Unusual shallow normal‐faulting earthquake sequence in compressional northeast Japan activated after the 2011 off the Pacific coast of Tohoku earthquake

Abstract

After the occurrence of the 2011 Mw9.0 off the Pacific coast of Tohoku earthquake, an unusual shallow normal‐faulting earthquake sequence occurred near the Pacific coast at the Ibaraki‐Fukushima prefectural border. We have investigated why normal‐faulting earthquakes were activated in northeast (NE) Japan, which is otherwise characterized by E–W compression. We computed the stress changes associated with the mainshock on the basis of a finite fault slip model, which showed that the amount of additional E–W tensional stresses in the study area was up to 1 MPa, which might be too small to generate normal‐faulting earthquakes in the pre‐shock compressional stress regime. We thus determined focal mechanisms of microearthquakes that occurred in the area before the mainshock, which indicated that the pre‐shock stress field in the area showed a normal‐faulting stress regime in contrast to the overall reverse‐faulting regime in NE Japan. We concluded that the 2011 Tohoku earthquake triggered the normal‐faulting earthquake sequence in a limited area in combination with a locally formed pre‐shock normal‐faulting stress regime. We also explored possible mechanisms for localization of a normal‐faulting stress field at the Ibaraki‐Fukushima prefectural border.

Shear-wave splitting in a region with newly-activated seismicity after the 2011 Tohoku earthquake

“…This suggests that the WNW-ESE orientation of the maximum stress axis was caused by the motion of the subducting plate. Earthquake focal mechanism solutions determined by the Full Range Seismograph Network of Japan (F-net) of NIED for this area indicated dominant reverse-type faulting along an E-W compression axis (Imanishi et al, 2012). These focal mechanism solutions are consistent with the regional stress field in the area, with the polarization direction in the western part of the study area also consistent with a WNW-ESE stress field.…”

“…However, the mechanism solution is not uniform, there is some variation of the mechanism (e.g. Imanishi et al, 2012), Then, we can detect the anisotropy of the area using the method.…”

“…This change is complex, with seismicity increasing in some areas and decreasing in others; these changes indicate that the significant deformation caused by the 2011 Tohoku earthquake resulted in a marked change in stress fields in Japan (Toda et al, 2011a). The southeastern Tohoku area, consisting of the northern part of Ibaraki prefecture and the southern part of Fukushima prefecture, is an area that underwent a significant change in seismicity after the 2011 event (Imanishi et al, 2012). Prior to this event, this area had a low background level of seismicity; however, after the earthquake, this area became seismically active with an increase in seismicity over an area of 50 km (north-south) × 30 km (eastwest) (Fig.…”

“…The focal mechanisms were difficult to determine. A few magnitude >3 earthquakes occurred in the west of the study area, and were associated with reverse faulting along an E-W compressional axis (Kato et al, 2011;Imanishi et al, 2012).…”

Simple Crack Models Explain Deformation Induced by Subduction Zone Megathrust Earthquakes

“…This has led to the recognition of several important factors influencing the deformation pattern in subduction zones. These factors include whether the main rupture breaks the surface (Lin and Stein, 2004) or displays a large shallow slip (Hasegawa et al, 2012), individual or combined effects of shear stress change and normal stress change (Yang et al, 2013), contributions from shallow seismic slip and deep aseismic slip (Gardi et al, 2006), along-dip location with respect to local slip centroid (Li et al, 2014), and the type of preexisting faulting in relation to the magnitude of stress perturbation (Imanishi et al, 2012). Despite great success, in practice, the initial setup (fault geometry, slip distribution, boundary condition, etc.)…”