The S to P convert wave from the bottom of sediment basin in the near-field seismic records

Introduction On October 18 and 19, 1989, a sequence of earthquakes with magnitude MS=5.5, 5.8 and 5.5 occurred in the southeast part of Datong basin, Shanxi Province, China. One year and five months later, on March 26, 1991, another strong earthquake with magnitude MS=5.8 strikes the Datong basin once more. After both October 1989 and March 1991 earthquakes, the Institute of Geophysics, China Seismological Bureau (IGCSB) made near-field observations for aftershocks. 5 DCS-302 digital…

The S to P convert wave from the bottom of sediment basin in the near-field seismic records

From the near-field records of aftershocks of October 1989 and March 1991 Datong earthquakes, an extra phase between P and S arrivals is found. High-precision epicenter location shows that some of these records are obtained with the incidental angle less than the critical angle. This excludes the possibility that the extra wave phase is a refractive wave from ground surface. Particle motion analysis shows that the characteristic of the extra wave is similar to that of P wave, therefore it is very possible that the extra phase is an S to P convert wave from the bottom of sediment basin. Suppose a low velocity layer covers on a high velocity basement. Successful simulation by synthetic seismogram conforms that the extra phase is an SP convert wave from the interface of basin bottom. Modifying the depth of interface at each ray path to match the waveform, we obtain an interface distribution in space. In this way a brief imagine of bottom could be shown, and Datong basin has a V shape basin bottom.