The use of “grand master” events for waveform cross correlation

This is for the 2012 AGU Fall Meeting.

More than 90% of seismic events recorded at teleseismic and regional distances are from a few relatively small geographic regions, causing the distribution of seismic events in the Reviewed Event Bulletin (REB) of the International Data Centre (IDC) to be inhomogeneous. When considering the waveform cross correlation technique for the detection, phase association and event building processes that are performed as part of monitoring compliance of the Comprehensive Nuclear-Test-Ban Treaty one is confined to the areas with historical seismicity. The backbone of the waveform cross correlation method is the set of master events (earthquakes or explosions) with high quality waveform templates that have been recorded at array stations of the International Monitoring System (IMS). These master events have to be evenly distributed and their template waveforms should be representative and pure (ie., with negligible noise input). The coverage and characteristic of historical seismicity observed by the IMS seismic network since 2001 does not match these requirements. The current REB allows selection of a number of master events in seismically active areas but even in these areas the quality of templates varies from master to master. In this study, we propose to replicate waveforms from the best master event over a regular grid expanding several hundred kilometers from its epicenter. We call this master event the “grand master”. For each grid point, i.e. replicated grand master event, the template has the relevant theoretical time delays between individual sensors at the involved array stations. Since the empirical deviations from the theoretical arrival times at these sensors are inherently related to seismic velocity structure beneath the station, they are fully retained for all replicated master events within several hundred kilometers. These empirical travel time residuals are small but play a key role in the waveform cross correlation method for weak signals. They define a higher sensitivity for the cross correlation method relative to beam forming method where the channels are stacked with the theoretical delays. As a result, the waveform cross correlation technique detects more valid signals at local, regional, and global levels.

In assessing the performance of the grand master approach the aftershock sequence of the April 11, 2012 Sumatra earthquake (Ms(IDC)=8.2) was used, with16 master events (actual aftershocks) distributed over an area of 500x500 km. Waveform templates from the best master event over a regular grid with 1o spacing have been replicated. There are two principal procedures in comparing the performance of actual and replicated master events as associated with various characteristics/distributions of detections, as well as with the number of event hypotheses built with the varying sets of stations and locations. Both methods have shown the superiority of the replicated events distributed over a regular grid. Such distributions also reduce the volume of calculations by two orders of magnitude. When appropriately chosen, the grand master allows a reduction in the magnitude threshold of seismic monitoring and improving the accuracy and uncertainty of event locations at the IDC to the level of the best located events. When a ground truth event is available, one can expand its influence over hundreds of kilometers.

Key words: array seismology, waveform cross correlation, seismicity, master events, IDC, CTBT

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