The Cascadia Subduction Zone vs. Seaside Having participated in school tsunami drills in Seaside, Oregon, the adequacy of practiced evacuation plans in the

event of a local earthquake along the Cascadia Subduction Zone (CSZ)—the fault along the West coast of the U.S. where

tectonic plates collide, building up extreme pressure—is lacking. The face of the land will change before anyone knows to

evacuate. And in light of the recent subduction-zone earthquake and tsunami in Tohoku, Japan, which was more deadly than

anticipated, the tsunami’s potential needs to be reevaluated also.

Because Seaside is a rather poor city, the best chance it has at a generally survivable evacuation plan is to

collaborate with privately-owned, large, new structures already in the area, whose building codes ensure a higher probability

that they will withstand an earthquake, and instruct people to run toward them in the event that they can’t quickly reach

higher elevations. This would require most of the population to run toward the ocean, where large hotels and parking

structures sit against the beach. However no one would follow such instructions without assertive publicity, especially

considering the high annual proportion of tourists.

The figure above shows the city of Seaside against several tsunami possibilities. The last earthquake along the CSZ was in the

1700’s, and the only remaining evidence of it is in the rock record and Native American history, which suggests that it was 10 meters high and

took 4 days to recede. This by itself would be discomfiting news, looking above at the devastation a 10-meter tsunami would cause, but the

most comparable earthquake in recent history was the Tohoku, Japan earthquake, whose tsunami was 40 meters high. This would entirely

submerge the city of Seaside. The residents and tourists of the city would have approximately 15 minutes to get to safety. It’s also alarming

that the official city evacuation routes citizens are instructed to take often cross multiple bridges before reaching official destinations of

safety; bridges are notorious for structural failure during earthquakes. In this case, failure would trap the greater mass of the population to

face the tsunami without protection. And only one of the city’s evacuation destinations would even survive a tsunami like Tohoku’s, should

anyone make it to them.

Some landmarks are included as both a spatial reference for those familiar with the area and as an implication of danger; most of

these landmarks are trapped between two rivers, at an almost strategic disadvantage , should the bridges fail. The downtown area that’s

usually the most populous is directly west of Broadway Middle School and the fire department.

This map shows perspective on the basis for Seaside’s evacuation plans. The Oregon Department of

Geology and Mineral Industries (DOGAMI) has reported that a much bigger area needs to be evacuated than the

Federal Emergency Management Agency’s (FEMA) expected tsunami would imply. An 8-meter contour is included to

show where a moderate tsunami might inundate, but this model is still more drastic than the report by FEMA. A 40-

meter contour is included to show where the recent tsunami in Japan would have reached. Only the southernmost of

the evacuation destinations designated by the city meet DOGAMI’s standards, much less the necessary elevation to

escape a tsunami similar to Tohoku’s.

‘Developed & Urban’ land is the type most vulnerable to a tsunami

for several reasons: developed land has very poor drainage (water can’t be

absorbed by asphalt and concrete like it can by soil), and the concentration

of people allows for greater loss of life no matter what the disaster is.

Vegetation can also act as a buffer against tsunami waves, but Seaside has no

vegetation to guard it from this destruction. Planting more vegetation,

allowing for the growth of more forest within the city or between it and the

ocean, might greatly reduce the impact of a tsunami.

This is a basic 3-dimensional view of the

modeled fault used to estimate the impact of a CSV

earthquake on the surface. The fault is approximately

120 kilometers west of seaside, dipping under the West

coast. Because the exact dipping angle of the fault

plane and amount of slip that will take place during

the earthquake are not certain, several models of

vertical displacement were considered, which show

where the land would uplift and subside at the surface:

5˚ dip, 12 meter slip 10˚ dip, 12 meter slip 12˚ dip, 2 meter slip

12˚ dip, 5 meter slip 12˚ dip, 7 meter slip 12˚ dip, 10 meter slip

12˚ dip, 13 meter slip 12˚ dip, 15 meter slip 15˚ dip, 12 meter slip

Although all the scenarios of slip and dip are possible, a dip of 12˚

and a slip of 12 meters were considered the most predictable and

consistent with the greatest number of models. It is shown above with

contours for greater accuracy in reading subsidence and uplift. Paired with

the figure below, modeling where the West coast falls on the vertical

displacement graph (with a star on Seaside), we can predict that the area

of study will most likely subside (fall) around 1.5 meters in elevation.

Although this scenario would produce a magnitude 8.8 earthquake and

older structures will likely crumble, 1.5 meters of subsidence luckily does

not pose much more danger to Seaside than the tsunami would otherwise

inflict.

To the right is a model of

the horizontal displacement

the modeled earthquake

would produce. Because

Seaside lies along the 120

kilometer line of the X axis,

we can predict that Seaside

will ‘jump’ to the west

approximately 2 meters, along

with its subsidence

A 3-dimensional representation of the

subsidence and uplift that would result from

the modeled earthquake. Seaside lies at

120 kilometers along the X-axis.

The map above models liquefaction potential in the area

of study. Liquefaction is a phenomenon that makes certain

rock-types behave liquidly during the shaking of an

earthquake. High-density objects, including buildings, cars,

and sometimes even people, will sink into the ground while

low-density objects might ‘float’ to the top, such as gasoline

tanks at gas stations and septic systems. Loose sand is

especially susceptible to this. Liquefaction poses the biggest

concern about evacuating people to the large, new structures

on the beach instead of instructing them to evacuate to the

hills.

12˚ dip, 12 meter slip

Sources: Nationalmap.gov, web.pdx.edu/~jduh/seasidegis/shapefiles/main.php, spatialdata.oregonexplorer.info, dingo.gapanalysisprogram.com/landcoverv2/DownloadData.aspx, ir.library.oregonstate.edu/xmlui/bistream/handle/1957/9402/Tolson_Patrick_m_1975_Plate 2 Geology.jpg?sequence=2,

www.oregongeology.org/sub/publications/IMS/ims-010/Maps/images/seaside?liq.jpg, nationalatlas.gov, google earth, seismo.berkeley.edu/seismo/annual_report/ar99_00/node28.html, oregon.gov/dogami/earthquakes/coastal/ofr95-67.pdf Created by C.S. Woolley of Brigham Young University-Idaho, mentored by Dr. J. Willis