The Berkeley Digital Seismic Network is ideally suited for the examination of the performance of a sparse regional network of broadband stations in the monitoring of nuclear explosions under the Comprehensive (nuclear) Test Ban Treaty (CTBT). The Seismological Laboratory has been involved in several studies with this objective. First, we have assessed the performance of a sparse network in the location of seismic events (Dreger et al., 1998). We have found that the addition of azimuthal information derived from the polarities of long-period Pnl waves considerably improves event locations using a sparse station geometry. Another result of this study is that it was possible to obtain locations with absolute mislocation levels (relative to ground truth) less than 18 km using stations in the 300 to 400 km distance range. Locations within 18 km of the true location is a stated goal of the CTBT. When more distant stations are used we find that it is not possible to locate events with this absolute accuracy using the technique that we employed.
Second, we have ported the two regional distance moment tensor codes operated by the Seismological Laboratory to the Prototype International Data Center (PIDC) (Romanowicz et al., 1998). We have tested the codes on a number of global earthquakes using the data available from the International Data Center and have obtained promising results. Future research on this project will focus on developing an automated procedure to assess solution quality and to "tune" the codes to operate in the very sparse monitoring environment afforded by the International Monitoring System.
Third, we have examined the performance of routine, regional distance, moment tensor methods in the discrimination of Nevada Test Site (NTS) nuclear explosions from earthquakes Dreger and Woods, 1998). Isotropic component restrained and unrestrained moment tensor inversions were performed on a reference earthquake and four nuclear explosions, namely the Bexar, Bullion, Dalhart and Junction tests. These events are 5.5 ( 4.5), good signal-to-noise levels, and represent a range of tectonic release. Our results show that as few as two stations are needed to distinguish the anomalous radiation pattern of explosions as compared to earthquakes. The explosions are characterized with shallow source depth 1 km, and a variety of anomalous moment tensors characterize them as "suspect events." The solution that is obtained depends upon the strength of the tectonic release. Figure 14.1 illustrates the difference between the isotropic restrained and unrestrained inversions for the Junction explosion. Note that the unrestrained inversion results have only a 2% double-couple component, and that the anomalous radiation pattern is split evenly between a vertically oriented CLVD and an isotropic component.
This research was supported by ARPA under contract DASG609610001 and by Lawrence Livermore National Laboratory through the Department of Energy's Comprehensive Test Ban Treaty Research and Development Program under the Inter-University (IUT) Agreement No. B331608.
Dreger, D., R. Uhrhammer, M. Pasyanos, J. Franck, and B. Romanowicz, "Regional and Far-Regional Earthquake Locations and Source Parameters using Sparse Broadband Networks: A Test on the Ridgecrest Sequence.", In press Bull. Seism. Soc. Am., 1998.
Dreger, D. and B. Woods, "Regional Moment Tensors as a Discriminant?", submitted to Bull. Seism. Soc. Am., 1998
Romanowicz, B., D. Dreger, M. Pasyanos, J. Durek, "Seismic Moment Tensor Inversion: Application to the International Data Center." Status report submitted to Nuclear Treaty Program Office, June 30, 1998.