

Earthquakes can cause liquids, such as water and liquidfuels, to slosh in tanks and reservoirs. Sudden ground motionand subsequent movement of the base of a tank can load a tankwall beyond capacity. An unanchored tank may rock resultingin connecting piping to break. As sloshing continues, rockingmay cause the tank to buckle or burst. Sloshing can also damagetank roofs and immersed components such as baffles andsludge rakes. In the Nisqually earthquake approximately 15water tanks were damaged, none catastrophically (Figure 5).Tanks containing liquid fuel have been damaged, and theircontents burned. Earthen reservoirs and dams can also bevulnerable to liquefaction and embankment failure. Forexample, the Lower Van Norman Dam was damaged byliquefaction in the 1971 San Fernando (California) earthquakealthough no catastrophic water release occurred.

Regional power systems went out of service followingthe 1995 Kobe, 1994 Northridge, and 1989 Loma Prietaearthquakes. Such failures are often due to self-protectingfeatures engineered into the system, and can often be restoredwithin 24 to 72 hours. Many of the power failures in the Seattlearea from the 2001 Nisqually earthquake were of this type.

The most vulnerable components of electrical powersystems typically are high voltage porcelain insulators. Thehigher the voltage, the larger and more vulnerable the insulatoris to strong shaking. As a result, high-voltage substations,particularly 230 kV and above, can be vulnerable to earthquakeground motion. Live tank circuit breakers, commonly used inindustry, have not performed well in earthquakes. Rigid bussesconnecting substation equipment can transfer dynamic loadsfrom other equipment, and exacerbate insulator failures. If wellanchored, lower voltage equipment functions well. Groundmotions from the Nisqually earthquake were not strong enoughto produce significant damage at most substations.

Power poles and towers have performed well, exceptwhen they are founded on unstable soils where landslides orliquefaction can occur. In the 1993 Landers (California)earthquake, a fault ruptured through the base of a four-leggedtransmission tower. The tower was distorted, but it did notcollapse. Ground shaking can cause low-voltage power lines toslap together causing short circuits. Higher voltage lines havegreater separation, and thus are less prone to short circuits.

Bridges are usually the most vulnerable components ofhighway systems. More robust bridge designs were developedin the 1970s and 1980s. Older bridges, built to lower designstandards, may be more prone to failure. Bridge decks can slideoff their seats if the seats are too narrow or the seats are notadequately restrained. Supporting columns can buckle if theyare over-loaded and not designed with adequate ductility.Single-span bridges supported on abutments perform better.Bridge foundations in liquefiable soils can move, allowing thespans they support to slide off.

The Nisqually earthquake caused significant damageto about a dozen bridges and highway structures (Figure 6), butnone collapsed. A major intersection at the junction ofInterstate 5 and Interstate 90 in downtown Seattle was closedfor several weeks while inspections and repairs were made.Bridge damage caused closure of northbound lanes ofInterstate 5 for 12 hours in Chehalis, and the Alaska Wayviaduct in Seattle was closed intermittently for weeks to assessand repair earthquake damage. The Deschutes Parkway wasclosed for weeks in Olympia due to lateral spreading (Figure 7).Landslides caused closure of highways 101, 202 and 302(Figure 8).

Railway bridges in general performed well as a resultof the very large loads they are designed to carry. Earthquakesin the U.S. and Japan have not tested the resistance of railroadbridges to liquefaction or lateral spreading but either mode ofground failure could cause loss of bridge approaches. Inaddition, a variety of hazards such as failed overpasses,building debris, and ground failures could affect railroad rightof ways.

Airport runways may be vulnerable to liquefaction. Inthe 1989 Loma Prieta Earthquake, 3000 feet at the end of themain runway of the Oakland Airport were taken out of servicewhen liquefied sand erupted through runway joints. TheNisqually earthquake caused a similar failure at Boeing Fieldwhere most of the largest liquefaction zones correlated with oldriver channels. Airport control tower glass is vulnerable, asmany tower structures are not adequately designed to transferthe roof load to the structure. Control towers at both the Seattle-Tacoma airport (Figure 9) and Boeing Field were damagedduring the Nisqually earthquake.

Pipelines: Water, Wastewater, Liquid Fuel, and Natural Gas

Tanks and Reservoirs

Electrical Power Facilities

Highways

Railways

Airports

Figure 8. Road failure on Washington highway 302 caused bylandsliding during the Nisqually earthquake (Ballantynephoto).

