Recent studies on the stress field in a seismogenic zone revealed a heterogeneous feature around a fault. To model the stress field around a fault zone, we have developed an inversion method that can be applied to focal mechanism data on microearthquakes and slip data on faults. The modeling scheme takes an objective approach without an a priori model such as the existence of faults or a magma source. The stress variation resulting from inelastic deformation in a medium (e.g., fault slip and magma intrusion) can be expressed as equivalent body forces in the medium. Thus, we attempted to model the stress field through the estimation of parameters of the regional stress and spatially distributed moment sources. The method was applied to the focal mechanism data of the aftershocks of the 2005 Fukuoka earthquake (M7.0) in Japan. The direction obtained for the minimum regional principal stress (i.e., NNW-SSE) was as expected from the general tendency of the focal mechanisms. The results revealed that the stress field was distorted by the fault slip in the middle segment of the earthquake fault. The slip detected at the deep part of the fault is located away from the coseismic slip area, suggesting a possibility of either preseismic or postseismic slip around the initiation point of the main shock rupture. In contrast, the stress accumulation rises at the folding point of the aftershock alignment, and the stress relaxation area is found at the deeper edge where the largest aftershock occurred and strong heterogeneous medium exists.
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