Earthquakes

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

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

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.