Lower Crust and Upper Mantle Structure in the Region of the 2000 Western Tottori, Japan Earthquake Estimated from Receiver Function Analyses

In this study, we image the P to S converted structure in the lower crust and upper mantle by receiver function (RF) analyses, for the source region of the 2000 Western Tottori, Japan earthquake (Mw 6.6). We use 21 teleseismic events recorded on a dense array of 54 stations that were deployed for aftershock studies following the main shock. The deconvolution technique used RF analyses essentially removes P waves, except for the direct arrival, and leaves conversions and reverberations of P to S types for a plane layered structure. We used data low-passed filtered at about 1 Hz. RF analyses usually use 1-D structures to determine the conversion depths along the assumed ray path of the incident P wave. In this study, however, we could not obtain a clear image by using this method. This is because that the target conversion waves propagate along different ray paths from that of P wave in a complex structure. Therefore, we estimate the arrival direction of the P coda, including the direct P wave, and the P to S converted waves, using a polarization analysis with multi-taper method (Park et al., 1987a). These results show that the arrival directions of P to S converted waves have variances of more than 20 degrees from that of P wave direction, indicating a strong influence from the 3-D velocity structure on the arrival directions of the converted waves. We estimated arrival directions of the converted phases from many teleseismic events over a large azimuthal range of incoming P waves. After we use this information to construct the 3-D surface where the P to S conversions occur near the recording station, we transform the time domain RF into depth domain one. Using this method with data from closely spaced stations, we can produce a clearer image of the upper mantle structure. The results show a surface that is located below the Moho discontinuity at depths of about 40 to 60 km and dipping toward the north, in the vicinity of the rupture area of the 2000 Western Tottori Earthquake. Recently, it has been suggested that there is particular deformation in the lower crust and upper mantle under active faults that cause large inland earthquakes. The structure imaged in this study may be related to these movements and important for understanding the preparatory processes of large crustal earthquakes.