Kosuke Heki

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.

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.

Horizontal and vertical crustal movements from three-dimensional very long baseline interferometry kinematic reference frame: Implication for the reversal timescale revision

Three-dimensional kinematic reference frame of geodetic very long baseline interferometry (VLBI) stations, tied to a geologic plate motion model, was established using the GLB907 solution by first selecting globally distributed stable plate interior stations and then applying a small translation and a rotation for the entire network in a three-dimensional space so that the differences in the “horizontal” velocities between the VLBI observations and the model predictions are minimized. Since the VLBI network is global, we only assume that the horizontal movements of technically stable stations obey a plate motion model; we need not introduce any unwarranted constraints to the vertical velocities of specific stations to realize the frame. A suggestive correlation was found between the estimated vertical velocities of North American stations and those predicted by a postglacial rebound model. The revision of the magnetic polarity timescale (MPTS) causes a uniform increase or decrease of the predicted velocities, which could be detected as the small difference between the measured and the predicted relative plate velocities. Direct estimation of the correction suggests that the VLBI data fit best to the model when the NUVEL1 model is corrected by +3.4% (±1.2%), which differs significantly from the −4.5% deduced from the astronomical MPTS calibration. This was further confirmed by estimating the rotation rates for individual plate pairs.

Snow load and seasonal variation of earthquake occurrence in Japan

Abstract Snow load along the western flank of the backbone range of the Japanese Islands causes seasonal crustal deformation. It perturbs the interseismic strain buildup, and may seasonally influence the seismicity in Japan. Intraplate earthquakes in northeastern Japan occur on reverse faults striking parallel with the snow-covered zone. In central and southwestern Japan, they occur on strike-slip faults striking either parallel with, or perpendicular to the snow cover. The snow load enhances compression at these faults, reducing the Coulomb failure stress by a few kPa. This is large enough to modulate the secular stress buildup of a few tens of kPa/yr. Past inland earthquakes with magnitudes ≥7.0 that occurred in regions covered with snow in winter, tend to occur more in spring and summer than in autumn and winter, while those in the snow-free regions do not show such variation. Although its statistical significance is not strong due to limited number of past earthquakes, it suggests that the spring thaw enhances seismicity beneath the snow cover.

Plate dynamics near divergent boundaries: Geophysical implications of postrifting crustal deformation in NE Iceland

The bulk of a tectonic plate is thought to move continously at a rate consistent with the geologic average. On the other hand, movements are highly episodic at plate boundaries. We study the plate dynamics that relate to these two different modes by modelling the displacements observed using the global positioning system in Northeast Iceland 1987–1990. These observations were made about 10 years after an episodic divergent movement between the North American and Eurasian plates 1975–1981. The horizontal displacement field fits well a two-dimensional model of postrupture stress relaxation assuming a thin elastic layer overlying a layer of Newtonian viscosity. This analysis indicates values of about 10 m2/s for the stress diffusivity and 0.3–2 × 1018 Pa s for the Newtonian viscosity of the lower layer. However, no significant correlation exists between the observed and modeled vertical displacements probably because of the relative inaccuracy of the vertical component observations. Assuming that contemporary plate motion is the sum of many displacements that have diffused from boundaries where episodic displacements occur periodically, we simulate the spatial transition from episodic to continuous plate movements. The plate “boundary zone” where movements are episodic or quasi-episodic is of the order of 100 km wide, depending on the stress diffusivity (which may be fairly uniform throughout the world) and the frequency of episodic movements.

Paleomagnetic study of the Central Andes: counterclockwise rotation of the Peruvian block

Kono, M., Heki, K. and Hamano, Y., 1985. Paleomagnetic study of the Central Andes: counterclockwise rotation of the Peruvian Block. Journal of Geodynamics, 2: 193-209. Paleomagnetic study was performed on Mesozoic and Tertiary rocks from Peru and northernmost Chile. Comparisons of these results as well as other data from the Central Andes with paleomagnetic poles from South American craton strongly support the orocline hypothesis of Carey for the formation of the Arica (Santa Cruz) deflection. Paleomagnetic declinations of Jurassic and Cretaeous rocks are quite similar to the direction of the present-day structural trend in the Central Andes, which suggests that the mountain belt has rotated in a coherent fashion (i.e., rigid body rotation) in sections of the Central Andes. The occurence of this deformation is certainly post-Cretaceous, with some suggestion that rotation still continued as recently as Neogene. The mechanism of this deformation is not well known, but a differential stretching of the Amazon Basin behind the Peruvian Andes is a possibility.

Short term afterslip in the 1994 Sanriku‐Haruka‐Oki Earthquake

The Sanriku-Haruka-Oki earthquake, that occurred on December 28, 1994 at the Japan Trench as a typical interplate thrust event, was followed by year-long afterslip as large as the slip in the high-speed rupture [Heki et al., 1997]. Here we report on the transition between these, inferred from crustal movements during the five days interval before and after the earthquake. Since this timescale is too long for seismometers but not suitably long for Global Positioning System (GPS), we rely primarily on strainmeter data taken ∼200 km southwest of the epicenter. To confirm that the recorded strain changes are not local disturbances, we compare them with crustal movements derived by high time resolution analyses of GPS data in the same period. The transition to the long term afterslip was gradually achieved by slow fault slip with a time constant of a few hours. The cumulative slow slip in 24 hours amounts to ∼1/3 of the coseismic slip, i.e. we may overestimate the coseismic displacement if we look at GPS data time series composed of daily solutions. The result presented here indicates that a single earthquake could have multiple aspects in its slowness, visible with different seismological and geodetic tools, and we need to use all such data to understand fully such a hybrid earthquake.

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.

Regional ionosphere map over Japanese Islands

Based on the high time and spatial resolution total electron content (TEC) data, which is estimated from the phase and code observables obtained by using GPS (Global Positioning System) Earth Observation Network (GEONET), the TEC distribution and its time variation over Japanese Islands are scaled into 0.5° × 0.5° grid data for each 10 minutes. The TEC daily map time series are arranged in an array to show the TEC evolution. Based on the spherical harmonics expansion of global ionospheric TEC model (GIM), which is estimated from global GPS observation, the TEC maps are expanded firstly through as high as 60 degrees and orders for a spherical harmonic function as a regional ionosphere map (RIM). The evolution history of medium scale traveling ionospheric disturbances (MSTIDs), i.e. polarward intense TEC enhancement and pre-noon rapid irregular fluctuations near the geomagnetic equator, are identified and confirmed in a quiet geomagnetic period.