HR: 0800h
AN: S31A-0215    [Abstracts]
TI: Strain Concentration Zones, Inland Earthquakes and Seismogenic Layers in the Crust of Central Honshu, Japan
AU: * Ito, K
EM: ito@rcep.dpri.kyoto-u.ac.jp
AF: Research Center for Earthquake Prediction, DPRI, Kyoto Univ., Gokasho, Uji, Kyoto, 611- 0011, Japan
AU: Ohmi, S
EM: ohmi@rcep.dpri.kyoto-u.ac.jp
AF: Research Center for Earthquake Prediction, DPRI, Kyoto Univ., Gokasho, Uji, Kyoto, 611- 0011, Japan
AU: Wada, H
EM: hiroo@rcep.dpri.kyoto-u.ac.jp
AF: Kamitkara Observatory, DPRI, Kyoto Univ., HOngo, Kamitkara-machi, Takayama, Gif 506- 1317, Japan
AB: Strain concentration zones have been found from surveys by a nation-wide dense GPS network in recent years in Honshu, Japan. The zones seem to have a large potential to cause disastrous earthquakes in the future, since large historical earthquakes occurred in and near the zones. Besides, Some moderate sized earthquakes took place for the last three years in one of the zones called the Niigata-Kobe Tectonic Line (NKTZ). However, no clear indication relating to the zones except for the abrupt variations in GPS displacement vectors along the zones. There exists an active fault system, called the Atotsugawa Fault System, in a part of NTTZ, in northwest of Chubu Honshu. In and around the fault zone, seismic observations have been continued for more than 30 years by the Kamitakara Observatory, Kyoto University. As a result, detailed seismicity has been revealed in the fault zone, showing a clear change in depth distribution across the fault zone; depths are shallow along the fault and become deep towards both sides of the fault, especially deeper in the NW direction. Furthermore, seismic velocity structure was also obtained from seismic refraction and wide-angle reflection studies. The depth of the upper and middle crust becomes deep towards NW from the fault zone. The variations in the cutoff depths of seismicity coincident well with the fault zone and eventually the stress concentration zone along the Atotsugawa fault system. The tendency is the same as in the area of the 2004 Chuetsu earthquake, occurred in NTTZ. In general, the stress concentration zones seem to locate in the areas along the change in depths of the seismogenic layer, although the reason of the strain concentration is not yet clear. The seismogenic layer is well correlated with thermal structure, as shallower the cutoff depth, the hotter the area. Furthermore, near both sides along the strike of a large inland event, cutoff depths of aftershocks become shallower. Thus, variations in depths of the seismogenic layer seem to affect the strain concentration in the upper crust. Dense network of earthquake observation made us enable to get high accuracy hypocenters on shore over Japanese Islands. Therefore, comparing the cutoff depths of seismicity with strain concentration areas from GPS over central Honshu, the latter seems to locate along the steeply changing areas of the cutoff depth of seismicity. The main strain concentration zones are NKTZ, the area along the Median Tectonic Line, and on the coast from Kanto to Tokai districts. However, since all steep changing areas of cutoff depths do not necessarily correspond to the stress concentration zone, the deeper crust, in which many dipping reflectors lie, also seems to play an important role to the formation of the strain concentration.
DE: 1734 Seismology
DE: 7205 Continental crust (1219)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8038 Regional crustal structure
SC: Seismology [S]
MN: 2007 Fall Meeting