Effect of a change in the state of stress on the likelihood of inland fault failure during the Mw 6.6 Iwaki earthquake resulting from the Mw 9.0 2011 Tohoku earthquake, Japan

A Mw 6.6 earthquake occurred on 11 April 2011 in the Iwaki area, northeastern Honshu arc, inland Japan (hereafter, the 2011 Iwaki earthquake). This event occurred just one month after the 2011 Mw 9.0 off the Pacific coast of Tohoku earthquake (11 March 2011; hereafter, the 2011 Tohoku earthquake). The 2011 Iwaki earthquake was produced by near-simultaneous slip on two faults (the Itozawa and Yunodake faults). Here, we examine the failure on the Itozawa and Yunodake faults with respect to the change in the state of stress in the Iwaki area produced by the 2011 Tohoku earthquake. Furthermore, we quantitatively evaluate the excess fluid pressure. Our analysis reveals that the antecedent state of stress played an important role in the occurrence of the 2011 Iwaki earthquake. We demonstrate the importance of excess fluid pressure in terms of the initiation of the 2011 Iwaki earthquake. Three end-member models of failure were identified based on our modeling with different friction coefficients. (1) High friction coefficients (> 0.75), in which case triggering of the Iwaki earthquake would have required fluid pressure levels greater than those of σ3. These pressures should have been regulated by fault-valve action, and evidence for such action should appear in the seismic record. (2) Low friction coefficients (< 0.4), in which case triggering of the Iwaki earthquake would have occurred in the absence of excess fluid pressure, although this is inconsistent with the relatively high competence of the crust in the area as inferred from the velocity structure derived from tomographic data. (3) Moderate friction coefficients (0.4-0.75), which would occur at moderate levels of excess fluid pressure, which are reasonable given the calculated magnitudes of crustal stress.