Crustal Structure using the tomography method, in the central part of Itoigawa-Shizuoka Tectonic Line (ISTL), Japan. Presenter: Hirata Naoshi 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

Introduction An active inland fault, the southwestern boundary of the Northern Fossa Magna sedimentary basin. Slip rate of: northern part 4-6 mm/yr (Sato et al. 2004) central part mm/yr (Miura et al. 2001) I.S.T.L N.F.M Paleozoic-Mesozoic rocks Miocene rocks  Yatsugatake volcano  Yatsugatake volcano Mt. Fuji  Triassic-Tertiary rocks Triassic-Tertiary rocks

Previous works about crustal structures along ISTL Low angle east dipping thrust fault (e.g. Sato and Hirata 1998) Major event intervals years (e.g. Okumura 2001) Crustal structure across northern ISTL (e.g. Matsubara et al. 2000, Takeda et al. 2005) West dipping thrust fault (Ikeda et al. 2004) Major event intervals years (e.g. Miura 2001) Northern ISTL Central ISTL

Objectives Derive a crustral structure model for the central part of ISTL and correlate it with the regional geology. Compare the regional tectonics between the northern and central parts of ISTL.

Tomography study in Central ISTL 58 stations south of Suwa lake with km spacing. 3 component 1 Hz seismometers and removable HD or DAT recorders. 2 months continuous recording at 100 Hz sampling rate, supported by li batteries (2003/08/25 – 2003/10/16).

Central ISTL velocity analysis Source: JMA catalogs from 09/02 to 10/13 of crustal events 10 deep events 2 vibroseis truck shots Stations: Relocate seismicity 24 routine network stations 58 temporary stations network Method: Double Difference Tomography (Zhang and Thurber 2003) tomoDD algorithm 4376 P and 3687 S arrivals Initial velocity model (ERI)

Events and grid node distribution Grid depth: 0, 2, 4, 8, 12, 18, 30 km From 09/02 to 10/13 of 2003 ( N, E) Total 89 events & 2 shots

DD tomography relocations Average 3 km shallower than the initial JMA hypocenter. The rms travel time residual was reduced from 0.29 s to 0.11 s after 12 iterations JMA and DD relocations difference near the station array

Checkers board resolution test Vp Vs Initial velocity model perturbations

Vp, Vs and Vp/Vs cross-sections S44W N44E Vp Vs Vp/Vs

Ryoke Belt Chichibu Belt Shimanto Belt Yatsugatake Volcano Izu-Bonin 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. Izu-Bonin: Oceanic crust Discussion 1 b a ISTL

b a Discussion 2: North vs Central ISTL After Takeda et al I.S.T.L

Conclusions 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.

Vp resolution with JMA events only Vs resolution with JMA events only Vp resolution with JMA & local events Vs resolution with JMA & local events

Vs with JMA events only Vp with JMA events only Vp with JMA & local events Vs with JMA & local events