Abstract
We carried out high density aftershock observations a week after the 2000 Western Tottori Earthquake for 40 days. We deployed 72 seismic stations in and around the aftershock area. The average spacing of the stations in the aftershock area was 4-5 km. We determined accurate hypocenters and focal mechanisms for -1, 000 aftershocks and obtained a high resolution 3-D velocity structure in the source region. High P and S wave velocity anomalies (> 4%) near the southeasternmost aftershock area at 2 km depth correlated with Jurassic to Late Cretaceous plutonic and high pressure metamorphic rocks. The depth distribution of the P and S wave velocities along the mainshock fault showed that high velocity anomalies were located at the shallow southeastern edge and the deeper central part of the aftershock area. The ratio between P and S wave velocities in the high velocity anomalies was a little higher (-1.75) than the average value (-1.70) in the upper crust. These results indicate that the high velocity anomalies could correspond to the plutonic or metamorphic rocks. The distributions of the high velocity anomalies and large slips of the mainshock were complementary. These suggest that the high velocity anomalies could be stronger than the surrounding materials and might behave as barriers to the mainshock rupture.
