Minoru Kasahara

The Amurian plate motion and current plate kinematics in eastern Asia

We use Global Positioning System (GPS) velocity data to model eastern Asian plate kinematics. Out of 15 stations in Korea, Russia, China, and Japan studied here, three sites considered to be on the stable interior of the hypothetical Amurian Plate showed eastward velocities as fast as ∼9–10 mm/yr with respect to the Eurasian Plate. They were stationary relative to each other to within 1 mm/yr, and these velocity vectors together with those of a few additional sites were used to accurately determine the instantaneous angular velocity (Euler) vector of the Amurian Plate. The predicted movement between the Amurian and the North American Plates is consistent with slip vectors along the eastern margin of the Japan Sea and Sakhalin, which reduces the necessity to postulate the existence of the Okhotsk Plate. The Euler vector of the Amurian Plate predicts left-lateral movement along its boundary with the south China block, consistent with neotectonic estimates of the displacement at the Qinling fault, possibly the southern boundary of the Amurian Plate. The Amurian Plate offers a platform for models of interseismic strain buildup in southwest Japan by the Philippine Sea Plate subduction at the Nankai Trough. Slip vectors along the Baikal rift, the boundary between the Amurian and the Eurasian Plates, are largely inconsistent with the GPS-based Euler vector, suggesting an intrinsic difficulty in using earthquake slip vectors in continental rift zones for such studies.

Initial results from WING, the continuous GPS network in the western Pacific area

To investigate tectonic deformation in the western Pacific, a continuous GPS tracking network has been established, and named the Western Pacific Integrated Network of GPS (WING). Between 1995 and March 1997 we establised ten new sites. Data for the period July 1995 to October 1996 were analyzed, together with data from International GPS Service for Geodynamics (IGS) global sites, to estimate daily coordinates. A fiducial-free approach was used to obtain the most accurate baseline estimates. To fix the estimated coordinates to the terrestrial reference frame, the Tsukuba IGS site is assumed to be moving westward relative to the stable Eurasian continent at ∼2 cm/yr according to Hekis [1996] estimate. We find that: (1) velocities of sites well within oceanic plates are in good agreement with rigid plate motion models; (2) sites close to plate boundaries are all affected by the deformation at those boundaries, among which back-arc rifting (spreading) is clearly visible at the Mariana and Okinawa troughs; (3) sites in eastern Asia are moving east to east-southeast relative to the stable Eurasian continent, suggesting long distance effects of the northward collision of India with Asia.

Velocity field of around the Sea of Okhotsk and Sea of Japan regions determined from a new continuous GPS network data

To investigate the current crustal movements in and around the Sea of Okhotsk and Sea of Japan regions, we have established a continuous GPS network. By the end of 1997, the network had been expanded to include 12 new stations. Data for the period from July 1995 to November 1997 were analyzed together with data from International GPS Service for Geodynamics (IGS) global stations. To fix the estimated coordinates to the terrestrial reference frame, the Tsukuba IGS station was assumed to be moving westward relative to the stable Eurasian continent at ∼2cm/yr according to Hekis[l996] estimate. We find that: (1) stations in the western margin of the Sea of Japan have eastward velocity vectors, (2) the pole position of the Okhotsk plate is located near Okha, which reconfirms the Okhotsk micro plate, (3) a plate boundary of the Okhotsk and Amurian plates between southen Sakhalin and Hokkaido is suggested.

Aftershock distribution of the October 4, 1994 Mw8.3 Kurile Islands Earthquake determined by a local seismic network in Hokkaido, Japan

On October 4, 1994, an earthquake with magnitude Mw8.3 occurred in the western part of Kurile Islands at 43.42°N, 146.81°E and 33 km in depth. The hypocenter parameters were determined by Hokkaido University in Japan. Aftershocks following this remarkable event were located using data from a local seismic network operated by Hokkaido University. We found that most of the aftershocks occurred (1) on the fault plane of the mainshock, (2) in the subducting plate around the fault plane of the mainshock, and (3) in the focal area of the largest aftershock, which occurred on October 9 with Mw7.3. Both (2) and (3) were not active immediately after the mainshock. Considering the time sequence of the aftershock activity, we identified one of the nodal planes of the Harvard quick CMT solutions as the fault plane of the mainshock; the strike is almost parallel to the trench axis and the dip angle is near vertical. It is obvious that this event is different from a low-angle thrust-type interplate earthquake. The distribution of aftershocks strongly suggests that it is an intraplate event.

Unbending and horizontal fracture of the subducting Pacific plate, as evidenced by the 1993 Kushiro-oki and the 1981 and 1987 intermediate-depth earthquakes in Hokkaido

