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A stress tensor inversion was carried out in the aftershock area of the 2003 Tokachi-Oki earthquake for a comparative study on the preseismic and postseismic stress state. The inversion technique was adopted from Gephart (1990) to pursue the analysis with earthquake focal mechanism data from National Research Institute for Earth Science and Disaster Prevention (NIED) of Japan. Results of the inversion revealed significant change in stress state before and after the 2003 Tokachi-Oki earthquake. The preseismic uniaxial compression in strike slip regime was changed to a transition from transpressional to a radial compression in a compressional regime.
Statistical analysis of P-axis populations showed that for the both preseismic and postseismic data sets, the major eigen vectors of the orientation matrices are subparallel to the convergent vector of the Pacific slab. Moreover, orientation of the maximum principal stress axis remained nearly unchanged after the 2003 earthquake and it is subparallel to the convergent vector of the Pacific slab indicating that the main source of tectonic stress in this region was supplied by the subduction of the Pacific slab beneath the Kurile trench.
Analysis of major aftershocks (M≧5.8) using teleseimic body waves revealed that the pole to one of the nodal planes subparallel to the main shock fault plane subscribes steep angle with the maximum principal stress axis. Based on the result of stress inversion and the angular relationship between the fault normal and the principal stress axis it is interpreted that the 2003 Tokachi-Oki earthquake occurred on a relatively weak fault zone resulted from elevated pore pressure. Since the minimum principal stress axis became vertical after the 2003 Tokachi-Oki earthquake, major aftershocks with one nodal plane subparallel to the main shock fault plane occurred so as to drain the excess fluid at the seismogenic depths.
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