Estimation of the strength of faults in the region of the 2000 Western Tottori Earthquake

To study the strength of the faults that caused large earthquakes, we tried to determine the stress state near the faults prior to and after the 2000 Western Tottori Earthquake using the focal mechanism data of the Joint Group for Dense Aftershock Observation. We investigated the spatial distribution of strikes of the nodal planes from the focal mechanisms of aftershocks. There are some aftershocks that have the nodal planes consistent with the fault strike of the mainshock (N132E) distributed near the mainshock hypocenter (35.2847N, 133.3459E), and these aftershocks are aligned in a N132E direction. To investigate the distribution of the P axis orientations of the aftershocks, the aftershock area was divided into four areas; the northern end of fault (area 1), the northern half of the rupture area (area 2), the southern half of the rupture area (area 3), the southern end of fault (area 4), based on the differences of the characteristic distributions of the P axes orientations and the principal axis of stress change due to the mainshock. P axes are mostly oriented in a northwest-southeast direction in areas 1 and 2. Around the large slip zone in area 3, the direction of the P axes vary widely, and in area 4, the P axes are mostly oriented in a east-west direction. Next we calculated the stress changes due to the slip distribution of the mainshock, estimated by Iwata and Sekiguchi (2002), using the formulas of Okada (1992). The direction of the maximum compressive stress axis calculated from the stress change was compared with that of the P axis. In area 1, the directions of the maximum compressive stress axes were almost consistent with those of the P axes. In the area 2, however, they were not consistent with those of the P axes, though the stress change was large (10-20 MPa.). Around the large slip zone in the area 3, the stress change was large (10-50 MPa.) and the directions of the maximum compressive stress axes were not consistent with those of the P axes. In the area 4, they were consistent with those of the P axes oriented in an east-west direction. From the results of the direction of the nodal planes, it is inferred that a weak plane with a direction consistent with the strike of the mainshock fault pre-existed near the hypocenter, and that these aftershocks occurred on this weak plane. From the comparison between the directions of the maximum compressive stress axes and the P axes, it is expected that the directions of the P axes were not affected by the stress change in areas 1 and 2, because they are not consistent with the maximum compressive stress axes, and almost oriented in a northwest-southeast direction. Thus, it is inferred that the maximum compressive stress axis prior to the mainshock was oriented in a northwest-southeast direction and the magnitude of the stress was large so that the direction of the maximum compressive stress was not modified by the stress change. Around the large slip zone in area 3, it is possible that the stress state became heterogeneous by the mainshock slip distribution. Finally, in area 4, it is possible that the magnitude of initial stress prior to the mainshock was smaller than that in areas 1 and 2, because the direction of the P axes was consistent with the maximum compressive stress axes, although the stress change was only a few MPa.. We can also explain these results, by the assumption that the magnitude of the maximum compressive stress was about 10 MPa. and was oriented in an east-west direction.