Shin'ichi Miyazaki

Anatomy of apparent seasonal variations from GPS‐derived site position time series

[1] Apparent seasonal site position variations are derived from 4.5 years of global continuous GPS time series and are explored through the ‘‘peering’’ approach. Peering is a way to depict the contributions of the comparatively well-known seasonal sources to garner insight into the relatively poorly known contributors. Contributions from pole tide effects, ocean tide loading, atmospheric loading, nontidal oceanic mass, and groundwater loading are evaluated. Our results show that � 40% of the power of the observed annual vertical variations in site positions can be explained by the joint contribution of these seasonal surface mass redistributions. After removing these seasonal effects from the observations the potential contributions from unmodeled wet troposphere effects, bedrock thermal expansion, errors in phase center variation models, and errors in orbital modeling are also investigated. A scaled sensitivity matrix analysis is proposed to assess the contributions from highly correlated parameters. The effects of employing different analysis strategies are investigated by comparing the solutions from different GPS data analysis centers. Comparison results indicate that current solutions of several analysis centers are able to detect the seasonal signals but that the differences among these solutions are the main cause for residual seasonal effects. Potential implications for modeling seasonal variations in global site positions are explored, in particular, as a way to improve the stability of the terrestrial reference frame on seasonal timescales. INDEX TERMS: 1223 Geodesy and Gravity: Ocean/Earth/atmosphere interactions (3339); 1247 Geodesy and Gravity: Terrestrial reference systems; KEYWORDS: seasonal variation, GPS, time series

Crustal velocity field of southwest Japan: Subduction and arc‐arc collision

We investigate crustal deformation in southwest Japan over a 3-year period revealed by a permanent dense Global Positioning System (GPS) array. Southwest Japan is a part of the Amurian Plate, a microplate moving about 10 mm/yr toward the east with respect to the Eurasian Plate. It overrides the Philippine Sea Plate at the Nankai Trough and collides with the northeast Japan arc in the central part of Japan. In this paper we first derive GPS site velocities relative to the stable part of the Amurian Plate in order to isolate signals of crustal deformation caused by the subduction and/or the collision. The velocity field has a conspicuous feature indicating the interseismic elastic loading by the Philippine Sea Plate slab at the Nankai Trough, characterized by the northwestward movements of points throughout the studied area. Their amplitudes are the largest at the Pacific coast and decay toward the Japan Sea coast with a subtle systematic shift of azimuths. A model assuming an elastic half-space, the convergence rate at the Nankai Trough based on a refined Euler vector, and the strength of the coupling inferred from a thermal model, could explain the velocity field in the western part of the studied area to a large extent. Those in the eastern part systematically deviate from them, and the residual components there show east-west shortening and north-south extension. This may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast Japan. This suggests that the collision between southwest and northeast Japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block. A velocity contrast was found across the Median Tectonic Line, the largest inland active fault in Japan, but the current permanent GPS network is not dense enough for us to discuss its coupling depth.

Full interseismic locking of the Nankai and Japan‐west Kurile subduction zones: An analysis of uniform elastic strain accumulation in Japan constrained by permanent GPS

We analyze permanent Global Positioning System (GPS) data obtained over Japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the Pacific and Philippine Sea plates below the Japanese islands. We first derive the GPS strain rate fields that characterize the crustal deformation of southern and northern Japan and invert them to determine the effective subduction velocity along the central Nankai trough on one side and the Japan-west Kurile trench on the other. These “reference free” velocities are close to those predicted by plate motion models with respect to Eurasia. We conclude that the Eurasian reference frame gives a good approximation to the subduction motion and that to first order, both subduction zones were fully locked during the period of measurements. We then test whether the coupling ratio shows local variations within the seismogenic zones. To do this, we divide the subduction interface into 35 km×30 km elements that we model by point source groups, and we invert the GPS velocity field referenced to Eurasia to derive the coupling ratio (between 0 and 1) on each fault element. The results are coherent over the 3 years and confirm that both the central Nankai and the Japan-west Kurile seismogenic zones are homogeneously fully locked. Most of the coupling ratios are close to 1 and a few are close to 0; intermediate values are rare. The zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 Sanriku-Oki interplate earthquake (Japan trench) or to a small overestimation of the actual lower limit of the locked zone. We conclude that within the resolution of the GPS data and the model, (1) partial coupling did not exist during these 3 years along the Nankai and Japan-west Kurile trenches; (2) the small seismic coupling ratio previously derived from earthquakes analysis for the Japan and Kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip; and (3) the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading. Finally, we discuss the nonelastic residual deformation within the frame of the long-term deformation of the Japanese islands.

