U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2010–3023

April 2010

U N D E R S T A N D I N G E A R T H Q U A K E H A Z A R D S I N T H E P A C I F I C N O R T H W E S T

A Magnitude 7.1 Earthquake in the Tacoma Fault Zone—

A Plausible Scenario for the Southern Puget Sound Region, 

Washington

 he U.S. Geological Survey 

 and cooperating scientists

have recently assessed the 

effects of a magnitude 7.1 

earthquake on the Tacoma 

Fault Zone in Pierce County, 

Washington. A quake of 

comparable magnitude struck 

the southern Puget Sound 

region about 1,100 years ago, 

and similar earthquakes are 

almost certain to occur in the 

future. The region is now home 

to hundreds of thousands of 

people, who would be at risk 

from the shaking, liquefaction, 

landsliding, and tsunamis 

caused by such an earthquake. 

The modeled effects of this 

scenario earthquake will 

help emergency planners and 

residents of the region prepare 

for future quakes.

T

Scientists with the U.S. Geological 

Survey (USGS) and cooperating organizations have recently simulated the 

shaking from a magnitude 7.1 earthquake in the Tacoma Fault Zone, in the 

southern Puget Sound region, Washington. This “scenario” models the groundmotion amplitudes for a hypothetical 

rupture along a fault plane that extends 

from the surface to 9 miles (15 km) deep 

and is 35 miles (56 km) in length, from 

Belfair through Vashon Island to near 

the city of Federal Way. These ground 

motions are derived using computer 

models with inputs from geological and

geophysical observations specific to the 

region and the fault zone.

The Tacoma Fault Zone

Local and regional experts have 

identified several strands of the Tacoma 

Fault Zone in the southern Puget Sound 

region, although the eastern extent of 

the zone remains uncertain. Changes in 

elevations of coastal marshes surrounding 

the Tacoma Fault Zone document a large, 

approximately magnitude 7 earthquake 

that occurred about 1,100 years ago. 

Studies are underway to identify evidence 

of earlier quakes and to more accurately 

describe the fault system. The ground 

-124 ̊ -123 ̊ -122 ̊

47 ̊

48 ̊

Aberdeen

Tacoma

Arlington

Auburn

STRAIT OF JUAN DE FUCA

Centralia

Everett

Puyallup

47 ̊

48 ̊

0 50

km

SHAKING: WEAK STRONG SEVERE

latitude ( ̊N)

longitude ( ̊W) PUGET SOUND

This senario “ShakeMap” shows hypothetical horizontal ground motions for a magnitude 7.1 

earthquake in the Tacoma Fault Zone. The strongest shaking (warm colors) occurs in a band 

close to the fault (black polygon) and in low-lying areas filled with sediments that amplify ground 

motions, such as the Tacoma Basin area located south of the fault zone. ShakeMaps show 

“instrumental” intensities, which are measures of the severity of shaking and its impacts. For real 

earthquakes, intensities are derived automatically by computers from the amplitudes of seismic 

waves recorded at monitoring stations located throughout the region affected by an earthquake. 

Although this ShakeMap is for a hypothetical earthquake, scientists have found evidence that 

a quake of similar magnitude struck the southern Puget Sound region about 1,100 years ago.
A magnitude 7 earthquake in the Tacoma Fault Zone could offset the floor 

of Puget Sound and generate a tsunami. Most of the low-lying areas in the 

Puyallup River delta will likely be subjected to flooding by tsunamis. The 

maximum tsunami runup for the simulated Tacoma Fault Zone quake shows 

water reaching an elevation of about 12 feet (~4 meters), and initial flooding 

in the Tacoma area would occur about 5 minutes after the earthquake. (Image 

courtesy of Washington Division of Geology and Earth Resources.)

 less than 0.5

0.5 to 2

2 to 5

Tsunami runup (meters)

Photo from the National Agriculture Imagery Program (NAIP)

PUYALLUP

RIVER

DELTA

TACOMA

GIG

HARBOR

N

 3 

0 2 miles

kilometers

1

0 1 2

shaking and fault offset of several yards 

(meters) during a future major earthquake 

on the Tacoma Fault Zone would cause 

landsliding, liquefaction, and possibly a 

tsunami in Puget Sound. These effects are 

summarized below. 

