Transcript (fig 1).
P079: Crustal Structure using the tomography method, in the central part of Itoigawa-Shizuoka Tectonic Line (ISTL). Panayotopoulos Yannis 1, Hirata Naoshi 1, Takeda Tetsuya 2, Kato Aitaro 1, 1, 1 Kurashimo Eiji Iwasaki Takaya 1: ERI, Univ. Tokyo, 2: NIED Institute 3. Data 1. Introduction The central part of the Itoigawa-Sizuoka Tectonic Line (ISTL) plays a crucial role 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. The double-difference tomography method (Zhang & Thurber, 2003) was applied to 89 earthquakes and 2 vibroseis shots in the central part of ISTL (fig 3). Many of these events have not been located by the JMA, and were estimated using our temporary stations. We have obtained 4376 P and 3687 S wave arrival time data, picked at 79 stations. 2. Observation Fig.3) Events used for the inversion. We analyzed earthquakes from 2003/09/02 to 2003/10/13. Fig.4) Grid node distribution. The nodes were set with a spacing of 3 km along the X direction, 10 km along Y and at depths of 0, 2, 4, 8, 12, 18, 30 km. All the results we present on this poster are from the middle cross-section. 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 (fig 2). 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. 4. Analysis We calculate the Vp, and Vs values at nodes of a 3D grid, with 19 x 5 x 12 nodes at XYZ directions respectively (fig 4). We first run a set of calculations using only the events published in the JMA catalogue (67) and next using all the events we have picked. For both cases, we run a checkerboard resolution test in order to locate the areas with sufficient resolution (fig 5). By adding the local micro-earthquakes into our dataset, we were able to achieve better resolution for shallow depths and also to expand to an area with better resolution towards the deeper parts of the crust. I.S.T.L Initial velocity model perturbations Mt. Fuji Fig. 1 ) Geological map of the surveyed area. Fig 2) Seismic stations used in this study. Blue stars: temporary network. Black stars: local network in the area 5. Results During the inversion, the P rms residuals drop from an initial 0.29 s to 0.11 s after 12 iterations. The relocated events near our station array appear to be approximately 3 km shallower than the initial JMA catalogue hypocenter (fig 6). In the Vp cross-section we image 2 areas with low Vp at the Northeast and Central parts (fig 7). We have achieved good P wave resolution at 0-13 Km depths for a 32 Km long area of the cross-section. Again we compare the results using only the JMA catalogue events and both JMA and local. Fig.6) Event distribution relocated by the DD tomography method. Green lines displays the sift of the hypocenters from the initial JMA catalog to that of the relocated determination. The relocated events appear to be ~3 km shallower than those by the JMA catalog. With blue color we display the local micro-earthquakes that are not reported by the JMA catalog but have been determined by our temporary array network data. Fig.7) DD tomography results using only the JMA catalog events (left) and those with both JMA and local micro-earthquake events (right). An area with low Vp can be identified at the northeastern part of the cross-section. In addition Vp displays low values to a depth of 7 km at the center of the cross-section. Vs seems to follow the same patterns as Vp. A low Vp/Vs area can be clearly seen in the eastern part of the cross-section. Vp with JMA events only Vs with JMA events only Vp resolution with JMA events only Vp resolution with JMA & local events Vs resolution with JMA events only Vs resolution with JMA & local events Fig.5) Checkerboard resolution test results: (left) inversion using only the JMA catalog events and (right) that with both JMA and local micro-earthquake events. Vp with JMA & local events Vs with JMA & local events 6. Discussion Our cross-section runs through 3 different geological units. The Palaeozoic-Mesozoic accretionary prism units to the southwest (Ryoke belt), the Triassic-Tertiary accretionary units in the centre (Chichibu-Shimanto belts) and the Yatsugatake volcano depositions to the northeast (fig 8). The low Vp area to the east can be attributed to the Yatsugatake volcano depositions, wile the low Vp/Vs zone is an indication of magmatic processes (fig 10). The relatively low Vp values in the centre of the cross section can be identified as the accretionary prism units of the Japanese arc (mostly flysch, shale and chert). Below that lies a high Vp area that represents the IzuBonin arc rocks (fig 10). The seismicity in the area is not suficient to trace the downwards evolution of the ISTL faul trace. Nevertheless, we believe that it represents the boundary between the accretionary prism units and the Izu-Bonin arc sequences. Fig 8) Major geological units in the area. Ryoke belt: Granitic rocks. Simanto belt: flysch, shale and chert. Chichibu belt: pillow basalt, reef limestone radiolarian chert. Yatsugatake volcano: Volcanic tuffs. ISTL b Vp/Vs with JMA events only Vp/Vs with JMA & local events 7. Conclusion a Yatsugatake Volcano Chichibu Belt Ryoke Belt Shimanto Belt Fig 9) Vp cross-section in the central ISTL. Two areas with low Vp can be identified in the cross-section. We can correlate these two areas with the Yatsugatake volcano depositions to the east (a) and the marginal depositions of the Japanese arc accretionary prism in the center (b). The deeper extension of the central ISTL is denoted by a broken line, estimeted by a reflection survey carried by Ikeda et al. 2004. We have conducted a temporary network array survey in the central part of ISTL from 2003/08/25 to 10/16. We have combined our stations with the routine network in the area in order to accurately estimate the local seismicity. We have processed these events according to the DD tomography method and estimated the crustal velocity structure in the area. The DD relocated hypocenters are shallower than those estimated by the JMA, because we take in account the lateral heterogeneities in the crust. In the central ISTL the extension of the local fault (Aoyagi fault) seems to be the boundary between the accretionary prism units and the Izu-Bonin arc sequences. The seismicity at the present is not directly related with fault activity in the studied area, because it lies deeper than the deepest extetion of the ISTL.