Abstract
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.
Highlights
A large earthquake occurred on 6 October 2000 in the western part of Tottori Prefecture, southwestern Honshu, Japan
High V p anomalies with 4% or greater are located in patches at the central part and the southeastern edge of the aftershock distribution
The changes of the hypocenters relocated in the tomographic inversion relative to those relocated by the Joint Hypocenter Determination (JHD) method were at the most 0.2 km horizontally and −0.05 – 0.75 km downwards, for more than 90% of the analyzed aftershocks
Summary
A large earthquake occurred on 6 October 2000 in the western part of Tottori Prefecture, southwestern Honshu, Japan. Following the mainshock we carried out a dense aftershock observation with 72 stations from 13 October that continued for one and a half months The purpose of this observation was to determine precisely the locations and the focal mechanism solutions of the aftershocks, to obtain a detailed 3-D velocity structure in the source region by means of tomographic inversions of P and S travel time data, and to image 3-D distributions of reflectors and scatterers by utilizing array analyses of reflected and scattered waves. Hirahara et al (1992) studied the relationship between the slip distribution of the 1984 Western Nagano Prefecture Earthquake (Mj = 6.9) and the 3-D velocity structure in the source region They found that the area with large dislocations corresponded to low velocity anomalies on the fault plane. Whose last activity was over 20,000 years ago (Tsukui, 1985)
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