A new spectral ratio method using narrow band coda envelopes: Evidence for non-self-similarity in the Hector Mine sequence

The use of local and regional S‐wave coda is shown to provide stable amplitude ratios that better constrains 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 Mw 7.0 Hector Mine earthquake and its aftershocks show a clear departure from self‐similarity, consistent with other studies using the same datasets; and (3) event‐pairs (Green's function and target events) can be separated by ∼25 km for coda amplitudes without any appreciable degradation, in sharp contrast to direct waves.