Distribution of S-wave reflectors in the crust and uppermost mantle beneath the source regions of large inland earthquakes in southwest and central Japan

It is important to estimate detailed heterogeneous structures beneath active fault zones, which will give us clues to understand the generating processes of large inland earthquakes. We estimated detailed seismic reflection properties in the crust and uppermost mantle in the Chugoku region, southwest Japan, and also in the Hokuriku region, central Japan, where several large (M>6) crustal earthquakes occurred within a few hundreds of years. In the Chugoku Region, we first analyzed the source region of the 2000 Western Tottori Earthquake with Mj 6.6, using dense aftershock observation data. Then, we extended the analysis to the whole Chugoku region, using routine network data. From the analyses of these two steps, we delineated the Conrad and the Moho discontinuities and also the top surface of the Philippine Sea Plate beneath the Chugoku region, at depths of 15-25 km, 30-45 km, and 50-70 km, respectively. We found that the strengths of seismic reflections have remarkable contrast between the two sides of the fault plane of the 2000 Western Tottori Earthquake. This suggests that the mainshock fault plane is located at the boundary of medium properties and that the fault plane has near-vertical extension down to the lower crust. In the Hokuriku Region, we conducted the same reflection analysis using waveforms from local earthquakes, including the source region of the 1948 Fukui Earthquake (Mj 7.1). We found two reflection zones at depths of 20-25 km and 40-50 km. Comparing with past studies of receiver function analyses, theses two reflection zones are thought to correspond to the Conrad and the Moho discontinuities, respectively. Reflection strength in the upper crust is higher on the ENE side of the fault plane of the 1948 Fukui Earthquake, while lower on the WSW side. In the source region of the 2000 Western Tottori Earthquake, crustal thickness changed along the fault strike just beneath the source. We will further look into such a relation between heterogeneous structures and earthquake generation in the Hokuriku region.