HR: 0800h
AN: S41A-0948    [Abstracts]
TI: Static Stress Field on a Branched Fault System: The 1891 Nobi, Japan, Earthquake (M8.0)
AU: * Mikumo, T
EM: mikumo@ollin.igeofcu.unam.mx
AF: Instituto de Geofisica, UNAM, Ciudad Universitaria, Mexico, D.F 04510 Mexico
AU: Fukuyama, E
EM: fuku@bosai.go.jp
AF: NIED, 3-1 Tennodai, Tsukuba, Ibaraki, Tsukuba, Iba 305-0006 Japan
AB: It has often been observed that earthquake rupture propagates along pre-existing faults with complex geometry such as fault bending, offsets, and branching. One of the remarkable examples is the 1891 Nobi earthquake (M8.0) in central Honshu, Japan, which appears to have ruptured branched faults (Muramatu, 1963, Res. Rep. Gifu Univ.; Mikumo and Ando, 1976, J. Phys. Earth), although one of the branches did not appear on the ground surface, and hence there has been much debate about its existence. In the present study, we investigate the possibility of dynamic rupture propagated on a branched fault system during the Nobi earthquake, by considering both static slip and dynamic rupture along the pre-existing fault traces. The surface fault breaks with unusually large displacements have been traced extending over 80 km, (Matsuda, 1974, Spec. Rep. Earthq. Res. Inst.), and the traces appear to consist of 4 segments. In addition, the possible existence of a burried fault, which branched off at near point (c) extending southwards through Gifu-Ichinomiya (e) possibly down to near Nagoya, has been suggested based on various observations (e.g. Muramatu, 1963). Recent stratigraphic surveys of pumice- and gravel-beds from many bore-hole records have revealed shallow underground vertical offsets at a depth of about 25-30 m along a line located 1.5 km east of the presumed location (Sugisaki and Shibata, 2004, Zisin). We include this burried branch into our fault model. We calculate the probable range of static stress drop on five fault segments using the horizontal fault displacements on the surface based on Matsuda's survey (1974). For the buried fault (c-e), we estimate its possible displacement during the 1891 event, by applying the triple junction kinematics proposed by Andrews (1989, JGR), which require that the ratio of slip to the sine of the opposite angle is the same for all three segments. The branch angle of 44$^\circ$ is wide enough for the rupture to propagate on two branched faults (Aochi et al., 2000, GRL). Since slip on the b-c segment drops down to about 2 m from 6.5 m (Matsuda, 1974) near the possible junction (c), slip on the c-e segment for the buried fault would be of the order of 1.5 - 1.8 m. The static stress drop was calculated at the center of each fault segment, referring to Chinnery (1969). We also estimate the principal stresses and their directions working in this fault zone from two-dimensional analysis. If we consider the stress change before and after the earthquake rupture for the two fault segments (b-c) and (c-d), numerical calculations show that the maximum principal stress direction lies between 105$^\circ$ and 109$^\circ$ and the stress change ranges between 7.7 and 12.4MPa, respectively. The direction of the maximum compressive stress is in good agreement with the direction of tectonic stress over this region, which has been inferred from recent GPS observations, but slightly deviated from the direction estimated from hydraulic fracturing stress measurements (Ikeda et al., 2002), which might include the co-seismic and post-seismic stress changes due to the earthquake. Based on the static parameters estimated above, we calculate spontaneous dynamic rupture propagation under various conditions (Fukuyama and Mikumo, 2004, AGU Fall Meeting).
DE: 7260 Theory and modeling
DE: 8010 Fractures and faults
DE: 7209 Earthquake dynamics and mechanics
DE: 3210 Modeling
DE: 3220 Nonlinear dynamics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting