A study of Puget Sound strong ground motion

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Abstract
An attempt was made to model strong motion velocities and displacements from the 1965 Puget Sound earthquake. Teleseismic P waves recorded at Tumwater, Washington, were also examined to place constraints on allowable interface contrasts and to determine whether lateral heterogeneity is a major factor affecting wave propagation. Although strong motion models qualitatively showed many of the characteristics of near-vertical wave propagation in layered structures, the amplitude behavior of individual stations was quite complex. Data from Tacoma and Seattle sites attained lower velocities and acceleration compared to Olympia. The amplitude behavior is consistent with higher attenuation under Tacoma and Seattle although this is not strictly required. The short-period P data recorded at Tumwater show evidence of large velocity contrast interfaces under the station consistent with those assumed in the crustal models. The teleseismic data also indicated that dipping structure or other lateral heterogeneity is important for Olympia structure. Irrespective of these wave propagation problems, the largest single factor which has affected the level of strong ground motions in Puget Sound is the large source depth of past earthquakes. Thus, estimates of seismic hazard based on a direct interpretation of the strong motion data of the 1965 and 1949 events will be erroneously biased toward less hazard if there is potential for shallow faulting in the Puget depression.

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