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Seattle 'intraslab' quake began deep in Earth


A rumble from far below

(CNN) -- The tremor that shook the Seattle and Olympia areas in Washington state Wednesday is the same kind of earthquake that killed dozens in Mexico in 1999 and claimed 1,000 lives in El Salvador this past January.

These poorly understood tremors, called "intraslab" quakes, pose a special threat to the Seattle area.

"The thing that's of interest here is that the larger of the intraslab earthquakes occur right beneath the urban corridor," said Steve Kirby from the United States Geological Survey. "So they're right beneath the feet of where people live."

Below is an interactive guide to earthquake magnitude and severity:

Intraslab quakes begin deep in the earth, in undersea areas called subduction zones.

In a subduction zone, one slab of the earth's surface is slowly sliding under another plate. As it sinks, it is under enormous pressure, and is heated by intense temperatures deep inside the earth.

That causes chemical and physical changes in the slab, which release trapped water. The water lets the rock crack along ancient fault lines, setting off an earthquake inside the slab anywhere from 30 to 180 miles below Earth's surface.

The quakes do less damage at that depth than they would at the surface, but that hardly means they are not powerful or deadly. Quakes of this type in Washington state in 1965 and 1949 each killed eight people.

Because they originate so deep below the Earth's surface, intraslab quakes are harder to study. Scientists believe this latest one in Seattle occurred about 30 miles underground.

Structural damage from Wednesday's Pacific Northwest quake
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Double dose of woe strikes historic Seattle neighborhood
March 1, 2001
'It made death a little more real,' eyewitness says
February 28, 2001
Businesses struggle to cope with quake
February 28, 2001
Bush says FEMA on top of quake situation
February 28, 2001
Moderate quake hits Southern California
February 10, 2001
Earthquake shakes southern California
December 24, 2000
Desert earthquake rattles Southwest
October 16, 1999
Moderate earthquake shakes Northern California
September 3, 2000

Washington State Department of Transportation Traveler Information
Washington Emergency Management Division
City of Seattle's Official Web Site
West Coast & Alaska Tsunami Warning Center
  • WC&ATWC Most Recent Event Message
  • Location Map
  • Close-up Map
Puget Sound Earthquake Origin and Occurrence
U.S. Geological Survey
The World-Wide Earthquake Locator
Understanding Earthquakes
USGS National Earthquake Information Center
Global Earthquake Response Center
Berkeley Seismology Lab

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4:30pm ET, 4/16

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