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

Abstract

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. Copyright 2007 by the American Geophysical Union.