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Slow slip events (SSEs) lasting for approximately 1 year occur every 6–8 years around the Bungo Channel in the southwest Japan subduction zone. The slip time evolution of the latest Bungo Channel SSE that occurred in 2018–2019 has been studied; however, the detailed spatial and temporal relationship between the slip process and other nearby phenomena, such as tectonic tremors and SSEs, is not well understood. Moreover, the migration of such long-term SSEs from Hyuganada to Shikoku through the Bungo Channel has been suggested, but a slip process connecting the SSEs has not been observed. In this study, we utilized 21 continuous global navigation satellite system (GNSS) stations around the Bungo Channel and Hyuganada that have been installed by us since 2014 in addition to GNSS Earth Observation System (GEONET) stations to improve the spatial resolution of such interplate slip. Based on these data, we estimate the spatial and temporal slip evolutions of the major SSE in 2018–2019, which was accompanied by tremor activity in the deep episodic tremor and slip (ETS) zone, and a smaller SSE in 2015–2016 without tremor activity. We show that the slip area of the major SSE overlaps the ETS zone, whereas that of the smaller SSE does not. This strongly suggests that synchronized tremor activity with an SSE requires a slip close to or overlapping the ETS zone. We also show two distinct slip propagation paths from the Oita area during the 2018–2019 sequence: one is a southward propagation to the Miyazaki area, leading to an SSE around the Miyazaki Plain, and the other is an eastward propagation to an area close to Cape Ashizuri, where “invading slip” is proposed to propagate from the ETS zone to a shallower megathrust source area. These slip propagations may be two of fundamental slip modes that connect slow-slip patch-like areas around the Bungo Channel and Hyuganada. Graphic Abstract

Using global navigation satellite system (GNSS) data to detect millimeter-order signals of short-term slow slip events (S-SSEs) and to estimate their source parameters, especially duration, is challenging because of low signal-to-noise ratio. Although the duration of S-SSEs in the Nankai subduction zone has been estimated using tiltmeters, its regional variation has never been quantitatively studied. We developed an S-SSE detection method to estimate both the fault model and duration with their errors based on the detection methods developed by previous studies and applied it to a 23-year period of GNSS data in the Nankai subduction zone. We extracted S-SSE signals by calculating correlation coefficients between the GNSS time series and a synthetic template representing the time evolution of an S-SSE and by computing the average of correlation coefficients weighted by the predicted S-SSE signals. We enhanced the signals for duration estimation by stacking GNSS time series weighted by displacements calculated from the estimated fault model. By applying the developed method, we detected 284 S-SSEs from 1997 to 2020 in the Nankai subduction zone from Tokai to Kyushu and discussed their regional characteristics. The results include some newly detected S-SSEs, including events accompanying very low-frequency earthquakes and repeating earthquakes in offshore Kyushu. Our study provides the first geodetic evidence for synchronization of S-SSEs and other seismic phenomena in offshore Kyushu. We estimated the cumulative slip and duration, and their error carefully. We also estimated the average slip rate by dividing the cumulative slip by the cumulative duration. This study clarified that the average slip rate in western Shikoku was approximately twice as that in eastern Shikoku and Kyushu. These regional differences were statistically significant at the 95% confidence interval. Multiple factors can influence the regional characteristics of S-SSEs, and we speculate that the subducting plate interface geometry is one of the dominant factors. Graphical Abstract

The Sumatran fault is an arc-parallel dextral strike-slip fault that accommodates much of the right-lateral component of oblique subduction of the Indian-Australian plate beneath the Sunda plate. T...

