Introduction

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 $M_{w}$ 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.

Figure 2.11: An example of amplitude ratios for a pair of events ($M_{w}$ 3.63 and 3.74) separated by 15.7 km in epicentral distance and identical depths of 7 km. Direct S-wave ratios (top) for 8 color-coded stations show significant scatter over the entire frequency range, whereas the coda wave amplitude ratios (bottom) are very stable from station to station.