1. Fig 3) Crustal Structure using the tomography method, in the central part of Itoigawa- Shizuoka Tectonic Line (ISTL). The central part of the Itoigawa-Sizuoka Tectonic Line (ISTL) plays a crucial factor in understanding the tectonic evolution of the ISTL fault system (fig 1). During the Pliocene the ISTL has been reactivated as a reverse fault due to tectonic inversion after the collision of the Izu-Bonin arc with the Japanese arc, with the northern part of ISTL behaving as a low angle east dipping thrust fault and the southern-central part as a west dipping thrust fault. The northern and central parts of the ISTL have a slip rate of 4 - 6 mm/yr and 1.3 – 2.5 mm/yr, respectively, while the southern part is considered to have ceased its activity. We deployed a temporary network for a 2 month period from 25 August to 16 October 2003; consisting of 49 linear array stations cutting across the ISTL fault trace and 9 stations scattered in the surrounding area. We used 3-component 1-Hz seismometers and long-term recorders with a sampling rate of 100 Hz. Using our temporary network and the permanent network in this area deployed by the Japanese Meteorological Agency (JMA), the National Research Institute for Earth Science and Disaster Prevention (NIED) and the Earthquake Research Institute, the University of Tokyo, we determined several local events. We analysed these events to locate and reveal the velocity structure across the ISTL, and connect them with seismic activity which may be related to present time movements of the fault system. The double-difference tomography method (Zhang & Thurber, 2003) was applied to 69 earthquakes and 2 vibroseis shots in the central part of ISTL. We have obtained 3523 P and 2907 S wave arrival time data, picked at 79 stations. In order to obtain a reasonable initial velocity model to use with the DD tomography inversion, we first analyzed these events according to the Joint Hypocenter Determination (JHD) method. Consequently we reprocessed these events to locate and reveal the velocity structure across the ISTL. In the Vp cross-section we image 2 areas with low Vp at the Northeast and Central parts (fig 3). We have achieved good P wave resolution at 0-12 Km depths for a 40 Km long area of the cross-section (fig 2), with P rms residuals dropping from 0.324 to 0.125 s. These low Vp zones can be identified on the geological map as the Yatsugatake volcano depositions and the marginal depositions of the Japanese arc accretionary prism. The relocated events are estimated to be 2-3 km shallower than the events published by the JMA. This is due to the large lateral heterogeneities in the area, expressed by the 2 low Vp zones. Fig 2) Fig 1) Map of the surveyed area. Fig 2) Checkerboard resolution test for P wave velocity perturbations. We have achieved good resolution for a 40 km log area to a depth of 10 km. Fig 3) Vp cross-section in the central ISTL. Two areas with low Vp can be identified in the cross-section. We can correlate these 2 areas with the Yatsugatake volcano depositions to the east (a) and the marginal depositions of the Japanese arc accretionary prism in the center (b). Panayotopoulos Yannis 1, Hirata Naoshi 1, Takeda Tetsuya 2, Kato Aitaro 1, Kurashimo Eiji 1, Iwasaki Takaya 1 1: ERI, Univ. Tokyo, 2: NIED Institute Fig 1) ba