Kazuro Hirahara

A slow thrust slip event following the two 1996 Hyuganada Earthquakes beneath the Bungo Channel, southwest Japan

We report a “slow thrust slip event” that occurred beneath the Bungo Channel region, southwestern Japan. On Oct. and Dec., 1996, two Hyuganada earthquakes (both Mw = 6.7), followed by afterslips, occurred. In addition, a crustal movement characterized by an extremely slow rise was observed around the Bungo Channel, about 200 km north from the epicenters, and continued for about 300 days long. Assuming a slow slip on the plate boundary, we estimate its duration and surface displacements from GPS time series data by curve-fitting, and then, determine the fault slip distribution. We found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the Hyuganada earthquakes. Occurrence of the slow thrust slip event suggests that this kind of event may be a characteristic mode of stress release at a transition region of interplate coupling.

Three-dimensional seismic structure beneath southwest Japan: The subducting Philippine Sea Plate

Abstract A three-dimensional seismic structure beneath southwest Japan has been investigated in detail by applying an inversion method. This analysis clearly reveals the present, and possibly the past, subduction of the Philippine Sea plate beneath southwest Japan. Beneath the Izu Peninsula, the Philippine Sea plate does not appear to subduct but collides against the continental Asian plate here, and its thickness appears thinnest, at most about 30 km. From the Tokai to the Shikoku regions, the high-velocity zone corresponds well to the zone of subcrustal earthquakes down to a depth of about 60–70 km, indicating that the subduction of the present Philippine Sea plate does not reach below this depth here. However, a small high-velocity portion appears at depths between 100 km and 150 km beneath a part of the Kinki region and probably extends to the Chugoku region, where no corresponding seismic activities exist. These high velocities might be an indication of the remnants of the past subduction of the Philippine Sea plate. In the Kyushu region, the high-velocity zone continues down to a depth of about 150 km or even to 200 km in the southern part, corresponding to a steeply inclined seismic zone. The Philippine Sea plate in this region appears to be thicker than in regions east of the Kyushu—Palau ridge. It was also found that there exist narrow low velocity zones intervening within the Philippine Sea plate, where two aseismic ridges, the Kyushu—Palau and Shichito—Iwojima ridges, extend northwards and the Nankai trough shows warping. These low-velocity zones give support to the “buoyant subduction” hypothesis. It also appears that lateral heterogeneities existing within and around the subducting plate are closely related to the mode of rupture of large earthquakes occurring around there.

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.

the thickness of upper mantle discontinuities, as inferred from short‐period J‐array data

The thickness of upper mantle discontinuities (at depths around 410 km and 660 km) provides a critical clue to our understanding of deep earth. Since the amplitude of reflected or S-P converted waves is controlled by the thickness of discontinuities, short-period studies of such waves may give a fine constraint on the thickness of upper mantle discontinuities. In this study, we analysed short-period and vertical waveform data from 14 deep events in the Fiji-Tonga region. The data were obtained from J-Array, a short-period and large-aperture seismic array in Japan. With slant stack analyses of these data, we detected short-period reflections or S-to-P conversions from 410 km and 660 km discontinuities near sources, which implies that both discontinuities are quite sharp. To directly measure the frequency content of these waves, linear slant stacks were made with various frequency pass-bands (0.2–0.5, 0.5–1.0 and 1.0–2.0 Hz). Synthetic traces were generated for the models with various thicknesses of the 410 km and the 660 km discontinuities. Comparison of slant stacks with synthetic traces indicates that the thickness of both discontinuities is at most 5 km.

Three-dimensional P-wave velocity structure beneath Central Japan: low-velocity bodies in the wedge portion of the upper mantle above high-velocity subducting plates

Abstract The three-dimensional (3-D) P-wave velocity structure beneath Central Japan has been investigated in detail by an inversion method. 7490 P-wave arrival times from 120 shallow and intermediate depth earthquakes that have occurred in this region are used to estimate velocity anomalies in 3-D subdivided blocks and hypocentral perturbations, simultaneously. The results reveal complex 3-D structures, with low-velocity zones in the wedge portion of the upper mantle above the high-velocity Philippine Sea and Pacific plates subducting beneath this region. Prominent low-velocity bodies exist just beneath active volcanoes, particularly in the Hida mountain range. Low-velocity bodies are spatially correlated with the low-Q zones estimated from seismic intensity data. One low-velocity body coincides with an anisotropic body detected from the study of shear-wave splitting. Dome-shaped low-velocity masses seem to represent partially melted mantle diapirs. No clear evidence on velocity contrast has been identified across the Fossa Magna, which is a tectonic boundary between Northeast and Southwest Japan.

