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On May 2, at 9:04 PM PDT (May 3, 04:04 UTC) a shallow earthquake with coda duration magnitude M_D 5.4 occurred 35 km east-northeast of Seattle, Washington, near the town of Duvall, in a narrow transitional area between the Puget Sound Lowland and the Cascade Range. Strong-motion sensors recorded a peak horizontal ground acceleration of 0.19g at the toe of the Tolt River Dam (16 km to the southeast).
Both P - wave first motions and three-component broadband seismograms recorded by the Pacific Northwest Seismograph Network (PNSN) show that the mainshock had a thrust mechanism with an east-west compression axis. This is unusual in that most earthquakes in the region (as well as borehole studies) indicate north-south compression. A least-squares moment tensor inversion using PNSN broadband seismograms yields a strike = 350 degrees, dip = 53 degrees (to the east), rake = 83 degrees, and M_0 = 4.1 X 10^23 dyne-cm (M_W = 5.0). Aftershock seismograms (3.0 <= M_D <= 3.4) were used as empirical Green's functions, indicating a source duration of t = 1 - 2 seconds.
Collecting felt reports over the Internet, the PNSN rapidly estimated Modified Mercalli intensity at 845 sites in the region, and produced an intensity contour map within a few days of the mainshock. This map and other pertinant information were available at a World Wide Web page receiving over 17,000 "visits" in the week following the mainshock (http://www.geophys.washington.edu/SEIS/EQ_Special/Duvall/duval.html).
For the first 100 days of aftershock activity the PNSN catalog contains 390 earthquakes, where 96 aftershocks are M_D => 2.0, and 9 aftershocks are 3.0 <= M_D <= 3.4. The spatial pattern of aftershocks from preliminary analyses suggests a complex structure; inclusion of local digital data recorded on portable instruments will be important in fully understanding the earthquake sequence.
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