Kenji Tachibana

Strain concentration zone along the volcanic front derived by GPS observations in NE Japan arc

A nationwide GPS array with more than 1,000 permanent stations operated by the Geographical Survey Institute of Japan, has provided many invaluable deformation data such as co- and post-seismic, volcanic, and ongoing secular deformations in the Japanese islands. Based on daily coordinate data of the GEONET stations together with results from a regional network operated by Tohoku University, we derived a map of the strain rate distribution in NE Japan showing that there exists a notable strain concentration zone of EW contraction along the Volcanic Front. The area demonstrates active seismicity including some disastrous earthquakes. Recent seismic tomography studies have revealed the existence of inclined seismic low-velocity zones (LVZ) at depths shallower than ~150 km in the mantle wedge sub-parallel to the subducted slab. The inclined LVZ reaches the Moho right beneath the Volcanic Front, indicating that the formation of the strain concentration zone is closely related to the rheological structure of the island-arc system.

Coseismic fault model of the 2008 Iwate-Miyagi Nairiku earthquake deduced by a dense GPS network

A large earthquake of Mj 7.2 occurred on June 14, 2008, beneath the border between Iwate and Miyagi prefectures in northeastern Japan. We propose a simple rectangular fault model based on a dense GPS network, including continuous GPS sites run by four agencies, to describe the coseismic deformation. The coseismic displacements are estimated by kinematic PPP (precise point positioning) analysis. Near the hypocenter, colocated independent instruments (integrated accelerogram and kinematic PPP) measure the same large displacement caused by the mainshock. The fault model explains the observations well and reproduces the observed complex spatial pattern, especially around the northern part of the focal area, which is the focus of a debate on whether or not the coseismic slip occurred on the Dedana fault system. Our results show that no major slip on the Dedana fault system occurred. The estimated amount of moment release was equivalent to Mw 6.9, and the maximum slip reached 3.5 m on the southern sub-fault.

Crustal deformation associated with the 1998 seismo-volcanic crisis of Iwate Volcano, Northeastern Japan, as observed by a dense GPS network

Mt. Iwate (2,038 m) is an active volcano located in northeastern Japan. Unrest of the volcano started in September, 1995 with intermediate-depth tremors. The shallow seismicity gradually became active in February, 1998, accompanying the notable crustal deformation observed by a dense GPS network. The pattern of the horizontal displacements is characterized by radially directing outward from the volcano. We estimated the source position by inversion analyses for every two-months period, assuming two models; a point pressure source (Mogi model) and a tensile fault. The comparison of AIC’s for the two models indicates that the latter is proper from February to April, while the former is preferable afterward. The tensile fault was located at about 5 km WSW of the summit and 3 km in depth, then a Mogi source was estimated at the western neighbor of the tensile fault in the successive period and moved westward as far as 10 km W of the summit with shallowing its depth. It should be noted that the seismic area also expanded westward in the same period. This synchronicity suggests that the both phenomena were caused by a movement of magma from the deeper part beneath the summit to the western shallower part.

Co- and post-seismic slip associated with the 2005 Miyagi-oki earthquake (M7.2) as inferred from GPS data

A large earthquake with M7.2 occurred on August 16, 2005 along the plate boundary off Miyagi Prefecture. Co- and post-seismic deformations associated with this event were investigated to reveal the causal interplate slips using continuous GPS data and geodetic inversion. The coseismic slip distribution shows good agreement with that estimated by seismic waveform inversions. The major slip area is limited to the southeastern part of the rupture area of the previous 1978 event. The post-seismic slip extended to the southwest of the co-seismic slip area. These distinctive features of both the co- and post-seismic slips might be caused by the existence of the locked plate interface, where seismogenic stress has not released yet, in the northern part of the 1978 rupture area.

Strain accumulation in and around Ou Backbone Range, northeastern Japan as observed by a dense GPS network

A dense GPS network was established in 1997 around Ou Backbone Range (OBR), northeastern Japan, by deploying 28 continuous GPS stations to complement the sparse portion of GEONET operated by the Geographical Survey Institute of Japan. The aim of the network is to investigate the present surface deformation and understand the relationship between earthquake occurrence and the deformation process of the island-arc crust. Our GPS data are analyzed using a precise point positioning strategy of GIPSY/OASIS-II. Results of GEONET in daily SINEX files have been supplied by the GSI. Producing grid data of horizontal velocities and taking spatial derivatives, we derived a map of strain rate distribution. The results show that the region between 38.8° and 39.8°N in the OBR experiences notable concentration of east-west contraction. The region coincides with the area of active seismicity, including the focal areas of large earthquakes occurring in 1896 (M7.2), 1900 (M7.0), 1962 (M6.5), 1970 (M6.2), and 1998 (M6.1). Observed strain is larger than can be explained by the total moment release of earthquakes that occurred in the same period as this study. Possible sources of strain concentration may be viscoelastic deformation due to large earthquakes and/or aseismic slip along the deeper extension of the active faults.