Abstract A large number of aftershocks of the 1993 Kushiro-oki earthquake ( M JMA = 7.8), 1987 Hidaka earthquake ( M JMA = 7.0), and 1981 Western Hidaka earthquake ( M JMA = 7.1) have been observed by the seismic network of Hokkaido University. These three earthquakes are the largest events that have occurred in the depth range from 100 km to 200 km under the Pacific side of Hokkaido for 109 years. They migrated from west to east in the last 12 years. Their focal mechanism solutions represent typical down-dip extension in the lower zone of the double seismic zone in the descending Pacific plate. We investigated aftershock distributions of the three intermediate-depth earthquakes to study physical properties of the descending slab. Aftershocks of the Kushiro-oki earthquake spread horizontally over an area of 50 km × 40 km. A vertical cross-section of this aftershock distribution shows that it extends from the lower seismic zone almost to the upper seismic zone. It shows a slightly concave shape of about 35 km in length. Aftershocks of the Hidaka and Western Hidaka earthquakes also extend horizontally over areas of 10 km × 10 km and 8 km × 12 km, respectively. These main shocks occurred in the lower seismic zone. The horizontal aftershock distributions of all three large earthquakes indicate that the horizontal nodal planes of the focal mechanisms, rather than the vertical nodal planes, are the fault planes. Such a characteristic suggests that the descending lithosphere under Hokkaido has been unbent macroscopically over the last 12 years. Unbending and the long horizontal faults of the intermediate-depth earthquakes may be caused by mantle flow in the asthenosphere beneath the Pacific plate. We suggest that the descending slab of about 100 km thickness constitutes two layers: a seismic, brittle upper layer and an aseismic, ductile lower layer. Small ruptures may tend to occur easily along both boundary surfaces of the upper layer under the action of forces, especially unbending, and form the double seismic zone.

Performance test on long-period moment tensor determination for near earthquakes by a sparse local network

We have investigated the accuracy of long-period moment tensor determination for local to regional earthquakes by a sparse local network consisting of three stations equipped with a very-broad-band, three-component seismometer. We present the analysis results for 15 events with seismic moments of 3 × 1017 to 9 × 1019 Nm, which occurred in and near the Japanese region during the period from January to December of 1989. The moment tensor solutions obtained for the events are compared with the solutions retrieved from global networks and reported in PDE (Preliminary Determination of Epicenters). The comparison shows that a sparse local network of three-component, very-broad-band seismometers can reliably determine the moment tensors of near-distance, moderate to large events.

On the Cause of Long-Period Crustal Movement

Abstract It has been found by continuous observation in the vaults of crustal-movement observatories that the progress of crustal deformation is accompanied by a long-period fluctuation. The migration of this fluctuation between the adjacent stations and its simultaneity over a wide area has also been recognized. Some interpretations of the cause have been proposed from the viewpoint that fluctuations originate substantially from the tectonic source. In this paper, it is concluded that this long-period strain fluctuation is fairly well explained by precipitation changes at each site; the simultaneity may be explained by the identical precipitation conditions over a wide area, while the migration may be deceptive, attributed to the differences in the response to the precipitation at each station. In other words, prior to the discussion of the long-period crustal movement in relation to tectonics, the effect of precipitation on the crustal strain at that station should be carefully examined.

Three-dimensional P and S wave velocity structures beneath the Hokkaido corner, Japan-Kurile arc-arc junction

We applied an inverse method developed by Zhao et al. (J. Geophys. Res., 97, 19909–19928, 1992) to 42,834 P and 18,263 S wave arrival time data observed at 152 seismographic stations for 1143 local earthquakes at depths between 0 and 200 km in order to estimate three-dimensional P and S wave velocity structures beneath the Hokkaido corner, Japan-Kurile arc-arc junction. High- and low-velocity zones were clearly imaged in the Hidaka Mountain Range at depths shallower than 35 km. The low-velocity anomalies of P and S waves were found to be distributed in the mantle wedge at depths between 35 and 100 km beneath the volcanic front, as also observed in the Tohoku region. Another low-velocity zone was found to exist in the fore-arc region at depths of 50–70 km above the plate boundary; this zone was not detected in Tohoku, suggesting that the dehydration process in the fore-arc region is different from that in the Tohoku region.

GPS observation of the first month of postseismic crustal deformation associated with the 2003 Tokachi-oki earthquake (MJMA 8.0), off southeastern Hokkaido, Japan

To investigate the postseismic crustal deformation associated with the Tokachi-oki earthquake (MJMA = 8.0) of 26 September 2003 in Japan Standard Time (JST), off southeastern Hokkaido, Japan, we newly established thirty GPS sites just after the mainshock in the eastern part of Hokkaido. Rapid data analysis for one month after the mainshock clearly indicated postseismic displacements only in the horizontal components. Observed maximum horizontal displacement was 6.6 cm from 28 September to 24 October, 2003. Absence of the vertical suggests that afterslip occurred in and around the coseismic fault rather than at downdip extension. Time series of coordinates are characterized by logarithmic decay functions with 4–11 days relaxation times. This suggests that postseismic deformation was due to afterslip on the fault following the large earthquake.

Postseismic displacements following the 2007 Noto peninsula earthquake detected by dense GPS observation

We have been conducting dense GPS observation in and around the epicentral region of the 2007 Noto peninsula earthquake since March 25, 2007, in order to detect postseismic displacements. Continuous observation has been underway at 12 sites to fill the gap of GEONET. Preliminary analysis of data up to early May shows that initial postseismic displacement rapidly decayed within 20 days after the occurrence of the mainshock. Horizontal displacements do not exceed 20 mm even at sites above the aftershock zone for this period. We also found a maximum uplift of about 20 mm there. Inversion of postseismic displacements with the variable slip model suggests a nearly right-lateral afterslip of less than 5 cm on the shallow portion of the source fault. Fitting a theoretical function to a time series of coordinate changes also suggests that the observed postseismic displacements might have been generated by afterslip.