Coulomb stress triggering is examined using well-determined aftershock focal mechanisms and source models of the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake. We tested several slip distributions obtained by inverting onshore GPS-derived coseismic displacements under different a priori constraints on the initial fault parameters. The aftershock focal mechanisms are most consistent with the Coulomb stress change calculated for a slip distribution having a center of slip close to the trench. This demonstrates the capability of the Coulomb stress change to help constrain the slip distribution that is otherwise difficult to determine. Coulomb stress changes for normal-fault aftershocks near the Japan Trench are found to be strongly dependent on the slip on the shallow portion of the fault. This fact suggests the possibility that the slip on the shallow portion of the fault can be better constrained by combining information of the Coulomb stress change with other available data. The case of normal-fault aftershocks near some trench segment which are calculated to be negatively stressed shows such an example, suggesting that the actual slip on the shallow portion of the fault is larger than that inverted from GPS-derived coseismic displacements.
A giant earthquake of M w 9.0 took place off the Pacific coast of Tohoku on March 11, 2011. It caused a large tsunami of 10-20 m and devastated the area along the Pacific coast in northeast Japan. The earthquake altered the stress field in the surrounding region immensely. We have calculated the change in Coulomb Failure Function ( CFF) due to this earthquake to evaluate the effect on aftershocks and future earthquake probabilities. The results suggest that the increased activity of normal-fault earthquakes after the main shock is explained by a large positive CFF of 1-5 MPa prevailing over a vast area in and around the main-shock fault zone. The areas adjacent to the northern and southern borders of the fault zone where other large interplate earthquakes might occur are occupied by a positive CFF of approximately 0.1 MPa. Based on the CFF result, the future probability of reverse fault earthquakes in the shallow crust is estimated to be decreased in the land area of Tohoku.
We investigated how the multiple inverse method developed by Yamaji works for the focal mechanism data. A numerical experiment with artificial data revealed that the stresses obtained for the focal mechanism data including auxiliary fault planes agree with those for the data consisting only of actual fault planes. Another numerical experiment showed that the method can retrieve the plural stresses when applied to artificial data that consist of focal mechanisms originating from di#erent stresses. When applied to the focal mechanisms of earthquakes in and around the rupture zone of the 2004 Sumatra-Andaman earthquake, the method could determine the stresses that were not resolved in the previous study, thus allowing us to discuss the tectonic stress field in more detail. We proposed an objective criterion for determining the significant stresses from among the plural inverted solutions. Although the criterion still needs to be refined, we consider that it provides a useful tool for evaluating the relative significance of the plural stresses inverted by the multiple inverse method.
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