Dynamical fault rupture processes in heterogeneous media

Abstract The fault rupture processes in a horizontally layered medium and also in a three-dimensionally heterogeneous structure are investigated on three-dimensional, spontaneous dynamic shear crack models. The wave equations for three-dimensional space are solved numerically by a finite difference scheme under the appropriate boundary conditions and the finite stress fracture criterion. The heterogeneous properties of the elastic medium, particularly the existence of low-velocity zones, give remarkable effects on the rupture process, yielding appreciably decelerated rupture velocities and large fault displacements in and around the zones and strong motions in the near-field. The large fault displacements are enhanced when the rupture breaks the ground surface. The static seismic moment in the case with a low-velocity zone is essentially the same as in a homogeneous half-space. An attempt is made to simulate the rupture process of a moderate-size earthquake, by applying the above shear crack model in a heterogeneous medium with depth-dependent and laterally heterogeneous stress drop. The model appears to explain the observed features to a satisfactory degree.

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.

Simulation of postseismic deformations caused by the 1896 Riku‐u Earthquake, northeast Japan: Re‐evaluation of the viscosity in the upper mantle

We examine the postseismic deformations which are induced by an inland earthquake at a subduction zone, and investigate especially the effect of the viscoelastic heterogeneity through Finite Element Method (FEM). We estimate the viscosity of the upper mantle beneath the northeast Japan with realistic structures based on the leveling data which includes the postseismic deformations due to the 1896 Riku-u earthquake. For this purpose, we construct two 3-D FEM models with different viscoelastic structures, namely Layered Model and Plate Model. The results show that spatial patterns of the postseismic deformations differ considerably, depending on the two viscoelastic structures. Postseismic deformations are strongly controlled by the effective thickness of the viscoelastic medium above descending plate. At the surface, if the structure is complicated, the relaxation time is outwardly longer and varies significantly with position. Considering the plate structure, the profile of surface deformation and decaying subsidence rate constrain the Maxwell time of the upper mantle to be 5 years, which is shorter than the value of 10 years estimated by Thatcher et al. [1980] assuming the layered structures.

Large surface wave of the 2004 Sumatra-Andaman earthquake captured by the very long baseline kinematic analysis of 1-Hz GPS data

The 26 December 2004 Sumatra-Andaman great earthquake had a −1500 km long rupture of more than 600 seconds duration, and may have involved a complex rupture process including slow slip. We processed International GNSS Service (IGS) 1-Hz Global Positioning System (GPS) data using kinematic analysis to investigate ground motion caused by this large earthquake. Since there are few 1-Hz stations, we had to process long baseline up to several thousand kilometers long. Long baselines degrade the GPS carrier phase ambiguity resolution. Nevertheless, clear seismic surface waves of the earthquake are recorded in our longdistance kinematic GPS solutions, which are in good agreement with response-corrected broadband seismic record. Our long baseline kinematic GPS solutions clearly indicated directivity of the seismic wave associated by rupture process of this earthquake. Also at the GPS stations that are 2,000 km away from the epicenter, dynamic displacements exceeding 5–10 cm were detected. In contrast, short baseline kinematic analysis shows large strain change caused by passage of surface wave, which reaches 6 × 10-6. Based on the comparison with seismometer and spectrum analysis of GPS results, it is difficult to discuss for very long time period displacement such as with a period more than 600 seconds in this study.

Local GPS tropospheric tomography

A formulation of local GPS tropospheric tomography for determining 4-D distribution of refractivity in the troposphere is presented together with a preliminary analysis of local dense GPS campaign data. Dividing the modeling space up to a height of 10 km above GPS receivers into cells, the refractivity in each cell is estimated in a successive time window by a tomographic reconstruction method in a quite similar manner like the seismic velocity in each cell in Earth’s interior is estimated in seismic tomography. The basic data for tomography are GPS slant delays for respective pairs of station and satellite, which are the sum of postfit phase residual, hydrostatic and wet slant delay. On the other hand, the slant delay from a station to a satellite is expressed by the summation of the product of path length and refractivity in each cell along the ray path. In a given time window, we have numerous observed slant delays corresponding to different ray trajectories, and the refractivity in each cell can be estimated by discrete inversion and least squares methods. The observational equations are usually singular so that we use a damped least squares method popular in seismic tomography. An example of real data analysis is given for the 1995 Shigaraki GPS campaign data, which reveals the spatial and temporal change of refractivity corresponding to the passage of ‘cold front’.