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.

Plate convergence and long-term crustal deformation in central Japan

Surveys by continuous Global Positioning System in and around Japan revealed that the Amurian Plate collides with the North American Plate in central Japan by ∼2 cm/yr. Long-term crustal deformation seems to be influenced mainly by this collision although subduction of oceanic plates governs short-term elastic deformation over the arc. Here we study the long-term deformation field by carefully removing the short-term signals inferred from a-priori plate convergence vectors and coupling strengths predicted by a thermal model. The obtained field shows that the change in velocities occurs along the longitude 135° ∼ 137°, and there exist a relatively rigid block and zones accommodating strains. Characteristic compressional deformation is found northwest of Izu due possibly to the collision of the Izu-Bonin arc with Honshu. Plate convergence rate along the Nankai-Suruga Trough is considerably smaller in eastern parts, due partly to the transition from the Amurian to the North American Plate of the landward side, and partly to the motion of the Izu Microplate relative to the Philippine Sea Plate. This accounts for longer recurrence intervals of interplate earthquakes in the Suruga Trough where the Tokai earthquake is anticipated to occur.

Afterslip and viscoelastic relaxation following the 2011 Tohoku‐oki earthquake (Mw9.0) inferred from inland GPS and seafloor GPS/Acoustic data

We simultaneously estimate 2.5 years of afterslip and viscoelastic relaxation, as well as coseismic slip, for the 2011 Tohoku-oki earthquake. Displacements at inland GPS and seafloor GPS/Acoustic stations are inverted using viscoelastic Greens functions for a model with an upper elastic layer and lower viscoelastic substrate. The result shows that afterslip is isolated from the rupture area and possibly asperities of historical earthquakes and has almost decayed by 10 September 2013, 2.5 years after the main shock. The inversion result also suggests that observed landward postseismic displacements at the seafloor GPS/Acoustic stations are caused by the viscoelastic relaxation, whereas trenchward displacements at inland stations are mainly an elastic response to afterslip.

Interplate coupling along the Nankai Trough off southwest Japan derived from GPS measurements

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.

Periodicity of strain accumulation detected by permanent GPS Array: Possible relationship to seasonality of major earthquakes' occurrence

Continuous GPS measurements at about 500 sites in Japan reveal a time periodic component of crustal strain accumulation within the island arc. The magnitude of this variability is about 20–30 % of the steady annual component of deformation driven by coupling across the subduction zone. The amplitude of temporal variability is strongly organized in space, and its correlation with the secular velocity field suggests a geodynamic origin for the phenomenon. The observed periodicity may help to explain the seasonality of occurrence of the major earthquakes (Magnitude >7.5) associated with the Japanase subduction zone.

GPS meteorology project of Japan —Exploring frontiers of geodesy—

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

GPS time series modeling by autoregressive moving average method: Application to the crustal deformation in central Japan

Autoregressive moving average (ARMA) method is applied to modeling the time series of position changes of GPS sites, obtained by the Geographical Survey Institute (GSI) of Japan during the period from April 1996 to March 1998. The present application is focused on denoising of the GPS time series data where only white noise is considered, and detection of data discontinuities and outliers in order to obtain time-averaged velocity and strain fields in central Japan. The data discontinuities are detected by a typical Kalman filter algorithm. The outliers are eliminated by using robust estimation techniques during the ARMA process. The averaged strain field, calculated by the least-squares collocation method from the improved two-year time series data, distinguishes clearly between the tectonically active and inactive regions. Higher maximum shear strain rates were detected in the southern area of the Kanto district. In the areas with very high seismicities, however, the difference between the maximum shear strain rates, that were estimated from the raw time series data and the ARMA-analyzed data, amounted to about 0.2 microstrain/yr. This indicates that the existence of noise and discontinuities can lead to an over-prediction of the strain field.