A PRELIMINARY-STUDY OF REGIONAL SEISMIC DISCRIMINATION IN CENTRAL-ASIA WITH EMPHASIS ON WESTERN CHINA

In support of a Comprehensive Test Ban Treaty, we have started to eval uate regional seismic-event discrimination capabilities for central As ia, emphasizing western China. We have measured noise and signal level s of over 260 earthquakes and 26 underground nuclear explosions record ed at the broadband, digital station WMQ in western China and over 120 earthquakes and five nuclear explosions at station AAK in Kyrgyzstan. The explosions are from the Kazakh Test Site (KTS) and Lop Nor, China . The earthquakes are mostly from northwest China. Event magnitudes (m easured as either m(b) or M-L) range between 2.5 and 6.5 and maximum e vent-station distance is about 1700 km. We formed phase, spectral, cro ss-spectral, short-period/long-period, and long-period ratios to test many possible event discriminants. All ratios were corrected for dista nce effects before forming ratio-versus-magnitude discrimination plots . The most consistent discriminants for separating earthquakes from ex plosions are P/S phase ratios for frequencies above 3 to 4 Hz. P-wave (f > 1 Hz) to Rayleigh-wave (0.03 Hz < f < 0.1 Hz) ratios are also eff ective, but surface-waves are difficult to measure for magnitudes belo w about 4.5. For the P/S ratios, separation between earthquakes and ex plosions increases with frequency, and for the P/R ratios, separation increases with increasing P frequency. The P (3 to 6 Hz)/S (0.75 to 1. 5 Hz) cross-spectral ratios also separate earthquakes and explosions. These cross-spectral ratios may prove quite useful because, due to wea k high-frequency L-g signals from explosions, the high-frequency P/L-g ratio sometimes cannot be evaluated, but the P (3 to 6 Hz)/L-g (0.75 to 1.5 Hz) can be evaluated successfully. P spectral ratios (3 to 6 Hz /0.75 to 1.5 Hz) also separate earthquakes and explosions but not quit e as consistently as the P/S ratios. However, P spectral ratios may pr ove useful in cases where path effects block or highly attenuate S-n a nd L-g waves, rendering the P/S ratios unavailable. In contrast to Nev ada Test Site (NTS) events, the L-g (3 to 6 Hz/0.75 to 1.5 Hz) spectra l ratio does not separate earthquakes and nuclear explosions in centra l Asia. This observation may be a source effect. as Asian explosions a re thought to be detonated in highly lithified rocks below the water t able, while most of the smaller (m(b) < 4.8) NTS explosions have been detonated in poorly lithified rocks above the water table. dFor statio n AAK, the short-period spectral and cross-spectral ratios identify a few earthquakes (not recorded at WMQ) from the north Pamir region that plot with the Lop Nor explosions. The waveforms from these earthquake s lack surface waves and a distinct L-g phase. These events, which we interpret as having occurred in a subducted slab, do separate from the explosions on high-frequency (f > 4 Hz) P/S discrimination plots. For station WMQ, long-period R/L (Rayleigh-wave/Love-wave) ratios separat e KTS explosions and Asian earthquakes. However, R/L ratios fail to id entify the large Lop Nor explosions recorded at station AAK because th e Love waves an as large as the Rayleigh waves, suggesting that strong tectonic release occurs with these explosions. Because we have examin ed only one path at a single station for these explosions, source-radi ation pattern may be influencing this observation.