Abstract
It is well known that a great earthquake causes stress changes to activate and/or suppress seismicity around the fault zone. Occurrence of a large inland earthquake might be accelerated by stress changes caused by the other preceding earthquakes and/or accumulation of back slip on plate boundaries. In order to verify the hypothesis, we investigated temporal changes in Coulomb Failure Function (ΔCFF) before the occurrence of the recent two major inland earthquakes: the 1995 Hyogoken-Nanbu earthquake (M7.2) and the 1984 Naganoken-Seibu earthquake (M6.8). For the calculation of ΔCFF, we employed a dislocation theory for a semi-infinite homogeneous perfect elastic body proposed by OKADA (1992). First, we found that the value of ΔCFF on the source fault of the 1995 Hyogoken-Nanbu earthquake had increased by about 1.4×10-2 MPa due to the preceding events (1965-1994) and back slip for the 30 years. We can conclude that the Hyogoken-Nanbu earthquake was mainly accelerated by stress accumulation due to interplate coupling. However, since the value is too small, other factors such as block motion bounded by intraplate faults, northward slow slip along a detachment fault, and pre-existing fluid in the source region may also be considered as generation mechanism of the event. Next, we found that the value of ΔCFF on the source fault of the 1984 Naganoken-Seibu earthquake decreased by about 9.0×10-2 MPa due to the preceding events (1965-1984) and back slip for the 20 years. Decrease of the value of ΔCFF for the Naganoken-Seibu earthquake indicates that other factors such as collision due to plate motion and block rotation of microplates must be taken into account in the model to explain generation mechanism of the event. We also attempted to investigate seismic potential in the Hokkaido-Tohoku districts in terms of spatial distribution of ΔCFF for two cases: strike-slip and reverse faults with P-axis in the direction of plate motion of the Pacific plate (N285°E). For the former, correlation between 13 inland earthquakes and calculated increase of ΔCFF is obscure. On the other hand, for the latter, we found fairly good correlation between them. Therefore, there is a possibility that ΔCFF may be useful to evaluate seismic potential in the Hokkaido-Tohoku districts for the case of reverse fault.
