UNDERSTANDING EARTHQUAKE HAZARDS IN WASHINGTON STATE

Modeling a Magnitude 9.0 Earthquake on theCascadia Subduction Zone off the Pacific CoastGeologic Description
The coastline of the northwestern U.S. and Canada isbordered by an active subduction zone where the Juande Fuca plate is subducting, or being pushed, beneaththe North American plate. Currently, the subduction
zone is considered locked (that is, it is not slipping).
Strain is therefore accumulating on the lockedinterface between the plates. Plate convergence isestimated to be between 3 and 4 centimeters per year
and possibly as high as 5.8 centimeters per year (thelong-term geologically estimated rate).
The M9.0 Cascadia scenario is based on an
approximately 1,000 kilometer (620 mile)-longrupture of the Cascadia subduction zone megathrustfault. The rupture extends from Cape Mendocino,
California, to central Vancouver Island, Canada. This
scenario is based on geologic evidence that indicatessuch ruptures occurred on the megathrust in the past:
The last rupture was on January 26, 1700. Geologic
evidence suggests that the average recurrence of~M9.0 earthquakes along the Cascadia megathrust isabout 500 years, but recurrence intervals vary,ranging from about 250 years to over 1,000 years.
The effects of these earthquakes include strong
ground shaking that goes on for several minutes,
subsidence and/or uplift of coastal areas, liquefaction,and tsunami. Aftershocks will be both strong andnumerous (possibly M7 or higher).

Figure 1. ShakeMap for a M9.0 earthquake on theCascadia megathrust. The black polygon offshore is
the modeled fault rupture for this scenario.

larger the waves and the longer they last. Severalfactors affect the strength, duration, and pattern ofshaking:The type of rock and sediment layers that thewaves travel through.

Type of Earthquake

Most earthquake hazards result from ground shaking
caused by seismic waves that radiate out from a fault
when it ruptures. Seismic waves transmit the energy
released by the earthquake: The bigger the quake, theThe dimensions and orientation of the fault
and the characteristics of rapid slippage alongit during an earthquake.

How close the rupture is to the surface of theground.

Washington Military DepartmentEmergency Management Division

FEMASubduction Zone Earthquakes: Subduction zone
earthquakes occur where the Juan de Fuca oceanic
plate is being forced under the North American plate.
An earthquake is produced when pressure that has
built up along this zone causes the plates to slip
suddenly and rapidly past each other. Shaking from
the M9.0 earthquake modeled in this scenario will be
felt over the entire region and may last for several
minutes. This event is similar to the 2011 Tohoku
earthquake and tsunami in Japan.
Aftershocks: Unlike deep earthquakes, such as theM6.8 Nisqually earthquake in 2001, which usuallyproduce few or no aftershocks strong enough to befelt, a M9.0 subduction zone earthquake will befollowed by thousands of aftershocks, a few of which
could be large enough to cause additional damage andproduce tsunamis.

Other Earthquake Effects
Tsunamis: A M9.0 Cascadia subduction zone
earthquake is expected to generate a massive tsunamithat will reach the coast of Washington about 20 to 30
minutes after the earthquake; waves may continue tostrike the coastline for the next 12 to 24 hours.
(Tsunami waves will also travel across the PacificOcean.) Delta failures and landslides caused by theshaking may also create or amplify tsunami waves.

Liquefaction: If sediments (loose soils consisting ofsilt, sand, or gravel) are water-saturated, strong
shaking can disrupt the grain-to-grain contacts,causing the sediment to lose its strength. Increased
pressure on the water between the grains cansometimes produce small geyser-like eruptions ofwater and sediment called sand blows. Sediment inthis condition is liquefied and behaves as a fluid.
Buildings on such soils can sink and topple, andfoundations can lose strength, resulting in severe
damage or structural collapse. Pipes, tanks, and otherstructures that are buried in liquefied soils will float
upward to the surface.
Artificial fills, tidal flats, and stream sediments areoften poorly consolidated and tend to have highliquefaction potential.
Landslides: Earthquake shaking may cause landslides
on slopes, particularly where the ground is watersaturated or has been modified (for example, by theremoval of stabilizing vegetation). Steeper slopes aremost susceptible, but old, deep-seated landslides may
be reactivated, even where gradients are as low as
15%. Catastrophic debris flows can move watersaturated materials rapidly and for long distances,mostly in mountainous regions. Underwater slides arealso possible, such as around river deltas.

Figure 2. Tsunami damage at
Onagawa, Ishinomaki, after theM9.0 Tohoku earthquake near
Japan’s east coast on March 11,
2011.
(Photo: NOAA/NGDC: ShunichiKoshimura)

BE PREPARED WHEREVER YOU ARE:
Develop a plan and a disastersupply kit. When you’reprepared, you feel more incontrol and better able to keep
yourself and your family safe.