Lack of observation network in the vicinity of oceanic intraplate earthquake lead the estimation of coseismic fault slip with high uncertainty. Satriano et al. [2] and Wei et al. [3] found NNE trending left-lateral slip as the primary features. In another hand, Yue et al. [4] and Hill et al. [5] proposed WNW trending right-lateral faults structure as the main characteristic. Here, we investigate the coseismic fault model that could explain the coseismic offset both vertical and horizontal in a 3-D heterogeneous earth structure. We constructed finite element model that include three-dimensional velocity structure, topography/bathymetry, spherical-earth and subducting slab. In this study, we employed scaling slip to adjust slip amount and total seismic moment. Instead of original slip amount, we preserve seismic moment as a basis comparison. Based on vertical and horizontal observation data, WNW trending right-lateral fault could fit better than NNE trending left-lateral fault. The present study demonstrates best-fit calculation using scaling slip optimized to the horizontal or vertical observation lead the both fault model worsen the misfit of vertical or horizontal component, respectively. This result analysis indicates a trade-off between vertical and horizontal component and reflects the importance of revisiting the fault slip modeling incorporating vertical and horizontal data equally.Lack of observation network in the vicinity of oceanic intraplate earthquake lead the estimation of coseismic fault slip with high uncertainty. Satriano et al. [2] and Wei et al. [3] found NNE trending left-lateral slip as the primary features. In another hand, Yue et al. [4] and Hill et al. [5] proposed WNW trending right-lateral faults structure as the main characteristic. Here, we investigate the coseismic fault model that could explain the coseismic offset both vertical and horizontal in a 3-D heterogeneous earth structure. We constructed finite element model that include three-dimensional velocity structure, topography/bathymetry, spherical-earth and subducting slab. In this study, we employed scaling slip to adjust slip amount and total seismic moment. Instead of original slip amount, we preserve seismic moment as a basis comparison. Based on vertical and horizontal observation data, WNW trending right-lateral fault could fit better than NNE trending left-lateral fault. The present study demonstrate...

We conducted yearlyGlobal Positioning System(GPS) campaigns in the eastern part of Mindanao from March 2010 to March 2014. The obtained station velocities with respect to the Sunda plate (SU) show that WNW motions are dominant due to the convergence of the Philippine Sea plate (PHS). However, it was found that elastic deformations caused by a full coupling of the plate interface down to 80 km could explain a maximum of only 29% of the observed station velocities. In order to interpret the displacement pattern, we applied a rigid block rotation model and determined the Euler vector. As a result, we determined that Mindanao Island could be divided into at least three blocks and that the Philippine fault is one of the block boundaries. Although it was not possible to determine the coupling ratio at the Philippine trench, the dislocation pattern of the Philippine fault showed along-strike variation in Mindanao Island.

We investigate the postseismic deformation of the 2004 Sumatra–Andaman earthquake (SAE) using 5 years of Global Positioning System (GPS) data located in northern Sumatra. Continuous GPS data from northern Sumatra suggest that the relaxation time in the vertical displacement is longer than horizontal displacements. This implies that there are multiple physical mechanisms that control the postseismic deformation, which refer to afterslip and viscoelastic relaxation. In this study, we introduce an analysis strategy of postseismic deformation to simultaneously calculate multiple mechanisms of afterslip and viscoelastic relaxation. The afterslip inversion results indicate that the distribution of the afterslip and the coseismic slip are compensatory of each other. Also, afterslip has a limited contribution to vertical deformation in northern Sumatra. In our rheology model, we use a gravitational Maxwell viscoelastic response and the result indicates that the elastic layer thickness is 65 ± 5 km and the Maxwell viscosity is 8.0 ± 1.0 × 1018 Pa s. We find that afterslip plus Maxwell viscoelastic relaxation are appropriate to explain the deformation in northern Sumatra. We also find that our rheology model reproduces the long-term features of the GPS time series in Thailand. Applying our rheology model to the data in Andaman Islands our afterslip estimation is located at the down-dip part of the plate boundary. Finally, we showed that our rheology model is applicable to the GPS datasets of postseismic deformation of the 2004 SAE located in northern Sumatra, Thailand, and Andaman–Nicobar, respectively.

