High-stress strike-slip faults in the Apennines: An example from the 2002 San Giuliano earthquakes (southern Italy)

Under the hypotheses that the high-frequency part of the seismic spectrum is controlled by source duration and by peak slip velocity, we applied a recent coda envelope methodology to obtain stable relative source estimates between selected mainshocks and their aftershocks. We computed stable mainshock/aftershock S-wave spectral ratios and used a simple source model in order to quantify the scaling of the seismic sources of the San Giuliano sequence (Southern Italy). From the analysis of the ratios obtained between the main shock of 10/31/2002, and 11 aftershocks, and of those computed between the other main event of the sequence, of 11/01/2002, and 10 aftershocks, we observe that the scaling relationships: M(0) proportional to f(c)(-(3+epsilon)) holds, with epsilon = 0.89 +/- 0.05. Despite the strong discrepancy between the moment magnitude and the high-frequency ground motion excited by the main shocks (M(L) was much lower than M(w)), that would indicate low-stress drop sources, we compute anomalously high stress parameters for both events. By comparison, the same analysis was carried out on seismic data of the Hector Mine seismic sequence (the main event of October 16, 1999, M(W) = 7.0, and six of its aftershocks). We found: M(0) proportional to f(c)(-(3+epsilon)), with epsilon approximate to 0.8 +/- 0.4.