CHARACTERISTICS OF SURFACE-WAVES GENERATED BY EVENTS ON AND NEAR THE CHINESE NUCLEAR TEST SITE

Our goal is to characterize surface waves at intermediate periods (5-30 s) generated by nuclear explosions and natural earthquakes occurring in and around the Chinese nuclear test site at Lop Nor observed at epicentral distances between 10 degrees and 20 degrees. We present observations of Rayleigh- and Love-wave group velocities, spectral amplitudes and polarizations for eight nuclear explosions and nine earthquakes recorded at IRIS and GEOSCOPE broad-band digital stations and at the Kirghiz Telemetered Seismic Network (KNET). The wave paths studied cross complex geological regimes. Waveforms are extremely complicated, yet observations made with frequency-time analysis are robust and repeatable. (1) Group velocity and spectral amplitude measurements vary strongly across the studied region, but display understandable systematics related to known tectonic features. (2) Both group velocity and spectral amplitude measurements across the KNET array are generally similar at periods above 10s, but differences are striking at shorter periods. Stacking methods dependent on waveform coherence across the network should only succeed at periods above 10s for this large-scale array. (3) Group velocity measurements for earthquakes and nuclear explosions that share a common wave path agree well, but spectral amplitudes differ appreciably, especially for Rayleigh waves. Typically, earthquake spectra are enriched in longer periods (12-20 s) relative to explosion spectra for events of the same magnitude. (4) Higher modes are observed on frequency-time diagrams for Rayleigh waves generated by earthquakes, but are not observed for explosions. (5) Multipathing is observed in both frequency-time diagrams and polarization analyses, but, again, is qualitatively understandable in terms of known structural features. In particular, the channelling of Love waves by the Tarim Basin is documented. (6) Large-amplitude Love waves are generated by nuclear explosions at Lop Nor, indicative of significant tectonic release. We discuss the relevance of the differences in spectral amplitudes between earthquakes and explosions and of the appearance of higher modes on earthquake frequency-time diagrams to the problem of discriminating nuclear explosions from earthquakes.