On April 11, 2012, a Mw 8.6 earthquake struck off the west coast of northern Sumatra approximately 300 km west of the Sunda trench following by Mw 8.2 two hours afterward. The 2012 Indian Ocean Earthquake sequence, which was the largest intraplate earthquake in recorded history, yielded seismic moment of 1.2-1.3 × 1022 N m and 0.2-0.3 × 1022 N m for the main shock (Mw 8.6) and the after shock (Mw 8.2), respectively. Aceh GPS Network for Sumatran Fault System (AGNeSS) observed a predominantly ENE coseismic offset up to 10 cm while the sites on the Andaman Island observed southward and southern part Sumatra GPS Array (SuGar) network observed northward. Limited far-field and inland GPS observation network might lead the homogeneous and half-space model insufficient to explain the coseismic offset. In that of sense, in order to estimate more realistic surface displacement due to complex subduction and far-field GPS station, we consider developing inhomogeneous three-dimensional finite element model incorporate subducting slab, three-dimensional velocity earth structure, realistic topography and bathymetry as well as sphericity of the earth. We calculate coseismic offset by forward modeling using slip distribution as reported from Wei et al. [7]. This study investigates the effect of inhomogeneous structure and spherical geometry model in reproducing actual coseimic offset due to 2012 Indian Ocean Earthquake.

© 2016 The Author(s). The concept of a weak asthenospheric layer underlying Earth's mobile tectonic plates is fundamental to our understanding of mantle convection and plate tectonics. However, little is known about the mechanical properties of the asthenosphere (the part of the upper mantle below the lithosphere) underlying the oceanic crust, which covers about 60 per cent of Earth's surface. Great earthquakes cause large coseismic crustal deformation in areas hundreds of kilometres away from and below the rupture area. Subsequent relaxation of the earthquake-induced stresses in the viscoelastic upper mantle leads to prolonged postseismic crustal deformation that may last several decades and can be recorded with geodetic methods. The observed postseismic deformation helps us to understand the rheological properties of the upper mantle, but so far such measurements have been limited to continental-plate boundary zones. Here we consider the postseismic deformation of the very large (moment magnitude 8.6) 2012 Indian Ocean earthquake to provide by far the most direct constraint on the structure of oceanic mantle rheology. In the first three years after the Indian Ocean earthquake, 37 continuous Global Navigation Satellite Systems stations in the region underwent horizontal northeastward displacements of up to 17 centimetres in a direction similar to that of the coseismic offsets. However, a few stations close to the rupture area that had experienced subsidence of up to about 4 centimetres during the earthquake rose by nearly 7 centimetres after the earthquake. Our three-dimensional viscoelastic finite-element models of the post-earthquake deformation show that a thin (30-200 kilometres), low-viscosity (having a steady-state Maxwell viscosity of (0.5-10) × 10 18 pascal seconds) asthenospheric layer beneath the elastic oceanic lithosphere is required to produce the observed postseismic uplift.

Since the 2004 Sumatra–Andaman earthquake (M w 9.2), the northwestern part of the Sumatran island has been a high seismicity region. To evaluate the seismic hazard along the Great Sumatran fault (GSF), we installed the Aceh GNSS network for the Sumatran fault system (AGNeSS) in March 2005. The AGNeSS observed co-seismic offsets due to the April 11, 2012 Indian Ocean earthquake (M w 8.6), which is the largest intraplate earthquake recorded in history. The largest offset at the AGNeSS site was approximately 14.9 cm. Two M w 6.1 earthquakes occurred within AGNeSS in 2013, one on January 21 and the other on July 2. We estimated the fault parameters of the two events using a Markov chain Monte Carlo method. The estimated fault parameter of the first event was a right-lateral strike-slip where the strike was oriented in approximately the same direction as the surface trace of the GSF. The estimated peak value of the probability density function for the static stress drop was approximately 0.7 MPa. On the other hand, the co-seismic displacement fields of the second event from nearby GNSS sites clearly showed a left-lateral motion on a northeast–southwest trending fault plane and supported the contention that the July 2 event broke at the conjugate fault of the GSF. We also calculated the Coulomb failure function ΔCFF caused by the first event to evaluate its effect on the second event. The results showed that the July 2 event was likely brought 0.1 MPa closer to failure by the January 21 event.

