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A preliminary study of regional seismic discrimination in central Asia with emphasis on western China

Geophysics Group, EES-3 Earth and Environmental Sciences Division Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (505) 665-8495; (505) 667-4739; hartse{at}lanl.gov

Abstract

In support of a Comprehensive Test Ban Treaty, we have started to evaluate regional seismic-event discrimination capabilities for central Asia, emphasizing western China. We have measured noise and signal levels of over 260 earthquakes and 26 underground nuclear explosions recorded 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 (measured as either m_b or M_L) range between 2.5 and 6.5 and maximum event-station distance is about 1700 km. We formed phase, spectral, cross-spectral, short-period/long-period, and long-period ratios to test many possible event discriminants. All ratios were corrected for distance effects before forming ratio-versus-magnitude discrimination plots.

The most consistent discriminants for separating earthquakes from explosions 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 effective, but surface-waves are difficult to measure for magnitudes below about 4.5. For the P/S ratios, separation between earthquakes and explosions 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 weak 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 quite as consistently as the P/S ratios. However, P spectral ratios may prove useful in cases where path effects block or highly attenuate S_n and L_g waves, rendering the P/S ratios unavailable. In contrast to Nevada Test Site (NTS) events, the L_g (3 to 6 Hz/0.75 to 1.5 Hz) spectral ratio does not separate earthquakes and nuclear explosions in central Asia. This observation may be a source effect, as Asian explosions are thought to be detonated in highly lithified rocks below the water table, while most of the smaller (m_b < 4.8) NTS explosions have been detonated in poorly lithified rocks above the water table.

For station 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 earthquakes 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 separate KTS explosions and Asian earthquakes. However, R/L ratios fail to identify the large Lop Nor explosions recorded at station AAK because the Love waves are as large as the Rayleigh waves, suggesting that strong tectonic release occurs with these explosions. Because we have examined only one path at a single station for these explosions, source-radiation pattern may be influencing this observation.

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