West Seattle Movie
Simulation for Figure 4 of Geophysical Research Letters manuscript:
Stephenson, W. J., A. D. Frankel, J. K. Odum, R. A. Williams, and T. L. Pratt (2006), Toward resolving an earthquake ground motion mystery in west Seattle, Washington State: Shallow seismic focusing may cause anomalous chimney damage, Geophys. Res. Lett., 33, L06316, doi:10.1029/2005GL025037.
Towards resolving an earthquake ground motion mystery in West Seattle, Washington State: Shallow seismic focusing may cause anomalous chimney damage
by
William J. Stephenson,
Arthur D. Frankel, Jack K. Odum, and Robert A. Williams
U.S. Geological Survey, Golden, Colorado, USA,
and
Thomas L. Pratt
U.S. Geological Survey, Seattle, Washington,
USA
Abstract A shallow bedrock fold imaged by a 1.3-km long high-resolution shear-wave seismic reflection profile in West Seattle focuses seismic waves arriving from the south. This focusing may cause a pocket of amplified ground shaking and the anomalous chimney damage observed in earthquakes of 1949, 1965 and 2001. The 200-m bedrock fold at ~300-m depth is caused by deformation across an inferred fault within the Seattle fault zone. Ground motion simulations, using the imaged geologic structure and northward-propagating north-dipping plane wave sources, predict a peak horizontal acceleration pattern that matches that observed in strong motion records of the 2001 Nisqually event. Additionally, a pocket of chimney damage reported for both the 1965 and the 2001 earthquakes generally coincides with a zone of simulated amplification caused by focusing. This study further demonstrates the significant impact shallow (<1km) crustal structures can have on earthquake ground-motion variability.
Movie Caption. Time slices for plane wave approaching from southern (left) side of bedrock fold model. The plane wave is dipping 45° north. Interpreted geologic boundaries are black and white solid lines. S-wave velocities for three interpreted layers are (shallowest to deepest) 485 m/s, 725 m/s and 1200 m/s. Seismograph stations HOLY and WEK are shown at top of model. WEK experienced a nearly three-fold greater amplification in horizontal peak ground acceleration than HOLY during the 2001 Nisqually earthquake. The direct SH-wave is initially arched by bedrock-sediment interface (white geologic boundary). As simulation progresses, the focused direct SH-amplitude increases. The highest wave amplification occurs when focused segment of direct SH-wave reaches region of station WEK.