The Philippine Sea plate has subducted beneath southwest Japan at the Nankai Trough, causing megathrust earthquakes every 100-140 years. The subduction process shows lateral variations along the trough, such as a bending of the plate interface beneath the Kii Peninsula and rapid changes of strike and dip off the east coast of Kyushu. We invert three-dimensional GPS velocities using ABIC geodetic inversion technique to estimate interseismic slip de.cit distribution at the Nankai Trough. The studied area is roughly from Kii peninsula to eastern Kyushu (131.5-137E, 31-35N). In the ABIC geodetic inversion some initial constraints are needed to stabilize solutions, such that the slip at the outer margin of the fault is set to zero. For the estimation of interseismic slip de.cit on the plate interface, however, these constraints are unrealistic because slip de.cit distribution continues later.ally to the outside of the target area. In this study we propose a repeated inversion method that shifts target area laterally along the plate boundary. This method averages slip de.cit estimates at the same point obtained from more than three consecutive inversions. The estimates obtained near the lateral margin, which may be strongly affected by the non-slip constraints at the outer margin, are excluded from the averaging. As a result final solutions are stabilized regardless the setting of the region. The final slip de.cit distribution at the Nankai Trough shows high contrast of strong and weak plate locking zones. Down-dip variation of the slip de.cit is emphasized more clearly but the lateral variation becomes moderate compared with the single inversion result.

We have studied crustal deformation in the Nankai subduction zone, southwest Japan, based on three-dimensional GPS velocity fields. Oblique subduction of the Philippine Sea plate has caused two different modes of deformation of the overriding plate: interseismic crustal shortening in the direction of plate convergence, and permanent lateral movement of the forearc. The block boundary dividing the forearc is the Median Tectonic Line (MTL); however, we assumed that its shallower portion is fully or partially locked to a certain depth. The plate boundary and the MTL are represented by many rectangular faults. We carried out inversion analyses with a priori information to estimate simultaneously slip deficit rates at those rectangular faults, together with the rate of lateral movement of the forearc. The results show that the seismogenic subduction faults at a depth of 5–25 km are strongly locked. As for the transition zone at 25–35 km, the inversion analysis results in stronger locking than shown in previous studies, especially when the vertical velocity data are weighted. The rates of lateral forearc movement and slip deficit at the MTL are nearly comparable but in a reverse sense to each other. This shows that the shallower portion of the MTL is strongly locked but that stationary aseismic slip is occurring in the deeper part.

In Antarctica, sea level rise and crustal uplift have occurred due to ice sheet melting and mantle response since the Last Glacial Maximum (LGM). The International GNSS Service (IGS) provides continuous data at nine sites on the Antarctic plate, and we analyzed data obtained from these sites between 1998 and 2003. Additional data were acquired by campaign observations around Syowa Station carried out by the Japanese Antarctic Research Expedition (JARE). Five sites around Syowa Station have been repeatedly occupied since 1998. Our analysis of the IGS data demonstrates that the Antarctic continent behaves as a rigid plate. Vertical components indicate an uplift of about 1.3–7.0 mm/year at almost all sites, which are attributed to the postglacial rebound. However, some observed velocities disagree with the predictions of glacial isostatic adjustment (GIA) models. It will be necessary to incorporate our results into the modeling of ice sheet melting. Around Syowa Station, the campaign GPS result is basically consistent with the IGS data analysis, thereby demonstrating that GPS observations properly represent the ongoing crustal movement around Syowa Station. However, the GPS results show some disagreement with the VLBI observations; this discrepancy will need to be sorted out using local tie observations that should be carried out in the near future.

