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To archive precious analog seismograms of major earthquakes that occurred in and around Japan, we have scanned selected analog seismograms from 113 stations deployed by the Japan Meteorological Agency (JMA) and are constructing a database of the scanned digital images accessible through the website of the Headquarters for Earthquake Research Promotion (HERP) of the Japanese government. This project started in 2006 and was conducted by the Association for the Development of Earthquake Prediction. The database of HERP now contains more than 185,000 digital images of seismograms recorded during the period from 1884 to 1995. It includes seismograms of earthquakes that occurred in and around Japan and of some major teleseismic earthquakes. The oldest analog seismogram was recorded on 2 January 1884 at a station in Tokyo. The seismograms are the intellectual property of mankind, and the database of these digital images will be an important material for studies of past destructive earthquakes. Technological progress such as artificial intelligence image recognition technology will help to further explore the data.

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.

[1] A magnitude 7.3 foreshock occurred at the subducting Pacific plate interface on March 9, 2011, 51 h before the magnitude 9.0 Tohoku earthquake off the Pacific coast of Japan. We propose a coseismic and postseismic afterslip model of the magnitude 7.3 event based on a global positioning system network and ocean bottom pressure gauge sites. The estimated coseismic slip and afterslip areas show complementary spatial distributions; the afterslip distribution is located up-dip of the coseismic slip for the foreshock and northward of hypocenter of the Tohoku earthquake. The slip amount for the afterslip is roughly consistent with that determined by repeating earthquake analysis carried out in a previous study. The estimated moment release for the afterslip reached magnitude 6.8, even within a short time period of 51h. A volumetric strainmeter time series also suggests that this event advanced with a rapid decay time constant compared with other typical large earthquakes.

[1] Real-time crustal deformation monitoring is extremely important for achieving rapid understanding of actual earthquake scales, because the measured permanent displacement directly gives the true earthquake size (seismic moment, Mw) information, which in turn, provides tsunami forecasting. We have developed an algorithm to detect/estimate static ground displacements due to earthquake faulting from real-time kinematic GPS (RTK-GPS) time series. The new algorithm identifies permanent displacements by monitoring the difference of a short-term average (STA) to a long-term average (LTA) of the GPS time series. We assessed the noise property and precision of the RTK-GPS time series with various baseline length conditions and orbits and discerned that the real-time ephemerides based on the International GNSS Service (IGS) are sufficient for crustal deformation monitoring with long baselines up to ∼1,000 km. We applied the algorithm to data obtained in the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0) to test the possibility of coseismic displacement detections, and further, we inverted the obtained displacement fields for a fault model; the inversion estimated a fault model with Mw 8.7, which is close to the actual Mw of 9.0, within five minutes from the origin time. Once the fault model is estimated, tsunami waveforms can be immediately synthesized using pre-computed tsunami Green's functions. The calculated waveforms showed good agreement with the actual tsunami observations both in arrival times and wave heights, suggesting that the RTK-GPS data by our algorithm can provide reliable rapid tsunami forecasting that can complement existing tsunami forecasting systems based on seismic observations.

After the 1995 Hyogo-ken Nanbu (Kobe) earthquake (M7.2), the Research Group for Explosion Seismology (RGES) conducted a refraction seismic experiment in the northern part of the Awaji Island in order to investigate a detailed structure around the source region. In the surveyed area, 145 temporal observation stations were deployed two-dimensionally at 200–250 meters intervals, and 7 shots of 150 kg charges were detonated. Using the first arrival times, we derived a three-dimensional P wave velocity structure of the shallow crust in this region. A dominant feature was the low velocity in the northern part of the region as compared with the southern part. The averages of velocities in the northern area were 4.1 km/s (at a depth of 0.09 km) and 4.5 km/s (at 0.49 km). In the southern region, these were 4.9 and 5.2 km/s, respectively. The study region was mostly within the Cretaceous granitic rocks, and we cannot find a difference in geology that corresponds to the velocity variation. On the other hand, the Bouguer anomaly and the resistivity distributions are consistent with the lateral velocity difference.

We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range - a strain concentration zone in the interseismic period - shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.

We propose a source fault model for the 2011, April 7, earthquake (M 7.1) deduced from a dense GPS network. The coseismic displacements obtained by GPS data analysis clearly show the spatial pattern specific to intraslab earthquakes, not only in the horizontal components but also the vertical ones. A rectangular fault with uniform slip was estimated by a non-linear inversion approach. The results indicate that the simple rectangular fault model can explain the overall features of the observations. The amount of moment released is equivalent to Mw 7.17. The hypocenter depth of the main shock estimated by the Japan Meteorological Agency is slightly deeper than the neutral plane between down-dip compression (DC) and down-dip extension (DE) stress zones of the double-planed seismic zone. This suggests that the depth of the neutral plane was deepened by the huge slip of the 2011, M 9.0, Tohoku Earthquake, and the rupture of the thrust M 7.1 earthquake was initiated at that depth, although more investigations are required to confirm this idea. The estimated fault plane has an angle of ∼60 degrees from the surface of the subducting Pacific plate. It is consistent with the hypothesis that intraslab earthquakes are thought to be a reactivation of the pre-existing hydrated weak zones produced in the bending process of oceanic plates around outer-rise regions.