The use of local and regional S-wave coda is shown to provide stable amplitude ratios that better constrain source differences between event pairs. We first compared amplitude ratio performance between local and near-regional S and coda waves in the San Francisco Bay region for moderate-sized events, then applied the coda spectral ratio method to the 1999 Hector Mine mainshock and its larger aftershocks. We find: (1) Average amplitude ratio standard deviations using coda are 0.05 to 0.12, roughly a factor of 3 smaller than direct S-waves for 0.2 f 15.0 Hz; (2) Coda spectral ratios for the 7.0 Hector Mine earthquake and its aftershocks show a clear departure from self-similarity, consistent with other studies using the same datasets; (3) Event-pairs (Greens function and target events) can be separated by 25 km for coda amplitudes without any appreciable degradation, in sharp contrast to direct waves.
Berkeley Seismological Laboratory
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