A time-dependent model of magma intrusion is presented for the Miyake–Kozu Island area in central Japan based on global positioning system (GPS) measurements at 28 sites recorded between June 27 and August 27, 2000. A model derived from a precise hypocenter distribution map indicates the presence of three dikes between Miyake and Kozu Islands. Other dike intrusion models, including a dike with aseismic creep and a dike associated with a deep deflation source are also discussed. The optimal parameters for each model are estimated using a genetic algorithm (GA) approach. Using Akaike's information criteria (AIC), the three-dike model is shown to provide the best solution for the observed deformation. Volume changes in spherical inflation and deflation sources, as well as three dikes, are calculated for seven discretized periods after GA optimization of the dike geometry. The optimization suggests a concentration of dike expansion near Miyake Island in the period from June 27 to July 1 associated with large deflation at a depth of about 7 km below Miyake volcano, indicating magma supply from depth below Miyake Island. In the period from July 9 to August 10, a huge dike intrusion near Kozu Island is inferred, accompanied by expansion of the lower parts of a central dike, suggesting magma supply from depth in the region between Miyake and Kozu Islands.

Anatahan Island is located at the southern end of the Mariana volcanic chain. On May 10, 2003, the eastern crater of the island erupted for the first time in recorded history. The Plinian eruption column reached an altitude as high as 13 km on May 11, and a thick layer of ash covered the island. The eruptive activity continued to June 2003, but most of the erupted material was expelled during the first week. The volcanic activity declined in the second half of 2003, but resumed in April 2004. In order to determine crustal deformation associated with the eruption, we conducted GPS measurements in July 2003 at a benchmark (ANAT) located approximately 7 km west–northwest of the active crater, where GPS campaign measurements had been repeated four times since 1992. In the period from January to July 2003 during the eruption, significant subsidence – as much as 21 cm – was detected, but horizontal movement was negligible. We began taking continuous GPS measurements at the same site in July 2003 to monitor the transient deformation that was probably associated with magma migration. To assess the spatial extent of the deformation more accurately, we established another permanent GPS site (ANA2) at a site approximately 3 km from the active crater in the northeastern part of the island in January 2004. The coordinates of this time series at ANAT probably show a change in trends at the beginning of 2004. Another subsidence of 2.8 cm and a westward motion of 2.1 cm were estimated to have occurred in the period from July to December 2003. This was followed by an uplift of 5.2 cm and movement in an eastward direction of 1.0 cm in the period from January to June 2004. We developed three preliminarily models of inflation/deflation sources for three different time periods. During the period from January to July 2003, a deformation source was located beneath the ANAT site and acted as a deflation source. Considering the gap in the GPS time series and errors in data (especially after July 2003), we expected that the deformation sources were located beneath the western part of Anatahan Island and not below the active crater.

We have conducted geodetic and geomorphic observations in the northernmost part of Sumatra, Indonesia to monitor strain accumulation in the vicinity of the northern Sumatran fault. Evaluation of the earthquake generation potential in this region is highly urgent because of a large fault slip rate, absence of major earthquakes for more than 100 years and recent Coulomb stress increase on the fault due to the 2004 Sumatra-Andaman earthquake (M w 9. 2). We have deployed Aceh GPS Network for the Sumatran Fault System (AGNeSS) since 2005. The data collected have been used for estimating slip/locking distribution of the Sumatran fault and constructing a comprehensive model for postseismic deformation after the 2004 event. Tectonic geomorphic features are also important to reveal long-term slip history of the fault. We have used high-resolution stereo-paired ALOS (Advanced Land Observing Satellite) PRISM (Panchromatic Remote-sensing Instrument for Stereo Mapping) satellite images to map the surface trace of the Sumatran fault and conducted field observations to ensure the trace by geomorphic and geologic evidence. We introduce preliminary results of the fault mapping together with a brief description of the crustal deformation field detected by GPS.

