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

This is for the 2012 AGU Fall Meeting.

Abstract
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

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

On comprehensive recovery of an aftershock sequence with cross correlation

The name of this blog suggests that I am a physicist trying to introduce some principal rules of  mechanics into economics. Economics is a hobby rather than every day activity. At some point, it's worth  to present my professional work. Here is our poster from the 2012  General Assembly  of the European Geophysical Union.  

Abstract and Poster

I. Kitov, D. Bobrov, J. Coyne, and G. Turyomurugyendo

CTBTO, IDC, Vienna, Austria (Ivan.Kitov@ctbto.org)

We have introduced cross correlation between seismic waveforms as a technique for signal detection and automatic event building at the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Treaty Organization. The intuition behind signal detection is simple – small and mid-sized seismic events close in space should produce similar signals at the same seismic stations. Equivalently, these signals have to be characterized by a high cross correlation coefficient. For array stations with many individual sensors distributed over a large area, signals from events at distances beyond, say, 50 km, are subject to destructive interference when cross correlated due to changing time delays between various channels. Thus, any cross correlation coefficient above some predefined threshold can be considered as a signature of a valid signal. With a dense grid of master events (spacing between adjacent masters between 20 km and 50 km corresponds to the statistically estimated correlation distance) with high quality (signal-to-noise ratio above 10) template waveforms at primary array stations of the International Monitoring System one can detect signals from and then build natural and manmade seismic events close to the master ones. The use of cross correlation allows detecting smaller signals (sometimes below noise level) than provided by the current IDC detecting techniques. As a result it is possible to automatically build from 50% to 100% more valid seismic events than included in the Reviewed Event Bulletin (REB).We have developed a tentative pipeline for automatic processing at the IDC. It includes three major stages. Firstly, we calculate cross correlation coefficient for a given master and continuous waveforms at the same stations and carry out signal detection as based on the statistical behavior of signal-to-noise ratio of the cross correlation coefficient. Secondly, a thorough screening is performed for all obtained signals using f-k analysis and F-statistics as applied to the cross-correlation traces at individual channels of all included array stations. Thirdly, local (i.e. confined to the correlation distance around the master event) association of origin times of all qualified signals is fulfilled. These origin times are calculated from the arrival times of these signals, which are reduced to the origin times by the travel times from the master event. An aftershock sequence of a mid-size earthquake is an ideal case to test cross correlation techniques for autiomatic event building. All events should be close to the mainshock and occur within several days. Here we analyse the aftershock sequence of an earthquake in the North Atlantic Ocean with mb(IDC)=4.79. The REB includes 38 events at distances less than 150 km from the mainshock. Our ultimate goal is to excersice the complete iterative procedure to find all possible aftershocks.We start with the mainshock and recover ten aftershocks with the largest number of stations to produce an initial set of master events with the highest quality templates. Then we find all aftershocks in the REB and many additional events, which were not originally found by the IDC. Using all events found after the first iteration as master events we find new events, which are also used in the next iteration. The iterative process stops when no new events can be found. In that sense the final set of aftershocks obtained with cross correlation is a comprehensive one.

Perspectives of cross correlation in seismic monitoring at the International Data Centre

This is a preprint of our paper submitted to the Pure and Applied Geophysics:

Abstract

We demonstrate that several techniques based on cross correlation are able to significantly reduce the detection threshold of seismic sources worldwide and to improve the reliability of IDC arrivals by a more accurate estimation of their defining parameters. More than ninety per cent of smaller REB events can be built in automatic processing while completely fitting the REB event definition criteria. The rate of false alarms, as compared to the events rejected from the SEL3 in the current interactive processing, has also been dramatically reduced by several powerful filters. The principal filter is the difference of arrival times between the master events and newly built events at three or more primary stations, which should lie in a narrow range of a few seconds. Two effective pre-filters are f-k analysis and Fprob based on correlation traces instead of original waveforms. As a result, cross correlation may reduce the overall workload related to IDC interactive analysis and provide a precise tool for quality check for both arrivals and events. Some major improvements in automatic and interactive processing achieved by cross correlation are illustrated by an aftershock sequence of a large continental earthquake. Exploring this sequence, we describe schematically the next steps for the development of a processing pipeline parallel to the existing IDC one in order to improve the quality of the REB together with the reduction of the magnitude threshold. The current IDC processing pipeline should be focused on the events in areas without historical seismicity which are not properly covered by REB events.