abstract

Intensity data from 14 historic earthquakes in or near Washington State, as reported at over 300 localities, are used to study the attenuation structure in Washington. The empirical relation of Evernden (1975) is used to determine the size and depth for each earthquake and the local attenuation factor, k, for two physiographic parts of the state. The value for k in the Puget Sound region and north into Canada is 134, while k=112 is more appropriate for eastern Washington and northern Oregon. Individual local amplification factors are computed for all localities at which four or more earthquakes have been felt by averaging the difference between the computed intensity and reported intensity at each site. Using these correction factors, the intensities for the North Cascade earthquake of 1872 are used to place constraints on its size and location. It appears this earthquake may be slightly larger (magnitude 7.4) and located south and west of the original epicenter determined by Milne.

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