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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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.
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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.
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