Analysis of the 2008 Chinese Earthquake Aftershocks Using Cross- Correlation.

摘要

Our objective is to assess the performance of a cross-correlation technique as applied to automatic and interactive processing of aftershock sequences at the International Data Centre (IDC). This technique allows a flexible approach to time windows, frequency bands, correlation thresholds and other parameters controlling the flux of detections. For array stations, we used vertical channels to calculate a unique cross-correlation coefficient. All detections obtained by cross-correlation were then used to build events according to IDC definitions. To investigate the influence of all defining parameters on the final bulletin, we selected the aftershock sequence of the March 20, 2008 earthquake in China with mb(IDC) = 5.41. As templates, fragments of P- and Pn-waves from two sets of the IDC Reviewed Event Bulletin (REB) events were selected: all 19 events from the second and third hour after the main shock and 50 events from the entire sequence with mb between 3.0 and 4.0. By varying the threshold of correlation coefficient and F-statistics which were applied to original waveforms and to the cross-correlation time series, we obtained several bulletins with different numbers of events, which could be compared to the original REB and also checked manually. These events were split into four categories: (1) new events having a counterpart (origin time within 10 sec) in the REB, (2) new valid events not having a counterpart in the REB, (3) valid REB events not having counterparts in any bulletin created by cross- correlation, and (4) bogus (invalid) events created by crosscorrelation and which are in the REB.