We investigate interplate coupling between the Philippine Sea and the Eurasian plates at the Nankai Trough off southwest Japan during the interseismic period. Plate coupling is modeled by imaginary backward normal slip superposed on the steady-state subduction. We divide the plate interface into several segments, and estimate coupling on each segment from GPS horizontal velocity data. Estimated coupling is in general agreement with the rate of steady-state subduction predicted from a plate motion model. This implies that current coupling at the Nankai Trough is so strong that much of the relative plate motion is taken up by shortening of the leading edge of the overriding plate. It seems that stronger interseismic coupling is realized on the segment where larger coseismic dislocation occurred, and vice versa. Time fluctuation of plate coupling from 1995 to 1996 is found on a small segment.

The interseismic strain accumulation process at the Nankai subduction zone, the plate-boundary region between the Philippine Sea and the Eurasian plates off Southwest Japan, has been revealed by GPS measurements spanning five years. Large strains characterized by a contraction of 2.2 ∼ 3.4 × 10−7/yr have accumulated in a narrow region extending parallel to the strike of the plate boundary. The contraction axes lie in a northwest-southeast direction nearly parallel to the direction of relative motion of the Philippine Sea plate with respect to the Eurasian plate. GPS-derived deformation data, including both rate and direction, are consistent with the expected earthquake deformation cycle at the Nankai subduction zone. Current interplate coupling in this zone is strong; much of the relative plate motion has been taken up by deformation of the marginal parts of the Eurasian plate.

Within 1 to 4 days following the 1995 Hyogo-ken Nanbu earthquake (M 7.2) that occurred on January 17, 1995, the Japanese University Consortium for GPS Research deployed more than 30 GPS receivers in the hypocentral area to investigate post-seismic activity. Of these stations, we analyzed data from 8 sites that were placed near the hypocentral area; six in Kobe and two in Awaji. Two methods of analysis were employed: 1) change of absolute geocentric coordinates were estimated by assuming Usuda global site, which is about 300 km northeast of the hypocentral area, as fixed, 2) analyses for short baselines were conducted within the hypocentral area using the automatic processing technique. Results suggest that 1) rapid displacements toward the southeast of more than 4cm/year were found at all of the analyzed sites, which might be part of a post-seismic deformation process, 2) largest of which was found at Rokko Farm site, at which displacement amounted to about 3 cm until the end of February, though its cause were not specified, and 3) local straining seems to be taking place at both Kobe and Awaji areas.

Post-seismic deformations in the focal region of the 1995 Hyogo-ken Nanbu earthquake (M=7.2), in southwest Japan, which occurred on January 17 (JST), were observed with GPS observations. We deployed more than 30 GPS stations in and around the area of aftershock activity within several days after the occurrence of the earthquake. First the data were stored and manually collected at the observation sites, but by the middle of February the data at most of sites were started to be telemetered through public phone lines to data centers. GPS observations continued until the end of March, except for 4 sites left active for monitoring long-term deformations. We used two types of GPS observations, usual static and real-time kinematic GPS observations. Static day-by-day analyses, which are based on 24-h data sampled every 30 s, show the deformation associated with post-seismic readjustment with a relaxation time of about 70 days and with an amplitude of 1-2 cm in the area close to the epicenter. Also, rapid movements were observed just after the occurrence of the earthquake. However, since our analyses are only preliminary, further analyses are needed to clarify these observations and to investigate models of post-seismic deformations. Real-time kinematic GPS observations, which is a new type of GPS observations, were made every 1 s from February 9 to March 22 for detecting possible rapid relative movements between two sites across the surface fault trace on Awaji Island and across the other fault, the Arima-Takatsuki Tectonic Fault on the Kobe side. Present real-time kinematic GPS observations show scatters with the amplitude of 0.5-3.0 cm depending on components and baseline lengths. Any motions with amplitudes larger than these noise levels could not be detected during the observation period.

