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

• Published in: Geophysical research letters Vol. 33; no. 6; pp. L06316 - n/a
• Main Authors: Stephenson, William J., Frankel, Arthur D., Odum, Jack K., Williams, Robert A., Pratt, Thomas L.
• Format: Journal Article
• Language: English
• Published: Washington, DC American Geophysical Union 01-03-2006; Blackwell Publishing Ltd
• Subjects: Continental crust; Earth sciences; Earth, ocean, space; Earthquake ground motions and engineering seismology; Exact sciences and technology; Exploration Geophysics; General or miscellaneous; Seismic methods; Seismology; United States; Washington; damage; folds; chimneys; simulation; Variability; S-waves; ground motion; earthquakes; North America; faults; Amplification; acceleration; plane waves; depth; bedrock; fault zones; deformation; strong motion; high resolution; seismic waves
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2005GL025037

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.