LEARN MORE ABOUT WHAT YOU
CAN DO: www.emd.wa.gov
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Hazus Results for theCascadia Subduction Zone Scenario
Hazus is a nationally applicable standardized
methodology developed by FEMA to help planners
estimate potential losses from earthquakes. Local,state, and regional officials can use such estimatesto plan risk-reduction efforts and prepare foremergency response and recovery.
Hazus was used to estimate the losses that could
result from a M9.0 earthquake on the Cascadia
subduction zone. Such an event is expected toimpact 23 counties in Washington. Among the mostaffected by the earthquake are Clallam, Grays
Harbor, Jefferson, King, Mason, Pacific, and Pierce.
(These estimates do not include losses due totsunami impacts.)
Injuries: The number of people injured in thisscenario will be high. Estimates vary by location,
ranging from several dozen (as in Jefferson County)
to nearly 2,000 (in King County). Although manyof the injuries will not be life-threatening, people inevery county will require medical attention and,in many cases, hospitalization. Potentially lifethreatening injuries and fatalities are expected; these
are likely to be more numerous if the earthquake
happens during the afternoon or early evening.
Damage: King County will have the greatest
number of damaged buildings (more than 130,000).
For other counties, the number is lower, but it often
represents a much greater proportion of the county’s
building stock (as in Clallam, Grays Harbor, Pacific,and Mason counties). Most of the damagedbuildings will be residential, but the number ofcommercial and industrial structures is also
extremely high. The degree of damage will vary,
but extensive damage to thousands of buildings is
expected in Clallam, Grays Harbor, King, Mason,
Pacific, and Pierce counties. Structural collapse(complete damage) of thousands of buildings is also
expected (more than 3,000 in Clallam County).
Economic Losses Due to Damage: Capital stock
losses are the direct economic losses associated withdamage to buildings, including the cost of structuraland non-structural damage, damage to contents, andCASCADIA SUBDUCTION ZONE SCENARIO EARTHQUAKE
End-to-end length of fault (kilometers)
Magnitude (M) of scenario earthquake
Number of counties impacted
Total injuries (*severity 1, 2, 3, 4) at 2:00 PM
Total number of buildings extensively damaged
Total number of buildings completely damaged
Income losses in millions
Displaced households
People requiring shelter (individuals)
Capital stock losses in millions
Debris total in millions of tons
Truckloads of debris (25 tons per truckload)

1,100
9.0
23
7,534
43,681
8,768$3,811
18,385
11,630$11,994
5.68
227,240

Table 1. Summary of significant losses in the M9.0
Cascadia subduction zone earthquake scenario. Amongthe most affected counties are Clallam, Grays Harbor,Jefferson, King, Mason, Pacific, and Pierce.
*Injury severity levels: 1—requires medical attention, but not
hospitalization; 2—not life-threatening, but does require
hospitalization; 3—hospitalization required; may be life-threatening
if not treated promptly; 4—victims are killed by the earthquake

loss of inventory. For this scenario, the estimates aresubstantial, ranging from more than $78 million inJefferson County to over $3 billion in King County.
Income losses, including wage losses and loss ofrental income due to damaged buildings, are high:King County alone accounts for over $1 billion.
Impact on Households and Schools: The numberof people without power or water will be highest inKing County (followed by Pierce, Grays Harbor,Pacific, and Clallam). King, Grays Harbor, andPierce counties will have the highest number ofdisplaced households and individuals in need ofshelter. The functionality of many schools will be
seriously affected by the earthquake. In PacificCounty, functionality will initially be as low as 12%.
Debris Removal: Following this earthquake, debris(brick, wood, concrete, and steel) will have to beremoved and disposed of. King County alone
accounts for more than 1 million tons, Grays Harborfor 740,000 tons, and Pierce for 583,000 tons.
Estimates vs. Actual Damage: Although this M9.0
earthquake was modeled using the best scientificinformation available, it represents a simplified
version of expected ground motions. The damage

Page 3 of 4 / 2012–2013resulting from an actual earthquake of similarmagnitude is likely to be even more variable andwill depend on the specific characteristics andenvironment of each affected structure.
Other Tools: Community planners can also look at
how a large earthquake may impact local resources
and people’s lives and livelihoods. The following
graphs illustrate variations in such impacts: The firstshows the levels of shaking that residents are likely
to experience; the second shows the possible impact
on different services and business sectors. Note thatin King County, a greater number of residents will
be exposed to very strong shaking, whereas GraysHarbor, Pacific, Mason, and Clallam counties,
although less populated, will experience even more
intense ground motions.

Figure 3. Number of residents and employees affected by the M9.0 earthquake projected for the Cascadia subduction zone.
Modified Mercalli Intensity (MMI) classes indicate peak ground acceleration (PGA) values and the impact of the shaking.
V. Rather Strong(PGA 3.9–9.2 g)
VI. Strong(PGA 9.2–18 g)

Felt outside by most. Dishes and windows may break. Large bells ring. Vibrations like large train passing close tohouse.
Felt by all; people walk unsteadily. Many frightened and run outdoors. Windows, dishes, glassware broken. Books
fall off shelves. Some heavy furniture moved or overturned. Cases of fallen plaster. Damage slight.
Difficult to stand. Furniture broken. Damage negligible in buildings of good design & construction; slight–VII. Very Strong
moderate in other well-built structures; considerable in poorly built/badly designed structures. Some chimneys(PGA 18–34 g)
broken.
VIII. Destructive Damage slight in specially designed structures; considerable in ordinary substantial buildings (partial collapse);(PGA 34–65 g) great in poorly built structures. Fall of chimneys, factory stacks, columns, walls. Heavy furniture moved.
IX. Violent
General panic; damage considerable in specially designed structures; well designed frame structures thrown out(PGA 65–124 g) of plumb. Damage great in substantial buildings: partial collapse. Buildings shifted off foundations.
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