西南日本のマイクロプレート・モデルと内陸大地震

1995年兵庫県南部地震 (M7.2) の発生を考える

抄録

Several boundaries of tectonic blocks can be defined as the lines linking large-scale active faults in Southwest Japan. Using available data concerning geology and geophysics, tectonic features of the block boundaries have been briefly reviewed. These features imply that Southwest Japan is separated into a few microplates. Based on the microplate model of Southwest Japan, sequential generation of large earthquakes including the 1995 M7.2 Hyogo-ken-nanbu earthquake can be interpreted. The Hyogo-ken-nanbu earthquake occurred in the middle of the 100-km long Takatsuki-Rokko-Awaji tectonic line (TATL). The aftershock distribution and the surface ruptures suggest that the main shock is caused by a 40-km long rupture of the central part of the TAIL. Thirty-kilometer long eastern and western parts of the TATL remain unruptured. The average moment-release rate of the 40-km central part of the TATL can be estimated from the slip-rate of constituent active faults. The magnitude of an earthquake generated at the central part can be predicted from the seismic moment, which is calculated from the moment-release rate and the elapsed time since the 1596 Keicho-Fushimi earthquake. The calculated magnitude of 6.8 to 7.2 is consistent with the actual magnitude of the Hyogo-ken-nanbu earthquake. If the eastern and western parts unruptured have the same slip-rate as that of the central part, the accumulated moments after the previous event amount to the earthquakes with a magnitude of 6.7 to 7.1. A two-meter right-lateral slip along the Nojima fault, which constitutes western-central part of the TATL, occurred at the time of the Hyogo-ken-nanbu earthquake. This slip may help a southward migration of the Kinki-Outer mircoplate. The movement may reduce normal stresses along the Median Tectonic Line and may increase stress concentration along the Nankai trough. The kinematics of the microplate and sequential generation of large earthquakes along the microplate boundaries suggest a high probability of generating an offshore earthquake along the Nankai trough in a period ranging from 2003 to 2040 A. D.