Slip distribution of the 2003 northern Miyagi earthquake (M6.4) deduced from geodetic inversion

On July 26, 2003, a disastrous earthquake with M6.4 struck the northern part of Miyagi prefecture, northern Honshu, Japan. GPS measurements and leveling surveys conducted by the Geographical Survey Institute (GSI) revealed clear evidence of coseismic deformation. We analyzed those data using a geodetic inversion to estimate the slip distribution on a curved fault plane, which is suggested by precise hypocenter determination performed by Tohoku University. The maximum slip area is located at the northern and shallower part of the fault plane, which is consistent with the slip distribution obtained by seismic waveform inversion. The spatial pattern of slip direction also shows good agreement with that of the focal mechanism.

Strain anomalies induced by the 2011 Tohoku Earthquake (Mw 9.0) as observed by a dense GPS network in northeastern Japan

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.

Coseismic and postseismic deformation related to the 2007 Chuetsu-oki, Niigata Earthquake

An intermediate-strength earthquake of magnitude Mj 6.8 occurred on July 16, 2007, centered beneath the Japan Sea a few kilometers offshore of Niigata Prefecture in central Japan. We constructed a dense GPS network to investigate postseismic deformation after this event, choosing our GPS sites carefully so as to complement the nationwide GPS GEONET array. Coseismic displacements caused by the mainshock detected at some GEONET sites were used to estimate coseismic fault parameters. The results indicate that the geodetic data can be explained by a combination of two rectangular faults dipping northwest and southeast. Minor but definite postseismic deformation was detected largely in the southern part of the dense network. The time series of site coordinates can be characterized by a logarithmic decay function, and the estimated time constant seems to be almost similar in range to that of the 2004 Mid-Niigata Prefecture Earthquake. We also found a possible site instability at 960566 (Izumo-zaki, GEONET) caused by a small, local landslide associated with the mainshock and therefore concluded that the data obtained at this site should not be used for coseismic or postseismic analysis.

Crustal strain, crustal stress and microearthquake activity in the northeastern Japan arc

Abstract Crustal movement and seismic activity in the northeastern Japan arc are produced by a reaction of the Pacific Plate sinking under the continental plate. We have investigated the time variation of crustal strain obtained from analysis of extensometer installed at eleven crustal-movement observatories in the northeastern Japan arc. We then investigated, using the finite element method, the spatial distribution and time variation of the crustal strain and stress caused by plate subduction. We finally compared the results with microearthquake activity observed by the seismic network of Tohoku University. The spatial distribution and time variation of microearthquake seismic energy were elucidated in order to know the seismic circumstances around the eleven observatories. A contour map of the seismic energy distribution was obtained, showing a lineation of the seismic activity. The time variation of various strain components from 1967 to 1981 were revealed. With the accumulation of additional data, it is possible to recognize the propagation of the maximum shear strain in the direction S50°E at 40 km/year from the Pacific Ocean side to the Sea of Japan. A similar time variation of areal-dilatation was found at the five observatories located at a distance of more than 100 km. The directions of the compression axes derived from the composite focal mechanism solution of microearthquakes coincide well with those of principal strain obtained from data gathered using extensometers, indicating E–W compression in most of the observatories and N–S compression around MYK. The spatial distribution and the accumulation of crustal stress caused by plate subduction were investigated using the finite element method, and a comparison with a hypocenter distribution of microearthquakes was performed. The distribution of the crustal stress is closely related to the crustal structure, but the amplitude of the observed strain is affected by local structures around the stations.

Postseismic slip associated with the 2007 Chuetsu-oki, Niigata, Japan, Earthquake (M 6.8 on 16 July 2007) as inferred from GPS data

Postseismic crustal deformation associated with the 2007 Chuetsu-oki Earthquake, which occurred on 16 July 2007 with a magnitude of 6.8 at the southeastern rim of the Sea of Japan, near the coast of Mid-Niigata Prefecture, Central Japan, are detected by GPS observations. We analyzed continuous GPS data from the sites of the Geographical Survey Institute of Japan (GSI) and another dense temporary network, which we established just after the main shock to reveal spatio-temporal evolution of postseismic slip for 50 days after the main shock by geodetic inversion methods. Four models of faults are configured following Ohta et al. (2008, this issue), and these are optimized based on ABIC (Akaike’s Bayesian Information Criterion). The results of the inversion analysis show that the postseismic slip on the faults occurred at a downdip and updip extension of the coseismically slipped portion. The slip in the shallower portion decayed to be negligible within 2 weeks, and the slip in the deeper portion was still large after the slip in the shallower portion had almost terminated.