The Great Sumatran Fault system in Indonesia is a major right-lateral trench-parallel system that can be divided into several segments, most of which have ruptured within the last century. This study focuses on the northern portion of the fault system which contains a 200-km-long segment that has not experienced a major earthquake in at least 170 years. In 2005, we established the Aceh GPS Network for the Sumatran Fault System (AGNeSS) across this segment. AGNeSS observes large displacements which include significant postseismic deformation from recent large megathrust earthquakes as well as interseismic deformation due to continued elastic loading of both the megathrust and the strike slip system. We parameterize the displacements due to afterslip on the megathrust using a model based on a rate- and state-dependent friction formalism. Using this approach, we are able to separate afterslip from other contributions. We remove predicted deformation due to afterslip from the observations, and use these corrected time series to infer the depth of shallow aseismic creep and deeper locked segments for the Great Sumatran Fault. In the northern portion of this fault segment, we infer aseismic creep down to 7.3 ± 4.8 km depth at a rate of 2.0 ± 0.6 cm/year. In the southwestern portion of the segment, we estimate a locking depth of 14.8 ± 3.4 km with a downdip slip rate of 1.6 ± 0.6 cm/year. This portion of the fault is capable of producing a magnitude 7.0 earthquake.

Transient deformation resulting from postseismic stress relaxation in underlying viscoelastic layers is examined. The main purpose of the present study is to understand spatial and temporal modes of postseismic deformation especially due to strike-slip faulting with a realistic geometry. The fault size as well as the viscoelastic structure of the earth proved to have large effects on the amount of postseismic deformation. Therefore, it should be taken into serious consideration in modeling the faulting. It is noteworthy that the concentration of shear strain into the near field continues long after the faulting. This strain concentration is similar to the preseismic strain accumulation and is contrary to the coseismic strain release. The viscoelastic structure of the earth, however, is hard to determine solely by the surface measurements performed at long time intervals. Crustal movements in the Inner Zone of Southwest Japan, in particular, those associated with the 1927 Tango earthquake of M=7.5, are then investigated in connection with postseismic stress relaxation. The discrepancies between the observed and calculated postseismic deformations can be reduced if the effect of tectonic stress is added to the calculated deformation field. It is widely accepted that the basic model to explain the physical behavior of the earth's crust and upper mantle generally consists of three layers with different rheological properties; namely, the uppermost elastic and brittle lithosphere, the intervenient ductile asthenosphere with low viscosity, and the substratum with relatively high viscosity. In addition, it is also recognized that there exist considerable regional heterogeneities in both thicknesses and properties of the layered structure of the earth. These heterogeneities probably characterize the mode of accumulation and release of tectonic stress in the region, and this mode is reflected in the crustal movements and seismic activity peculiar to the region.

Immediately after the 1984 Yamasaki fault earthquake of M=5.6, remarkable postseismic deformation, represented by an exponential function, was observed at Ikuno in the northwestern part of the Kinki district, southwest Japan. The exceptional amounts of both strain and tilt changes suggest that the local conditions of the observation site played an important role in disarranging and amplifying local strain fields. The postseismic deformation at Ikuno can be explained as an elastic deformation caused by a large pressure decrease at a disturbing source with a thrust-type pressure distribution. It can be thought of as a characteristic phenomenon occurring under the peculiar conditions in which the observations have been carried out at a closed mine that has now been fully filled up with underground water to a depth of about 1, 000 m. The disturbing source was probably caused by the diffusion of underground water, which has a high hydraulic pressure due to the high water-level in the mine. We think that the movements of underground water sometimes have an important effect on the local ground deformation in the period immediately after an earthquake.

