SEISMICITY AND CRUSTALL STRUCTURE ALONG THE SOUTHERN JAPANESE ALPS
SEGMENT OF THE ITOIGAWA-SHIZUOKA TECTONIC LINE
Panayotopoulos Y.[1]; Hirata N.[1]; Sato H.[1]; Iwasaki T.[1]; Kato A.[1]; Imanishi K.[2]; Cho I.[2];
Kuwahara Y.[2]
[1] Earthquake Research Institute, The University of Tokyo, P.O. 113-0032, 1-1-1 Yayoi, Bunkyo
ward, Tokyo Japan [2] Geological Survey of Japan, AIST Tsukuba Central 1,Tsukuba, Ibaraki
305-8561, Japan
Tel: +81358411764, Fax: +81358411759 Email: yannis@eri.u-tokyo.ac.jp
The wide area along the Itoigawa-Sizuoka Tectonic Line (ISTL) is considered to be with one of the
highest probability of a major event occurrence in the near future. In order to understand the active
tectonics that control the earthquake genesis mechanisms, it is essential to investigate the microseismic
activity near the faults, the deep layout of active faults and the seismic velocity structure of the earth
crust in the surrounding area. To reveal seismic activity, which may be related to the ISTL activities,
we deployed 60 offline stations in the south-central part of the ISTL from 15 Sep. 2005 to 23 Dec.
2005. We manually repicked 348 M > 0.5 events published by the Japan Meteorological Agency
(JMA) and reprocessed them using the Double Difference tomography method (Zhang and Thurber,
2003). The relocated events have lined up on a low / high Vp boundary between 5 to 20 km depth. The
deeper extension of the ISTL in Southern Alps region seems to be the boundary between these low Vp
and high Vp zones, which can be identified as the accretionary prism units and the Izu-Bonin arc
sequences. The ISTL forms a thrust fault that dips approximately 450~550 towards the west. The
seismicity in the area may occur on the deeper extension of the ISTL; and if so, this could be evidence
that the deeper part of the geological ISTL is connected to the active fault at depth.