Frequency-dependent coda-Q values for subregions of the state of Washington have been determined using a time-domain coda-decay method. A total of 95 earthquakes digitally recorded on a regional seismograph network of over 100 short-period stations were selected to obtain 206 on-scale, high signal-to-noise ratio traces for analysis. Grouping the data by structural regions of the state showed only small variations in coda Q by region other than in a very localized area at Mount St. Helens. At Mount St. Helens coda Q was significantly lower than for all other areas. An average of all of the data except that from the local Mount St. Helens area results in a Q relationship of: Qc = 63 f0.97 for the frequency band 2 to 16 Hz. There is little variation of this coda-Q relationship when using sources down to 50 km depth and thereby uniformly sampling the top 50 to 100 km of the crust and upper mantle.

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