Inland Normal Faulting during the Mw 6.6 Iwaki Earthquake Induced By Temporal Change of Stress State after the Mw 9.0 2011 Tohoku Earthquake, Japan

Abstract:

A Mw 6.6 earthquake hit Iwaki area, northeastern Honshu arc, Japan on April 11, 2011 (here 2011 Iwaki earthquake). The 2011 Iwaki earthquake occurred one month after the 2011 Mw 9.0 earthquake off the Pacific coast of Tohoku (March 11, 2011; hereafter, 2011 Tohoku earthquake) (Ozawa et al., 2011; Simons et al., 2011). The 2011 Iwaki earthquake was followed by simultaneous slips of multi faults (the Itozawa fault and the Yunodake fault) (e.g. Toda and Tsutsumi, 2013). We examine the activity of the Itozawa fault and Yunodake fault and the mechanical preparation of the 2011 Iwaki earthquake from the temporal change of the stress state before and after the 2011 Tohoku earthquake. Furthermore, we quantitatively evaluate the role of crustal fluid beneath the faults and its supply mechanisms when the 2011 Iwaki earthquake occurred. We revealed that the generation of the slip on the Itozawa fault and simultaneous activity of faults cannot be occurred under the state of stress before the 2011 Tohoku earthquake, even though the both the state of stress is normal-faulting stress regimes before and after 2011 Tohoku earthquake. We show the importance of excess fluid pressure on the 2011 Iwaki earthquake, and we estimated the magnitude of excess fluid pressure ~30 MPa and ~20 MPa on the Itozawa and Yunodake fault respectively. We also show that the state of stress after the 2011 Tohoku earthquake favorably change the permeability of the Itozawa fault to be supplied the crustal fluid beneath the fault. A part of this research project has been conducted as the regulatory supporting research funded by the Secretariat of Nuclear Regulation Authority (Secretariat of NRA), Japan.