Propagation or Source? Analysis of a Moderate-Magnitude Seismic Event in the Qinghai Province, China

We investigate a moderate-size (mb 4.5) seismic event that occurred in the Qinghai Province, China on 6 December 1997. Observations of large Pn and little Sn energy at high frequency (>3 Hz) show explosion-like characteristics at the Chinese Digital Seismic Network station WMQ. We perform high- and low-frequency analyses using both body and surface waves. The longer-period surface wave analysis shows that the event was indeed an earthquake. Several factors appear to be contributing to the large Pn amplitude and small Sn amplitude. A zone of high Sn attenuation identified in the Tibetan plateau appears to be contributing to the lowamplitude Sn observations if the zone lies further north than previously identified. The focal mechanism and rupture directivity may be contributing to the large Pn observation. Furthermore, focusing of the Pn wave is possibly given propagation from a source region with a thick crust to thinner crust at station WMQ.