Imaging of the Philippine Sea Plate beneath the Chugoku and Shikoku Regions in Southwest Japan - Reflection and Forward-scattering Analyses of Local Earthquakes and Teleseismic P Codas -

In this study, we investigate how the Philippine Sea (PHS) Plate subducts beneath the Chugoku and Shikoku regions in southwest Japan, which will be essential for understanding the tectonic settings and the origin of the volcanoes in southwest Japan. Nakamura et al. (1997) showed that there are seismic activities accompanied by the PHS plate subduction to the depth of ~40km beneath the northern part of the Shikoku region. Yamane et al. (2000) detected the leading edge of the high-velocity slab at depths of ~60-80km in the middle of the Chugoku region through travel time tomography. Siomi (2002) found the velocity discontinuity due to the PHS plate subducton to the depth of ~50km by receiver function analysis. However, it is not clear how deep and how far the aseismic slab reaches beneath the Chugoku region. We detected a highly reflective layer at depths of 50-60km beneath the northern part of the Chugoku region by reflection analysis using aftershocks of the 2000 Western Tottori Earthquake (Doi et al., 2003). In the present study, in order to elucidate how deep the PHS plate subducts as well as the relationship between the reflective layer stated above and the PHS plate, we applied the reflection analysis using the waveforms of local earthquakes and forward-scattering analysis of the teleseismic P coda in the wide region from Shikoku to Chugoku. In the reflection analysis, we used 3,364 seismograms from 492 local earthquakes observed at 55 Hi-net stations with the epicentral distances less than ~70km. We applied normal moveout (NMO) correction to the waveform data after correcting the geometrical spreading and anelastic attenuation. We detected a southeast-to-northwest dipping reflective layer in the Shikoku region at depths almost the same as those shown by Nakamura et al. (1997) and Siomi (2002). We also found another reflective layer dipping southeast to northwest at depths of ~50-70km beneath the entire Chugoku region which is considered to be the same as that Doi et al. (2003) found at similar depths. In the analysis of teleseismic P coda, we assumed the coda waves as P to P forward-scattered waves as Revenaugh (1995) did in southern California. The analysis procedure is as follows. First, we averaged the P wave part (~10s) over the stations, by summing all the waveforms with taking the waveform correlation. We assumed that the averaged P waves mostly represent the source and near-source structure effects. Then, we deconvolved the averaged P wave from each waveform to obtain waveforms containing heterogeneities below the station-network. We divided the analysis region into blocks with 5km in horizontal and 2.5km in depth direction and assigned the amplitudes of the envelope of the deconvolved waveforms at the corresponding travel time of scattered waves to each block. We stacked the envelope amplitudes for all the waveform traces, in order to estimate the relative scattering strength in every block. The result thus obtained shows that the high-scattering zone exists in Shikoku region below the reflective layer stated above. There is also another high-scattering area at depths of ~70-80km beneath the Chugoku region where the reflective zone was detected at similar depths. We are going to add more data to obtain images of the aseismic slab in Chugoku region with higher resolution.