Simulation of Seismic Energy Focussing in West Seattle

This simulation is a supplement for Figure 4 of the GRL paper listed in the references below.

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.

Description of Simulation

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.

References