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Earthquake
Hot Topics
Earthquakes are one of the most significant threats facing Seattle. Large quakes have occurred here in the
past and will happen again. Many residents remember the Nisqually earthquake of 2001.
While it was a big event, it is small compared to potential "Big Ones" that we face. A fault zone
runs through the middle of the City. Off the coast a chunk of oceanic crust being pushed
under North America. When it slips it will produce a massive quake similar to the one that caused the 2004
Indian Ocean Tsunami. Luckily these types of earthquakes are not as common as the "Nisqually"
type that occur about every 30 to 50 years deep under Puget Sound. The origins of these quakes are so
deep, the seismic waves are weaker by the time they reach the surface.
Major Incidents
Year |
Epicenter |
Magnitude / Category |
Impacts |
2001 |
Nisqually Delta |
6.8 (deep) |
Regionally: 1 death, 407 injuries and $2 billion worth of damage. |
1965 |
South Puget Sound |
6.5 (deep) |
Regionally: 6 fatalities regionally and nearly $100 million worth of damage in 2006 dollars. |
1949 |
South Puget Sound |
7.1 (deep) |
Regionally: 8 fatalities regionally and $292 million worth of damage in 2006 dollars. |
1946 |
Mid-Puget Sound |
6.3 (deep) |
Light damage in Seattle mainly in the Duwamish Valley. |
1700 |
Washington Coast |
9.0 (subduction) |
Massive megathrust quake drops the Washington coast by several feet. Learn
here how scientists
can date this quake to the hour it happened. |
About 900 AD |
Seattle |
7.4 (shallow) |
This quake would be destastating for Seattle. Find out more
here. |
Issues to Note
Pacific Northwest quakes fall into three categories: shallow, deep, and subduction:
- Shallow: Depths vary from 0 to 30km. They are usually felt very intensely near their epicenter, but their effects usually diminish quickly. There is an active shallow fault system running through the middle of Seattle (the Seattle Fault).
- Deep: Depths between 35 and 70km. Since they are farther from the surface, they are not felt as intensely, but are experienced over a wider area than shallow quakes. Deep ruptures in the Juan de Fuca Plate produced the 1949, 1965 and 2001.
- Subduction: Result when the interface between the San Juan plate and North American plates ruptures. They occur from off the Washington coast. This area is over 1,000 km long. Subduction earthquakes are the largest type of quake, with magnitudes from 8.0 to over 9.0. They have occurred at intervals ranging from 200 to 1,100 years (State of Washington, 2001).
The Earthquake Engineering Research Institute conducted a Seattle Fault Scenario
that modeled a magnitude 6.7 quake. It is the worst quake Seattle is likely to face in the near future. Ground motions would be two
to five times that of the Nisqually Earthquake. A rupture on the Seattle Fault
zone could severely disrupt north-south lifeline systems, including utilities
and transportation routes (EERI, 2003).
A major Seattle Fault earthquake could cause over 1000 deaths and devastate the City's infrastructure. Older brick buildings (unreinforced masonry) and
tilt up construction is the most vulnerable. Areas of the City on soft soils like the Duwamish and Rainier Valleys are the most vulnerable. Seattle's
geography and our dependence on bridges make us vulnerable to transportation disruptions.
On the Web
How to Prepare for Earthquakes (PDF) - Information from the City of Seattle, King County and Washington State.
Pacific Northest Seismic Network - The best site for information on earthquake in our region.
United States Geological Survey Earthquake Hazards Program - Collects quake information
from the whole world. Excellent source of general earthquake information.
Nisqually Earthquake Information from the Pacific Northwest Seismic Network.
Seattle Post-Intelligencer Special Report on the Nisqually Earthquake
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