Ground Motions

Most earthquake hazards result from 

the shaking, or ground motions, caused 

by seismic waves that radiate out from a 

fault as it ruptures. Several factors affect 

the resulting pattern of shaking. Seismic 

waves transmit the energy released 

by the earthquake, and bigger quakes 

generate larger and longer lasting waves. 

The rock and sediment layers that the 

waves travel through affect their strength 

and duration. The pattern of shaking is 

affected by the dimensions and orientation of a fault and by the characteristics 

of the rapid slippage along it that occurs 

during an earthquake. The proximity of 

the rupture to the ground surface also 

significantly affects the strength and 

pattern of shaking. Deep quakes, like 

the magnitude 6.8 Nisqually, Washington, earthquake in 2001 and others like 

it in 1965 and 1949, tend to be much 

less damaging than shallow ones of 

comparable magnitude, like the surfacebreaking scenario earthquake modeled 

in the Tacoma Fault Zone. This difference is primarily due to the fact that in 

deeper quakes the seismic waves have 

lost more energy by the time they reach 

the surface. Unlike deep earthquakes 

that usually have few to no aftershocks, 

a magnitude 7.1 earthquake like the one 

in this scenario would likely have many 

aftershocks, a few of which could be 

large enough to be damaging.

Other Earthquake Effects

Tsunamis—Scientists anticipate 

that a magnitude 7.1 earthquake in the 

Tacoma Fault Zone that ruptures a fault 

reaching the ground surface could offset 

the floor of Puget Sound and generate 

a tsunami. Delta failures and landslides 

caused by the shaking will also likely create or amplify tsunamis. Geological and 

Puyallup

 River

East 

Passage

Vashon

Island

Belfair

TACOMA 

BASIN

SEATTLE

UPLIFT

Tacoma

Federal

 Way

Gig Harbor

Nisqually 



N

0

0 3 kilometers

1 2 miles

1 2

700

500 500

400

800

800 600

700

WASHINGTON

AREA OF MAP

historical evidence shows that landslides 

and failures of the sediments in river deltas have generated tsunamis within Puget 

Sound in the past.

Computer simulations of tsunamigenerating earthquakes show that most 

of the low-lying areas in the Puyallup 

River delta are vulnerable to flooding by 

tsunamis. The maximum tsunami runup 

for the simulated Tacoma Fault Zone 

quake shows water reaching an elevation of about 12 feet (~4 meters) and 

that initial flooding in the Tacoma area 

would occur about 5 minutes after the 

earthquake.

Liquefaction—Severe shaking during an 

earthquake can cause sediments to lose 

strength. In water-saturated sediments 

Map showing the location of several 

strands of the Tacoma Fault Zone (red 

lines) near Tacoma, Washington. The 

subsurface structure of the Tacoma 

Basin is shown by the depth of the 

unconsolidated sediments (green 

contours in meters; 1 m is about 3.28 

ft); these sediments may amplify 

ground shaking during an earthquake. 

Star shows the epicenter of the 2001 

Nisqually earthquake.
Shaking during earthquakes can cause water-saturated sediments to fail in a process called “liquefaction,” resulting in 

damage to overlying buildings and structures. This map shows liquefaction susceptibility for the greater Tacoma urban 

area. Some locations are particularly susceptible, such as the abandoned channels (short black meandering lines) 

of the Puyallup River and Wapato Creek, and have liquefied repeatedly in past earthquakes (stars show sites of past 

liquefaction). (Image courtesy of Washington Division of Geology and Earth Resources.)

strong shaking during an earthquake can 

disrupt the grain-to-grain contacts and 

cause the sediments to compact. When 

compaction occurs, pressure in water-filled 

pores (the water between sediment grains) 

increases, which causes the sediment to 

lose its strength. This sometimes results in 

small geyser-like eruptions of water and 

sediment called “sand blows.”