Figure 6. Short-column damage at Holgate overcrossing of

Interstate 5 in Seattle caused by Nisqually earthquake. (Photo

courtesy of Mark Eberhard, University of Washington,

Figure 9. Control tower failures at Seattle-Tacoma InternationalAirport during the Nisqually earthquake. No one was seriouslyinjured by the debris. (photo courtesy Carl Nelman, BoeingCompany, ).www.maximus.ce.washington.edu

Figure 7. Road failure at Capital Lake in Olympia caused bylateral spreading. This roadway also failed in the 1949 and1965 earthquakes. (Photo courtesy Steve Kramer, University ofWashington, ).www.maximus.ce.washington.edu

The lifeline systems and geology shown on the accompanying map have been greatly simplified. Mostsystems are shown in a general way for graphical purposes and may not be accurate in detail. In severallocations where one system overlies another, system elements have been adjusted so that they are moredistinctly visible on the map. The surface geology has been greatly simplified for the purposes of providingregionally consistent geological characteristics throughout the entire study area (Cottage Grove, Oregon,to Vancouver, British Columbia). This map should not be used for any site-specific purpose. Any site-specificconsideration requires more detailed geotechnical and geological data than are presented in this map.

INTRODUCTION

The Interstate 5 highway corridor, stretching fromMexico to Canada, is not only the economic artery of thePacific Northwest, but is also home to the majority ofOregonians and Washingtonians. Accordingly, most regionalutility and transportation systems, such as railroads andelectrical transmission lines, have major components in the I-5 corridor. The section of I-5 from Cottage Grove, Oregon, toBlaine, Washington, is rapidly urbanizing, with populationgrowth and economic development centered around the citiesof Eugene, Salem, Portland, Olympia, Tacoma, Seattle,Everett, and Bellingham. For the purposes of this map, werefer to this area as the I-5 Urban Corridor.

Economic success in this urban corridor heavilydepends on essential utility and transportation systems, calledlifeline systems, such as highways, railroads, pipelines, ports,airports, communications, and electrical power.Consequently, natural disasters that disrupt these lifelinesystems can cause economic losses. For example, a majorwinter windstorm may disrupt an electrical system causingloss of power at smaller distribution substations andwidespread power outages due to falling trees breaking powerlines. As a result, hundreds of thousands of residents andbusinesses may be without power for a day or longer. Largerscale natural disasters, such as earthquakes, can present morecomplex challenges because they tend to affect and disablemany lifeline systems at once. For example, failures in thehighway system after an earthquake may make restoration ofcritical electrical power substations or sewer treatment plantsmore difficult. Subsequently, determining priorities andstrategies for recovery becomes increasingly difficult due tothe potential simultaneous failures of several systems.

As the 2001 Nisqually earthquake reminded us, thePuget Sound region is earthquake country. Large-magnitude,damaging earthquakes struck Olympia in 1949 and Seattle in1965, and the 2001 Nisqually earthquake occurred very nearthe epicenter of the 1949 event. In addition to these largeevents, earthquakes are felt in the Puget Sound region aboutonce a month. In contrast, the southern part of the I-5 UrbanCorridor, the Eugene and Salem areas in particular, hasexperienced very few felt earthquakes this century. However,during the last decade earth scientists have uncoveredconvincing evidence suggesting that the entire UrbanCorridor, from Eugene to Vancouver, B.C. is at risk fromgreat off-shore subduction zone earthquakes, perhaps ofmagnitude 9.

Understanding where major lifeline systems arelocated in relation to earthquake hazards and populationcenters is an important first step in developing mitigationstrategies that can make the I-5 Urban Corridor moreearthquake resistant and expedite economic recovery after anearthquake. Lifeline systems are complex webs that crossthrough many communities and areas of higher and lowerearthquake hazards. The result of the geographic relationshipsbetween the lifelines and underlying geology is a complicatedmulti-layered network that can be difficult to visualize forplanners, emergency response providers, elected officials,and other non-specialists.

