Earth Planets Space, Vol. 60 (No. 2), pp. 105-110, 2008LETTER
Aitaro Kato1, Shinichi Sakai1, Takashi Iidaka1, Takaya Iwasaki1, Eiji Kurashimo1, Toshihiro Igarashi1, Naoshi Hirata1, Toshihiko Kanazawa1, and Group for the aftershock observations of the 2007 Noto Hanto Earthquake
1Earthquake Research Institute, University of Tokyo, Tokyo, Japan
(Received June 30, 2007; Revised October 12, 2007; Accepted October 15, 2007; Online published February 19, 2008)
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 (Vp) of the hanging wall in the southeast appears to be higher than that of the footwall in the northwest, and the high-Vp body of the hanging wall has a relatively high Vp/Vs ratio. Conversely, the low-Vp body in the footwall appears to have a low Vp/Vs 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 Vp and high Vp/Vs 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-Vp and low-Vp/Vs 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.
Key words: Tomography, the Noto Hanto Earthquake in 2007, aftershocks, inversion tectonics.