Three‐Dimensional Electrical Resistivity Structure Beneath a Strain Concentration Area in the Back‐Arc Side of the Northeastern Japan Arc.

Intraplate earthquakes occur more frequently in the Japanese islands than in other regions. Large intraplate earthquakes in the island arc preferentially occur in strain concentration zones detected by geological and geodetic studies. Crustal heterogeneity plays a crucial role in generating large intraplate earthquakes and strain concentrations. Thus, we elucidated the crustal heterogeneities beneath a strain concentration area on the back‐arc side of the northeastern Japan Arc based on electrical resistivity, which is sensitive to weak zones in the crust. By deploying wideband magnetotelluric surveys, we revealed the three‐dimensional electrical resistivity structure in the crust, suggesting the coexistence of two different types of strain‐concentration mechanisms in the strain‐concentration area. The shallow conductive layers and lower‐crustal conductors appear to act as low‐elastic‐modulus and low‐viscosity areas, respectively, and are responsible for the strain concentration. We found shallow and lower‐crustal conductors in the strain concentration zone revealed by geological studies. Those conductive areas are considered to act as mechanically weak zones and cause stress loading on the brittle pars of pre‐existing faults, resulting in large intraplate earthquakes. The resultant earthquakes presumably contribute to strain accumulation on a geological timescale. In addition, a spatial correlation between the epicenters of large intraplate earthquakes and edges of lower‐crustal conductors implies the contribution of the fluids in the lower crust to the generation of large earthquakes. We also identified vertical conductors ranging from the lower crust to Quaternary volcanoes, which may indicate trans‐crustal magmatic systems under these volcanoes. Plain Language Summary: Intraplate earthquakes occur more frequently in the Japanese islands than in other regions worldwide. Large intraplate earthquakes in this region preferentially occur in high‐deformation zones, as detected by geological studies and global positioning system (GPS) measurements. Heterogeneous structure in the crust plays a crucial role in the generation of large intraplate earthquakes and high crustal deformations. Thus, we revealed the three‐dimensional crustal electrical resistivity structure beneath a strain‐concentration area in the northeastern Japan. The resistivity structure suggests the coexistence of two different mechanisms of high deformation. Shallow conductive layers and lower‐crustal conductors appear to act as low‐stiffness and low‐viscosity areas, respectively, leading to high crustal deformation. We found shallow and deep conductors in the high‐deformation zones revealed by geological studies. As both can act as weak zones, those conductors presumably play a key role in loading on faults, resulting in intraplate earthquakes. Because the epicenters of large intraplate earthquakes are positioned near the edges of lower‐crustal conductors, the fluids in those deep conductors possibly contribute to the generation of large earthquakes. We also revealed vertical conductors ranging from the deep crust to Quaternary volcanoes, which may indicate magma‐rich areas under these volcanoes. Key Points: The electrical resistivity structure suggests the coexistence of two strain‐concentration mechanisms in the strain‐concentration areaThe shallow conductive layers and lower‐crustal conductors act as low‐elastic‐modulus and low‐viscosity areas, respectivelyWeak zones in the crust, imaged as conductors, presumably cause stress loading on faults and contribute to large intraplate earthquakes [ABSTRACT FROM AUTHOR]