Magnitude 6.8 WASHINGTON
2001 February 28 18:54:32 UTC
Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver
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The preliminary mechanism for this earthquake is tensional (normal) faulting in the subducting (downgoing) Juan de Fuca Plate, caused by bending of the plate. Damage and injuries have occurred in the Olympia-Seattle area. This earthquake is located in the same general area as a magnitude 7.1 earthquake on April 13, 1949. The location for this earthquake was furnished by the Geophysics Program, University of Washington, Seattle. About 400 people injured and major damage in the Seattle-Tacoma- Olympia area. Maximum intensity (VIII) in the Capitol Hill area of Olympia and in the Pioneer Square area south of downtown Seattle. Preliminary estimates of damage are between 1 and 4 billion U.S. dollars. Felt from central Oregon to southern British Columbia and as far east as northwestern Montana. The maximum recorded acceleration was 0.3g at Seward Park. Landslides occurred in the Tacoma area and near Renton. Liquefaction and sand blows occurred in parts of Olympia and South Seattle. No aftershocks were recorded by the University of Washington network in the first 6 hours following the magnitude 6.8 main shock. In a typical aftershock sequence, the highest rate of aftershock activity occurs during this period. Earthquakes of comparable magnitude with nearby epicenters and similar depths occurred in 1949 and 1965. The 1949 earthquake had only one aftershock in the following six months, and little aftershock activity was observed following the 1965 earthquake. On the basis of this information, we do not expect significant aftershocks from today's earthquake. The depths (50-70 km) of these three earthquakes may be a factor in their low aftershock productivity. Shallow (less than 20 km) mainshocks are more likely to have energetic aftershock sequences. |
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