The Median Tectonic Line (MTL) is the longest arc-parallel fault system in southwest Japan whose right-lateral strike-slip is related to oblique subduction of the Philippine Sea plate (PH). We constructed a dense Global Positioning System network along a 200 km-long traverse line across the MTL in 1998 to estimate deep fault structure and slip distribution. Horizontal velocities were determined at 65 sites through campaign measurements and show crustal shortening in the direction of the plate convergence. Using multi-rectangular segments and depth-dependent coupling at the plate interface, we calculate and remove elastic deformation caused by the PH subduction. The residual velocity field shows right-lateral strike-slip block motion of about 5 mm/yr across the MTL, consistent with geological estimates. However, the block boundary does not coincide with the surface trace of the MTL, being displaced 20–30 km to the north. The residual velocity field is reproduced by a model with a 35–45° northwarddipping fault plane, full locking of the upper portion to a depth of 15 km, and steady slip of 5 mm/yr below. GPS results are supported by imaging of an inclined fault plane revealed by seismic profiling and currently low activity of shallow earthquakes.

In 2004, we started monitoring crustal deformation at Kumano-nada in the Nankai trough using the GPS/Acoustic technique. We observed a large southward seafloor displacement of ∼30 cm associated with the off Kii Peninsula earthquake, which occurred in September 2004, between our two survey campaigns in August and November 2004. The observed seafloor displacement is larger than that predicted from a slip model derived solely from GPS measurements on land. This may indicate the earthquake fault is slightly shallower and extends move to the NW than previously estimated.

1Kyoto University, Uji, Kyoto 611-0011, Japan 2National Astronomical Observatory, Mizusawa, Iwate 023-0861, Japan 3Geographical Survey Institute, Tsukuba, Ibaraki 305-0811, Japan 4Meteorological Research Institute, Tsukuba, Ibaraki 305-0052, Japan 5Nagoya University, Nagoya 464-0814, Japan 6Kochi University, Kochi 780-8520, Japan 7Kyushu University, Fukuoka 812-0053, Japan 8Tsukuba University, Tsukuba, Ibaraki 305-0006, Japan 9Shizuoka University, Shizuoka 422-8017, Japan 10University of Tokyo, Tokyo 113-0032, Japan

Although the Kozu Island Volcano, one of the Izu Islands Volcanoes in the south part Central Japan, is an active volcano, there is no record of the eruption for about 1100 years since the last eruption in 833 A.D. Since 1988, frequent earthquake swarms are observed around the Kozu Island, and the uplift of 2–4 cm/yr is observed on the island by tidal observations. Station velocities detected by GPS measurements since 1989 show velocities that differ from the convergent velocity of the Philippine Sea plate calculated from plate motion models. A local GPS network with 12 stations is occupied around the volcano, and the GPS measurements are repeated every about six month since July 1996. Inflated deformation of 2–4 cm/yr are detected from the GPS measurements and the pressure source is estimated to be located in the northeastern part of the island at a depth of 2.1 km using Mogi solution. Negative gravity changes of more than 30 microgal are also measured above the pressure source in the period November 1998 to July 1999, consistent with uplift.

We estimate long-term/permanent crustal deformation in the Nankai forearc, southwest Japan, that accumulated over one earthquake deformation cycle in the Nankai subduction zone. A short-term deformation due to an oblique subduction of the Philippine Sea plate is modeled based on coseismic fault slips associated with interplate thrust earthquakes at the Nankai Trough, and subtracted from the interseismic crustal velocity field observed by GPS. The long-term/permanent deformation left in the data shows arc-parallel movement of the forearc at a rate of 5— 10 mm/yr. We interpret that the forearc movement, driven by the oblique plate convergence, is accommodated by steady aseismic slip on the deep portion of the fault plane of the Median Tectonic Line (MTL) on the rear boundary of the Nankai forearc. The rate of the forearc movement is consistent with the geological slip rate of the MTL in the late Quaternary, and also with that expected from deflection of the slip vector of the 1946 Nankai earthquake from the current plate convergence vector.