Shallow inland earthquakes in NE Japan possibly triggered by the 2011 off the Pacific coast of Tohoku Earthquake

Abstract

Shallow seismic activity in the crust of the overriding plate west of the source area increased significantly after the 2011 M w 9.0 Tohoku earthquake which ruptured the plate boundary to the east off northern Japan beneath the Pacific Ocean. In order to understand the cause of this distinctive change in seismicity, we have precisely relocated earthquake hypocenters for several earthquake sequences that occurred during the period March 11-April 6, 2011, following the Tohoku earthquake by the double-difference method. Hypocenter distributions were used to discriminate the fault plane from the auxiliary plane of the focal mechanisms for those earthquake sequences. We then calculated the Coulomb stress change on those fault planes caused by the 2011 M w 9 earthquake. In all cases, the estimated Coulomb stress changes at the plausible fault planes for those post-mainshock sequences are positive. The positive Coulomb stress change is mainly due to the reduction of normal stress on the fault plane of the earthquake sequences by the large, low-angle thrust fault responsible for the 2011 M w 9 earthquake. The present observations suggest that static stress transfer possibly triggered those post-mainshock earthquake sequences.

Stable Forearc Stressed by a Weak Megathrust: Mechanical and Geodynamic Implications of Stress Changes Caused by the M = 9 Tohoku‐Oki Earthquake

“…It is remarkable that such a seemingly small stress drop changed the state of stress in the offshore forearc from predominantly margin‐normal compression to tension. The stress reversal was documented by a number of studies through analyses of focal mechanisms of small earthquakes off the megathrust before and after the earthquake (Asano et al, ; Hasegawa et al, ; Nakamura et al, ; Okada et al, ; Yoshida et al, ). Results from Hasegawa et al () and Nakamura et al () along a central corridor (Figure a) are shown in Figure .…”

Complex Ruptures of the 11 April 2011 Mw 6.6 Iwaki Earthquake Triggered by the 11 March 2011 Mw 9.0 Tohoku Earthquake, Japan

“…The 2011 M w 9.0 Tohoku earthquake, Japan, generated more than 50 m of slip on the interface between the subducting Pacific plate and the overriding plate (e.g., Ito et al, 2011;Simons et al, 2011;Yamazaki et al, 2011), and caused significant stress changes in the eastern half of Honshu Island, resulting in the excitation of widespread seismic activity (e.g., Okada et al, 2011; and changes in the type of focal mechanisms in and around the focal area (Asano et al, 2011).…”

Spatial distribution of the faulting types of small earthquakes around the 2011 Tohoku‐oki earthquake: A comprehensive search using template events

“…The Tohoku‐oki earthquake brought stress changes and consequent seismicity changes in and around the source region [e.g., Toda et al , ; Hasegawa et al , ; Okada et al , ]. For example, in the area that underwent large coseismic slip during the 2011 Tohoku‐oki earthquake, the number of interplate and repeating earthquakes has been reported as having dramatically decreased after the M 9 earthquake [e.g., Asano et al , ; Kato and Igarashi , ; Uchida and Matsuzawa , ].…”