HR: 0800h
AN: T11D-1285    [Abstracts]
TI: Shallow Subsurface Geometry of Active Thrust Faults Along the Itoigawa-Shizuoka Tectonic Line, Central Japan, Determined From Closely Spaced Gravity Survey and Fault Dislocation Model.
AU: * KURIYAMA, M
EM: mataza_masa@yahoo.co.jp
AF: Dept. Earth Sciences, Fac. Science, Okayama Univ, Tsushimanaka 3-1-1, Okayama, 7008530 Japan
AU: KUMAMOTO, T
EM: tkuma@cc.okayama-u.ac.jp
AF: Dept. Earth Sciences, Fac. Science, Okayama Univ, Tsushimanaka 3-1-1, Okayama, 7008530 Japan
AU: ISHIHARA, K
EM: constanta_capricornus@yahoo.co.jp
AF: Dept. Earth Sciences, Fac. Science, Okayama Univ, Tsushimanaka 3-1-1, Okayama, 7008530 Japan
AU: FUTAGAMI, Y
EM: futa@tobunken.go.jp
AF: National Research Institute for Cultural Properties, Uenokoen 13-43, Taito-ku, Tokyo, 1108713 Japan
AU: IKEDA, Y
EM: ikeda@eps.s.u-tokyo.ac.jp
AF: Graduate School of Science, Univ. Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 1130033 Japan
AB: The Itoigawa-Shizuoka Tectonic Line (ISTL) is one of the most active intraplate faults in central Japan with a slip rate as high as 6-9 mm/yr in the Late Quaternary. Although slip rate is rather uniform along the entire length of the ISTL, characteristics of Quaternary faulting change remarkably along strike. The northern portion of the ISTL is thrust with the east side upthrown (i.e., Okubo et al., 2000), whereas the southern portion is left-slip or thrust with the west side upthrown (Matsuta et al., 2000). First, we conducted closely-spaced gravity measurements along three lines across the southern portion of the ISTL: these are, from south to north, Ichinose upland, Fujimi, and Chino lines. We analyzed the gravity data using the two-dimensional Talwani's method, and determined the density structures along these three lines. In the Ichinose upland section, back tilting of 100 ka alluvial fans in the foothills suggests thrust component, whereas no evidence exists for strike slip. The density boundary between Middle Miocene rocks and Quaternary basin fill sediments lies in front of the foothills, not at the topographic range front. The boundary fault dips about 18 degrees west. Elongated pressure ridges develop along the range front in the Fujimi section. The pressure ridges are bounded on the west by left-slip faults; shallow excavations have revealed these faults nearly vertical. However, the density boundary between pre-Miocene rocks and Quaternary sediments dips about 15 degrees west, and is situated a few kilometers east of the pressure ridges, where middle Pleistocene alluvial fans have been back tilted. Thus the vertical faults bounding the pressure ridges are secondary faults formed as a result of oblique slip on the west-dipping, low-angle thrust. In the Chino section, 50 ka terrace risers have been displaced left-laterally by more than 400 meters, and hence strike slip seems dominant here. However, the density structure is essentially the same as the above two sections, although more detailed terrain corrections are needed for such a section with rugged topography. Next, we conducted try-and-error parameter calculations of the fault dislocation model in order to clarify whether the low-angle thrust fault could create observed land-surface tectonic features, such as backward tilting fan and asymmetric bulge in the Ichinose upland and the Fujimi regions. The result shows that the model of low-angle thrust fault at 200m beneath surface with short high-angle back thrust indicate good agreement with observed geomorphic features under the condition of fixed total slip amount derived from average slip rate and tephra chronology, compared to another candidate model of high angle strike-slip master fault with flower-structure type short faults. Our results indicate that the ISTL south of Suwa basin is basically a west-dipping fault, formed as one of west-dipping imbricated thrust faults due to collision of the Izu-Bonin arc against Honshu in Miocene-Pliocene time. Our results provide an example of how pre-existing structures rejuvenate under new stress fields.
DE: 7223 Seismic hazard assessment and prediction
DE: 8122 Dynamics, gravity and tectonics
DE: 8123 Dynamics, seismotectonics
DE: 7215 Earthquake parameters
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting