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We conducted long-period magnetotelluric observations in northeastern Japan from 2010 to 2013 to investigate the three-dimensional electrical resistivity distribution of the subduction zone. Incorporating prior information of the subducting slab into the inversion scheme, we obtained a three-dimensional resistivity model in which a vertically continuous conductive zone is imaged from the subducting slab surface to the lower crust beneath the Ou Backbone Range. The conductive body indicates a saline fluid and/or melt pathway from the subducting slab surface to the lower crust. The lower crust conductor is less than 10 Ωm, and we estimate a saline fluid and/or melt fraction of at least 0.7 vol. %. Other resistivity profiles in the across-arc direction reveal that the conductive body segregates from the subducting slab surface at 80–100 km depth and takes an overturned form toward the backarc. The head of the conducting body reaches the lower crust just beneath Mt. Gassan, one of the prominent backarc volcanoes in the system.

Recent studies indicate that the structure of the lower crust affects the occurrence of large intraplate earthquakes. Therefore, for understanding the generating processes of intraplate earthquakes, it is important to reveal the detailed structure of the lower crust. After the 1995 Hyogo-ken Nanbu (Kobe) earthquake (M 7.3), the Research Group for Explosion Seismology (RGES) conducted a seismic refraction experiment in the northern part of Awaji Island in order to investigate the detailed structure around the focal region. In the surveyed area of 5 km × 10 km, 128 temporal observation stations were deployed two-dimensionally, and seven explosive shots with a 150-kg charge were detonated. The recorded waveforms contained many later phases with lapse times of 6–11 s from the shots. These later phases imply that strong inhomogeneity exists in the lower crust. Waveforms of the later phases were very complicated and appeared to be coherent only among 8–11 adjacent stations. In this study, we considered the later phases as P-to-P scattered waves and estimated the spatial distribution of scatterers by using the following procedure. We divided the stations into 18 groups, each consisting of 8–11 stations. For each group and each shot, the semblance coefficient is calculated using the seismograms in a frequency range of 4–8 Hz to pick up coherent later phases. The location of a scatterer was estimated from travel time, incoming azimuth, and incident angle of the phase. By applying this procedure, we obtained an image of the scatterer distribution. The scatterers were distributed in regions around 25 km depth in the central part and 30 km depth to the northwest of Awaji Island with a horizontal extent of about 10 km. These two regions were located on the edge of the low-Vp, low-Vs, and high-Vp/Vs zones. The spatial relationship between the source fault, scatterer distribution, and velocity anomalies found in this study was similar to that obtained in other focal regions of intraplate earthquakes, the 2002 western Tottori earthquake (M 7.3) and the 1962 northern Miyagi earthquake (M 6.5).

Real-time monitoring of crustal deformation is important in achieving rapid understanding of earthquake magnitude and fault model. Recently, an algorithm called Real-time Automatic detection method for Permanent Displacement (“RAPiD”) has been developed to detect/estimate static ground displacements due to earthquake faulting from real-time kinematic (RTK)-GPS time series. We applied this algorithm to the 2011 off Ibaraki earthquake (Mw 7.7), which occurred only 30 min after the 2011 Tohoku-Oki earthquake (Mw 9.0). The RAPiD algorithm worked well with the long baseline RTK-GPS time series for quasi real-time coseismic displacement detection and estimation. A quasi real-time fault determination was also attempted with an automatic detection/estimation displacement field. We found that the estimated moment release reached Mw 7.7 60 s after the origin time, almost the same as the actual seismic moment for this earthquake. We also assessed the long-term stability of the RTK-GPS time series under a 200-km baseline condition. We found the time series precision degraded slightly in summer compared with winter. However, the total stability is good for monitoring crustal deformation. These results suggest clearly that using real-time GPS data in conjunction with the RAPiD algorithm can provide rapid coseismic fault determination, even for consecutive large earthquakes.