LOCAL AMPLIFICATION OF SEISMIC WAVES FROM THE MW7.9 ALASKA EARTHQUAKE AND DAMAGING WATER WAVES IN LAKE UNION, SEATTLE, WASHINGTON

The Mw7.9 Alaska earthquake of 3 November, 2002, caused minor damage to at least 20 houseboats in Seattle, Washington, by initiating water waves in Lake Union. These houseboats were damaged by water waves likely initiated during the seismic surface waves, which produced the largest ground motions from this earthquake. Because Lake Union is situated on the Seattle sedimentary basin, the water waves may have been initiated in part from local amplification of the seismic waves by the basin. To better understand the causes of these water waves and estimate the hazard from them in future earthquakes, we examined ground shaking on the strong motion recorders from the Pacific Northwest Seismic Network (PNSN). Maps of peak ground acceleration recorded during the Alaska earthquake on the PNSN strong-motion instruments show substantially increased surface wave amplitudes coincident with the Seattle sedimentary basin. We computed spectral ratios with respect to nearby bedrock sites at periods of 1 sec to 100 sec. Amplifications of surface waves by the basin were greater than a factor of 4 at periods from 2.5 to 10 sec, with peak amplifications of about 10 at periods of 3 to 5 sec. Shear wave arrivals showed similar amplification values, but their absolute amplitudes were much smaller than the surface wave amplitudes. Modeling of water waves indicates a single surface wave arrival would produce water waves with a peak amplitude of several cm in Lake Union. Resonance initiated by multiple cycles of surface waves, focusing, and near-shore effects could further amplify the water waves, creating potentially damaging waves.