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Toward resolving an earthquake ground motion mystery in west Seattle, Washington State: Shallow seismic focusing may cause anomalous chimney damage

W. J. Stephenson·A.D. Frankel·J. K. Odum·R. A. Williams·T. L. Pratt
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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.

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W. J. Stephenson, A.D. Frankel, J. K. Odum, R. A. Williams, T. L. Pratt. 2006-03-30. Toward resolving an earthquake ground motion mystery in west Seattle, Washington State: Shallow seismic focusing may cause anomalous chimney damage. https://doi.org/10.1029/2005gl025037

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