Sediment in this condition is liquefied and behaves as a fluid. Buildings can 

sink and topple, and foundations can lose 

strength, causing structural collapse or 

severe damage. High liquefaction potential exists for areas covered by artificial 

fill, peat deposits, tidal flats, and stream 

sediments because they are often poorly 

consolidated. Some locations, such as 

the abandoned channels of the Puyallup 

River and Wapato Creek, are particularly 

susceptible and have liquefied repeatedly in past earthquakes. Liquefaction 

failure from the 1949 earthquake killed 

two people at the Port of Tacoma, despite 

that quake’s much greater depth and less 

severe shaking than the magnitude 7.1 

quake modeled in this scenario.

7

7

161

512

167

167

161

167 99

16

509

509

509

LIQUEFACTION SUSCEPTIBILITY OF THE

GREATER TACOMA URBAN AREA, PIERCE AND KING COUNTIES, WASHINGTON

by

Stephen P. Palmer, William J. Perkins, and W. Paul Grant

2003

HIGH

LOW TO MODERATE

VERY LOW

Peat deposits

Area of filled tide flats

Division of Geology and Earth Resources

Ron Teissere - State Geologist

Liquefaction Susceptibility

122 ̊37’30’’ 122 ̊30’00’’ 122 ̊22’30’’ 122 ̊15’00’’

47 ̊07’30’’

47 ̊15’00’’

WASHINGTON DIVISION OF

GEOLOGY AND EARTH RESOURCES

GEOLOGY MAP GM-51

2003

TACOMA

PUGET SOUND

Commencement

Bay The Narrows

0 2 miles

0 3 kilometers

1

1 2
Joan Gomberg, Brian Sherrod, Craig Weaver, 

and Art Frankel

Edited by James W. Hendley II

Graphic design by Judy Weathers

COOPERATING ORGANIZATIONS

Pacific Northwest Seismic Network, 

Dept. of Earth and Space Sciences, 

University of Washington

Washington Department of Natural Resources

For more information contact:

Joan Gomberg

U.S. Geological Survey

University of Washington, 

Dept. of Earth and Space Sciences,

 Box 351310, Seattle, WA 98195

gomberg@usgs.gov

http://earthquake.usgs.gov/

This fact sheet and any updates to it are avalable 

on line at

http://pubs.usgs.gov/fs/2010/3023/

Earthquake shaking may cause landslides on slopes, particularly where the ground is water saturated or has been 

modified (for example, by having stabilizing vegetation removed). This photograph shows landslides at Salmon Beach, 

near Tacoma, that were caused by the 2001 magnitude 6.8 Nisqually earthquake, which resulted in an estimated $2 billion 

in damage and is one of the largest recorded quakes in Washington’s history. Slopes along coastal areas of the southern 

Puget Sound region are particularly vulnerable to landsliding during powerful earthquakes.

Landslides—Earthquake shaking may 

cause landslides on slopes, particularly 

where the ground is water saturated or 

has been modified (for example, had 

stabilizing vegetation removed). Steeper 

slopes are most susceptible, but old, 

deep-seated landslides may be reactivated, even where gradients are as low 

as 15%. Catastrophic debris flows can 

move water-saturated materials rapidly 

and for long distances, mostly in mountainous regions. Underwater slides also 

have occurred in the Puyallup River 

delta in past earthquakes.

Although the magnitude 7.1 earthquake scenario for the Tacoma Fault Zone 

was modeled using the best scientific 

information available, it still represents 

a simplified version of expected ground 

motions. The damage resulting from 

ground motions in an actual earthquake of 

similar magnitude is likely to be even more 

variable than that modeled, depending on 

the specific characteristics of each affected 

structure and its surrounding environment.

The information from this hypothetical but plausible earthquake scenario is being used to help better prepare 

residents of the southern Puget Sound 

region for future earthquakes in the 

Tacoma Fault Zone. The work of USGS 

scientists in the Puget Sound region 

is only part of the USGS Earthquake 

Hazards Program’s ongoing efforts to 

protect people’s lives and property from 

the earthquakes that are an inevitable 

part of life in the Pacific Northwest.