HR: 08:50h
AN: T31G-04    [Abstracts]
TI: Microearthquake Distribution along the Itoigawa-Shizuoka Tectonic Line by the 2003 Temporary Array Observation
AU: * Panayotopoulos, Y
EM: yannis@eri.u-tokyo.ac.jp
AU: Hirata, N
EM: hirata@eri.u-tokyo.ac,jp
AU: Takeda, T
EM: takeda@ni.aist.go.jp
AB: The Itoigawa-Sizuoka Tectonic Line (ISTL) is a major geological structure that divides Japan into NE and SW parts. It was formed as a normal fault in the early Miocene and represents the southwestern boundary of the northern Fossa Mangna sedimentary basin. During the Pliocene the ISTL is reactivated as a reverse fault due to tectonic inversion, after the collision of the Izu-Bonin arc with the Japanese arc. It comprises an active inland fault system with a high slip rate. The central and northern parts of the ISTL have a slip rate of 8.6-9.5 mm/yr and 3.0 mm/yr, respectively, while the southern part is considered to have ceased its activity. To reveal seismic activity, which may be related to the ISTL activities, we deployed seismic arrays in northern, middle, and southern parts. These arrays consisted of 4-9 stations in each area. At the southern part, across the ISTL, we also deployed a linear station array across the ISTL, consisted of 49 stations. We used 3 -component 1-Hz seismometers and long-term recorders with a sampling rate of 100 or 200 hz Hz. The observation lasted for 3 months starting from August 4 to November 21, 2003. In the north part of ISTL, using our temporary network and the local network in this area deployed by the Japanese Meteorological Agency (JMA) and the Earthquake Research Institute, the University of Tokyo, we determined 281 events with 2.7 $>$ M $>$ -1. The JMA network has located only 103 of these events. Most of which are concentrated in a cluster at a depth of about 15 km. In order to locate these events accurately, we have applied the Joint Hypocenter Determination method. We estimated station corrections for our temporary network and the regional permanent network. The main cluster is located inside an area of 1 x 1 x 1.5 km approximately. We can clearly see a migration in the hypocenter distribution from a starting depth of about 15.5 Km km to a depth of 14 km, which is much deeper than the deeper extension of the ISTL. Some previous studies in the area attributed this kind of cluster activity to water migration through cracks in the rock formations. The hydro-fracturing can be one of the triggering mechanisms of an inland large earthquake which occurs far away from a plate boundary.
DE: 7230 Seismicity and seismotectonics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting