Seismology makes an important contribution toward monitoring compliance with the Comprehensive Test Ban Treaty (CTBT). An important task at the testbed of the Center for Monitoring Research (CMR, Washington DC, USA) and the International Data Center (IDC) of the Comprehensive Test Ban Treaty Organization (CTBTO, Vienna, Austria) is to detect, locate and characterize seismic events in order to distinguish between natural sources of seismic waves such as earthquakes, and other sources which might possibly be nuclear tests. For large events, this is not particularly difficult. However, small events, whether natural or man-made, present a greater challenge. While their epicenters and magnitudes can be determined fairly precisely using standard seismological methods, seismic moment tensor analysis can help in two ways. It gives information about the size and mechanism of a source in terms of its seismic moment and the moment tensor components. It provides, in addition, an estimate of the source's depth, which cannot always be reliably determined using normal location techniques. Thus, if an event has a large non double-couple component () its source may be an explosion, possibly a nuclear explosion, while tectonic earthquakes typically have more than 70-80% double couple movement (Dreger and Woods, 2002). The source depth determined from moment tensor analysis may also help to weed out deep tectonic events from among the more than 100000 events of magnitude 4 and greater that occur annually. Only events at shallow depths need be scrutinized as part of the monitoring process of the Comprehensive Test Ban Treaty (CTBT).
This project's goal is to implement the procedure for automatically determining seismic moment tensors routinely used in real-time at the University of California at Berkeley (UCB, Romanowicz et al., 1993; Dreger and Romanowicz, 1994; Pasyanos et al., 1996) on the testbed at CMR. Although the moment tensor procedure will not run in real-time on the testbed, in its final implementation it will run automatically, triggered from the Reviewed Event Bulletin (REB) and will be an additional, potentially powerful method for screening events (Pechmann et al., 1995; Dreger and Woods, 2002).
The earthquakes which occurred in Alaska in October and November, 2002, provide an excellent opportunity for testing the moment tensor procedures.
This research is sponsored by the Defense Threat Reduction Agency under contract DTRA01-00-C-0038.
Dreger, D. and B. Romanowicz, Source characteristics of events in the San Francisco Bay Region, USGS Open-file report, 94-176, 301-309, 1994. Dreger D. and B. Woods, Regional Distance Seismic Moment Tensors of Nuclear Explosions, Tectonophysics, 356, 139-156, 2002.
Pasyanos, M., D. Dreger, and B. Romanowicz, Toward real-time estimation of regional moment tensors, Bull. Seism. Soc. Am., 86, 1255-1269, 1996.
Pechmann, J.C., W.R. Walter, S.J. Nava, and W.J. Arabasz, The February 3, 1995, 5.1 seismic event in the Trona mining district of southwestern Wyoming, Seis. Res. Lett., 66, 25-34, 1995. Ratchkovski, N.A., Hansen, R.A., New Evidence for Segmentation of the Alaska Subduction Zone, Bull. Seism. Soc. Am., 92, 1754-1765, 2002.
Romanowicz, B., M. Pasyanos, D. Dreger, and R. Uhrhammer, Monitoring of strain release in central and northern California using broadband data, Geophys. Res. Lett., 20, 1643-1646, 1993.
Berkeley Seismological Laboratory
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