Receiver Function Analysis in and around the Boso-Peninsula, Central Japan

We analyzed receiver functions to estimate a crustal structure in and around the Boso Peninsula, which is located east of Tokyo, Japan. The area includes a part of the source region of the 1923 Kanto Earthquake (M=7.9). The seismicity beneath the Boso Peninsula is reported to be lower than that around the area, and large events did not occur in recent 50 years. Several slow-slip events were also detected by continuous GPS observations. We applied the receiver function estimation technique using multiple-taper methods [Park and Levin, 2000]. We used seismograms of earthquakes occurred in teleseismic distances (epicenter distances between 30 to 90 degrees) and regional event that occurred in the subducting Pacific plate. In the Boso Peninsula, we installed a dense seismographic array which consists of 30 stations along a line of the northeast-southwest direction with a station spacing of about 2 km to 10 km. The array has observed teleseismic and regional events since February 2004. We also used seismograms recorded by regional seismographic networks from August 2002. A velocity structure beneath the Boso area is very complex. The Philippine Sea plate is subducting northward and the Pacific plate is subducting westward. We show some preliminary results although a recording period of a half year is not long enough for reliable results. First, we suggest that positive pulses at 4 -6 s correspond to the upper boundary of the northward subducting Philippine Sea plate. The depths expected from the positive pulses are relatively consistent with those estimated by the controlled source experiment. However, the direct P wave at 0 seconds is not clear and a thick sediment layer with a low velocity near surface delays the arrival by about 1-2 s. Some converted phase observed in local events also seems to appear on a common boundary. We expect to clarify the seismic velocity structure of deeper part of the crust to estimate the location of the upper plate boundary of the subducting Philippine Sea plate from these conversion phase analyses.