Abstract
Abstract The velocity structure and accurate aftershock distributions of the Noto Hanto Earthquake in 2007 (thrust type) are elucidated by inverting the arrival times from 917 aftershocks using double-difference tomography. P-wave velocity (V p ) of the hanging wall in the southeast appears to be higher than that of the footwall in the northwest, and the high-V p body of the hanging wall has a relatively high V p /V s ratio. Conversely, the low-V p body in the footwall appears to have a low V p /V s ratio at depths greater than 3 km. Aftershocks associated with the mainshock fault are roughly distributed along this velocity boundary between the hanging wall and footwall. Near-surface thin layers with significantly low V p and high V p /V s are imaged in a northwest direction from the mainshock epicenter. A likely explanation is that the mainshock fault plane was reactivated as a reverse fault in terms of the inversion tectonics due to the crustal shortening which initiated from the late Miocene. Both the mainshock hypocenter and the vertical alignment of aftershocks beneath it are located in the low-V p and low-V p /V s zones, indicating the potential presence of water-filled pores. Crustal stretching and shortening in and around the Noto Peninsula have created complex structures, including weak high-dip angle faults, almost vertical faults, and low velocity zones, which can potentially affect the seismic activities around the source region.
Highlights
IntroductionWhile online seismic networks have been operated on the Noto Peninsula by the NIED, Japan Meteorological Agency (JMA), universities, and the Japanese University Group of the Joint Seismic Observations at NKTZ (Niigata-Kobe Tectonic Zone) (2005), the average spacing of these stations is not sufficient for resolving the detailed velocity structure in the source region with high accuracy
A shallow Mw = 6.7 inland earthquake occurred on the west coast of the Noto Peninsula in Japan on March 25, 2007, inflicting serious damage in the surrounding areas
The shallow high-Vp/Vs bodies with significantly low Vp correspond to the sediments that have piled over the half grabens formed by crustal stretching at the time of expansion of the Japan Sea (Fig. 4)
Summary
While online seismic networks have been operated on the Noto Peninsula by the NIED, Japan Meteorological Agency (JMA), universities, and the Japanese University Group of the Joint Seismic Observations at NKTZ (Niigata-Kobe Tectonic Zone) (2005), the average spacing of these stations is not sufficient for resolving the detailed velocity structure in the source region with high accuracy. The differential arrival times were derived from both manually picked data and the waveform cross-correlation method This method simultaneously solves for the three-dimensional velocity structure of both P- and S-wave velocities (Vp, Vs) and seismic event locations. They appear to be distributed along a minor velocity boundary in the footwall
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