Thursday, July 31, 2014

Sumatera 2012 aftershocks: REB vs. waveform cross-correlation bulletin

I am working on an exiting problem associated with my professional duties – waveform cross correlation. Unfortunately, have no time to blog on economics. This post is to attract attention to our poster to be presented at the Monitoring Research Review 2012.

 Our objective is to assess the performance of waveform cross-correlation technique, as applied to automatic processing of the aftershock sequence of the 2012 Sumatera (Mw=8.6) earthquake, relative to the Reviewed Event Bulletin (REB) issued by the International Data Centre. The REB includes ~1150 aftershocks between April 11 and May 23 with (IDC) body wave magnitudes from 3.05 to 6.19. The aftershocks cover a slightly unusual V-shaped area. The cross correlation technique allows a flexible approach to signal detection, phase association and event building. To automatically recover the sequence, we selected sixteen aftershocks with mb(IDC) between 4.5 and 5.0 from the IDC Standard Event List (SEL3) available on April 13. These events evenly but sparsely cover the whole area. After a superficial manual review these aftershocks were designated as master events. Waveform templates from only seven array stations with the largest SNR for the signals from the main shock were used to calculate cross-correlation coefficients. All detections obtained by cross-correlation were then used to build events according to the IDC definition, i.e. at least three primary stations with accurate arrival times, azimuth and slowness estimates. The qualified events populated the cross-correlation Standard Event List (XSEL). The XSEL was compared with two IDC products: the final automatic bulletin (SEL3) and the interactive bulletin (REB). There are some valid events missed in the REB but found in the XSEL. As a bootstrap exercise to confirm the significance of the XSEL findings, a large portion of the newly built events was reviewed interactively by experienced analysts. In order to investigate the influence of all defining parameters (cross correlation coefficient threshold and SNR, F-statistics and F-K analysis, azimuth and slowness estimates, relative magnitude, etc.) on the final XSEL we have constructed relevant frequency distributions for all detections and only for those which were associated with the XSEL events. These distributions are also station and master dependent. This allows the introduction of accurate threshold for all defining parameters.

Key words: cross-correlation, IDC, REB, Sumatera



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