Site Response in West Seattle, Washington
The largest recorded earthquakes in the Puget sound area occurred in 1949 (mb 7.1) and 1965 (mb 6.5). Although the 1949 event occurred near Olympia and the 1965 event was 25 km south of Seattle, both earthquakes caused widespread damage in Seattle. For both earthquakes, the worst damage, corresponding to an intensity of Modified Mercalli VIII, was at Harbor Island and West Seattle. Harbor Island is man-made fill, so the high intensities are easily explained, but the high intensities in West Seattle are more difficult to understand.
We deployed an array of seismographs in the Seattle urban area to investigate the correlation of intensity with site response. Estimates of site response are indicated on the maps as circles.
We conducted a special study in West Seattle to take advantage of the seismic air gun sources used in the SHIPS experiment. Nine seismographs were deployed on the Pleistoscene outwash sand deposits that underlie most of West Seattle, and a tenth station, ALD, was located at Alki Point on Tertiary sandstone and was used as the reference rock site. ALD recorded significantly smaller ground motion than the other sites.
The array recorded 47 pops of the SHIPS air gun as the ship passed around the West Seattle peninsula from the south into Elliot Bay. The ship's motion provided us with records from sources over a 180-degree azimuth range. The array also recorded a magnitude 3.1 ML earthquake near Bremerton.
