Automated Moment Tensor Software for Monitoring
the Comprehensive Test Ban Treaty

Margaret Hellweg, Douglas Dreger, Barbara Romanowicz, Jeffry Stevens (SAIC)

Introduction

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 ($> 50\%$) 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 Denali Sequence

The earthquakes which occurred in Alaska in October and November, 2002, provide an excellent opportunity for testing the moment tensor procedures.

Figure 25.1: A: Epicentral locations of the Denali mainshock (large star), foreshock (medium-sized star) and the four aftershocks (small stars) analyzed. The dots show the epicenters other events in the sequence. Moment tensor solutions north of the epicenters were calculated using data filtered between 20 s and 50 s, those to the south were calculated from data filtered between 30 s and 100 s. The second solution south of FS was calculated using data for all stations. B: Mechanisms for recent Alaska seismicity given by Ratchkovski and Hanson (2002). The dotted lines mark the areas of overlap between the two maps. Stars mark the locations of the mainshock, foreshock and the two aftershocks in the region of overlap.

On the map in Figure 25.1A, the dots represent aftershocks in the sequence, while the locations of the mainshock, the foreshock and the four aftershocks we analyzed are shown as the large, medium-sized and small stars, respectively. We applied the CW moment tensor method to data from the foreshock on 23 Oct 2002, (FS, $m_{b}$(NEIC) 6.1 and $m_{b}$(REB) 5.5), and four aftershocks (AS1: 5 Nov, 07:50 UTC, $m_{b}$(NEIC) 4.9 and $m_{b}$(REB) 4.6; AS2: 08 Nov, 04:04 UTC, $m_{b}$(NEIC) 5.3 and $m_{b}$(REB) 5.1; AS3: 08 Nov, 17:34 UTC, $m_{b}$(NEIC) 5.2 and $m_{b}$(REB) 5.0; AS4: 08 Nov, 20:29 UTC, $m_{b}$(NEIC) 5.0 and $m_{b}$(REB) 4.7). The moment tensor calculated for the foreshock using data from three primary stations of the IMS network that had been filtered between 30 s and 100 s (mechanism south of FS) agrees well with that given in the Harvard CMT catalog. The fits between the synthetics and data are very good. It was notable that to achieve this fit, we had to use GFs for distances 100 km too short. This is very likely due to the fact that we use Greens functions calculated from the iasp91 velocity model. The actual seismic velocities under the North American continent are probably faster than those given in the model. Ratchkovski and Hanson (2002) have investigated recent seismicity in Alaska, producing mechanisms of many events in a region overlapping with some of the seismicity from the Denali sequence. Figure 25.1B shows their results. Both the foreshock and two of the aftershocks lie within the limits of their study. It is notable that the mechanisms north of the epicenters in Figure 25.1A, determined using data and GFs filtered between 20 and 50 s agree very well with the solutions for previous events located along the Denali Fault.

Acknowledgements

This research is sponsored by the Defense Threat Reduction Agency under contract DTRA01-00-C-0038.

References

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