To meet the need for a simple and integrated graphicalrepresentation of lifeline systems and earthquake hazards, theUnited States Geological Survey, in cooperation with publicagencies and private companies, has been developing a seriesof maps for the I-5 Urban Corridor. We have divided the I-5Urban Corridor into four regions from Cottage Grove, Oregonto southern British Columbia. This map covers Cottage Groveto Woodburn, Oregon (from about I-5 mileposts 160 to 274).The intent is to provide an overview of the lifeline systems andthe corresponding earthquake hazards for the citizens,engineers, planners, and decision-makers who live and workin this region. Please note that the map does not provide site-specific information for engineering or environmentalpurposes.

The base of the I-5 Corridor maps is a shaded-reliefbackground that provides a quick, qualitative depiction ofslopes and river valleys. The regional geology is generalizedand categorized as probably less hazardous (green) orprobably more hazardous (beige) ground in the event of anearthquake. Simplified lifeline system elementssuperimposed on the geological base are shown for: majorelectric power transmission lines, water supply pipelines,major sewer pipelines and treatment plants, liquid fuelpipelines, natural gas pipelines, and major ports and airports.Each map also shows recent earthquakes of magnitude 2.0 andlarger and historically important earthquakes estimated to belarger than magnitude 5. On this map from Cottage Grove toWoodburn, Oregon, the only seismic event known to begreater than magnitude 5 is the magnitude 5.7 Scotts Millsearthquake east of Salem in 1993.

Lifelines in Earthquake country

The lifelines and earthquake hazards map

U.S. Department of the Interior

U.S. Geological Survey

Figure 2. Faults and earthquakes in the Willamette Valley and vicinity. Solid lines show crustalfaults identified by Yeats and others (1996). Darker lines indicate faults discussed in text.Earthquakes are plotted in 3 magnitude ranges and two shades. Smallest symbols representevents between magnitude 2.0 and 3.4, medium symbols events between 3.5 and 4.9, and largestsymbol size is over 5.0. Only the 1993 Scotts Mills earthquake (magnitude 5.7) is greater thanmagnitude 5. Light symbols are intraplate events, dark symbols crustal earthquakes. (AfterBlakely and others, 2000 )

Figure 3. Earthquakes in Cascadia. Known earthquakes greater than magnitude 6 since about 1870,magnitude 5 since 1950, and earthquakes of magnitude 2 and greater located by the modernseismographic networks and catalogued by the University of Washington (www.ess.washington.edu).Smallest circles are magnitude 2, intermediate circles are between magnitude 5 and 6, and the largestcircles are greater than magnitude 6. Earthquakes are grouped into two broad zones. The redearthquakes occur in the intraplate zone, along with events that occurred within the shallow portion ofthe Juan de Fuca plate. Shallow crustal events are the green earthquakes. Triangles are volcanoes.

Salem

Springfie ld

Eugene

Corvallis

1993 Scotts MillsEarthquake Mag 5.7

Lebanon

Sweet Home

Corvallis

Fault

Mt Angel Fault

Albany

Willa

me

tte

Riv

er

Willa

me

tte

Riv

er

Wald

oHill

sFa

ult

Cottage Grove

1963 Marys Peak 4.2

Woodburn

5

Figure 3. Earthquakes in Cascadia. Known earthquakes greater than magnitude 6 since about 1870,magnitude 5 since 1950, and earthquakes of magnitude 2 and greater located by the modernseismographic networks and catalogued by the University of Washington (www.ess.washington.edu).

Earthquakes are grouped into two broad zones: red earthquakesoccur in the intraplate zone, along with events that occurred within the shallow portion of the Juan deFuca plate, and shallow crustal events are the green earthquakes. The 1700 AD Cascadia earthquakezone is shown in pink. The red triangles are the Cascade volcanoes.

Smallest circles are magnitude 2, intermediate circles are between magnitude 5 and 6, and the largestcircles are greater than magnitude 6.

Mill

Creek and

Turn

er

Faults

39

51

131

0 200 Kilometers

119122125128

41

43

45

47

49

116o

o

o

oo

o o o

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WASHINGTON

VancouverIsland

GORDAPLATE

Ju

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Blanco Fracture Zone

PACIFIC

PLATE

JUAN DE FUCA

PLATE

Go

rda

Rid

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IDA

HO

OREGON

NEVADA

WASHINGTON

CALIFORNIA

MendocinoCape

EXPLO

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PLATE

BRITISH COLUMBIA

NORTH AMERICA

PLATE

Cascad

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