Tamao Sato

Three‐stage rupture process of the 28 December 1994 Sanriku‐Oki Earthquake

We investigated the rupture process of the 28 December 1994 Sanriku-Oki earthquake (Mw = 7.7) using broadband seismograms recorded at local distances. The earthquake rupture nucleated at the eastern end of the aftershock area. As it propagated towards the west, a major subevent occurred near the center of the aftershock area about 26 s after the initial rupture, releasing most of the seismic energy of this earthquake. About 24 s later, another subevent followed at the western end of the aftershock area, emitting large high-frequency waves. The whole rupture time is about 55 sec. The three-stage rupture process is very similar to the earlier-stage rupture process of the 1968 Tokachi-Oki earthquake (Mw = 8.2). The difference is that the rupture of the Tokachi-Oki earthquake further propagated towards the north in the following stage instead of terminating at the western end. The occurrence of the Sanriku-Oki earthquake is critical to the validity of the characteristic earthquake so far believed to exist in this part of the plate boundary.

Spatial distribution of intermediate-depth earthquakes with horizontal or vertical nodal planes beneath northeastern Japan

Abstract The 1993 Kushiro-oki (off-Kushiro) earthquake ( M JMA = 7.8) is one of the largest intermediate-depth earthquakes that have occurred in northeastern Japan. This earthquake is a notable event that has a nearly horizontal fault plane extended from the lower seismic plane of the double-planed seismic zone toward the upper seismic plane. We have examined the spatial distribution of focal mechanism solutions of deep and intermediate-depth earthquakes beneath northeastern Japan concerning this peculiar event. Analyses of 127 new solutions along with existing solutions confirm the previous results on essential features of their spatial distribution. The predominant types of solutions are down-dip compression in the upper plane except beneath eastern Hokkaido, and down-dip extension in the lower plane. P-Axes in the upper seismic plane and T-axes in the lower plane are in the direction of plate convergence. Beneath Hokkaido, however, T-axes show significant deviation from the direction of plate convergence. The stress axes beneath the junction between the northeastern Japan arc and the Kuril arc exhibit slight deviation from those in the adjacent areas, suggesting slab distortion beneath the junction. We next investigate the spatial distribution of events with horizontal nodal planes as candidates for events with similar focal mechanisms to the Kushiro-oki earthquake. The activity of earthquakes with horizontal nodal planes is definitely high in the lower plane, especially beneath Hokkaido. Many of them have similar focal mechanisms to the Kushiro-oki earthquake, indicating that the Kushiro-oki earthquake is not exceptional but is a typical event in the region. However, the fault planes of these events are variable in space; some events are known to have vertical fault planes. There are shallow and deep depth bounds and a southwestern boundary for the events with horizontal fault planes. These facts suggest that the horizontal faulting is probably caused by some regional stress concentration owing to the unbending and the distortion of the segmented slab at the junction.

Tomographic inversion for P wave velocity structure beneath the northeastern Japan arc using local and teleseismic data

We have determined three-dimensional P wave velocity structure up to a depth of 296 km beneath northern Tohoku, Honshu, Japan, by inverting arrival times of local and teleseismic events simultaneously. Velocities in the upper crustal layers correlate well with the surface geology and Bouguer gravity anomalies. Low-velocity zones are ubiquitous in the crust and the uppermost mantle beneath the Quarternary volcanic areas. A well-known feature of a pronounced low-velocity mantle wedge underlain by the dipping high-velocity (high-V) Pacific plate is generally recovered over the study area. Strong heterogeneity within the high-velocity Pacific plate is suggested by the presence of a pronounced low-velocity anomaly confined to the depths of 130-200 km. The decreased seismicity of intermediate-focus events may be correlated with the low-velocity zone. A two-dimensional inversion scheme was invoked to determine stably the average mantle structure perpendicular to the island arc. In the depths of 50-150 km, the inclined high-V zone has a thickness of 80-100 km with its top coinciding with the upper plane of the double-planed deep seismic zone. Below the depth of 200 km, the high-V zone is shifted toward the bottom of the 80-km-thick reference slab. In the intermediate-depth layer (150-200 km), the high-V zone is not clearly visible within the 80-km-thick reference slab.

Coulomb stress change for the normal-fault aftershocks triggered near the Japan Trench by the 2011 Mw 9.0 Tohoku-Oki earthquake

Coulomb stress triggering is examined using well-determined aftershock focal mechanisms and source models of the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake. We tested several slip distributions obtained by inverting onshore GPS-derived coseismic displacements under different a priori constraints on the initial fault parameters. The aftershock focal mechanisms are most consistent with the Coulomb stress change calculated for a slip distribution having a center of slip close to the trench. This demonstrates the capability of the Coulomb stress change to help constrain the slip distribution that is otherwise difficult to determine. Coulomb stress changes for normal-fault aftershocks near the Japan Trench are found to be strongly dependent on the slip on the shallow portion of the fault. This fact suggests the possibility that the slip on the shallow portion of the fault can be better constrained by combining information of the Coulomb stress change with other available data. The case of normal-fault aftershocks near some trench segment which are calculated to be negatively stressed shows such an example, suggesting that the actual slip on the shallow portion of the fault is larger than that inverted from GPS-derived coseismic displacements.

Stress due to the interseismic back slip and its relation with the focal mechanisms of earthquakes occurring in the Kuril and northeastern Japan arcs

Assuming a uniform and inhomogeneous distribution of back slip, based on a realistic configuration of the plate boundary that was newly determined from the distribution of earthquakes, we have investigated if the stress due to the interseismic back slip can explain the focal mechanisms of earthquakes occurring in the Kuril and northeastern Japan arcs. The inhomogeneous back-slip model was taken from the distribution of slip deficit estimated from GPS data collected in recent years. We examined the change in the Coulomb Failure Function (ΔCFF) for several target faults to evaluate the fitness of the models to the observed focal mechanisms. The results of ΔCFF are clearly different in the two back-slip models near the down-dip end of the locked zone. Despite this difference, the models can generally explain the earthquake focal mechanisms in the overriding plate as well as those on the megathrust plate boundary. However, they cannot explain the focal mechanisms of intermediate-depth earthquakes in the double-planed deep seismic zone. Neither can they fully explain the focal mechanisms with the pressure axis parallel to the trench that are observed at the junction of the Kuril and northeastern Japan arcs. Other tectonic processes, such as the transcurrent movement of the forearc sliver along the Kuril arc, may be involved in producing the stress anomaly.

Moment release budget at oblique convergence margin as revealed by the 2004 Sumatra-Andaman earthquake

Based on the model of oblique convergence, we predicted the slip rate on the megathrust plate boundary in the western Sunda arc and compared it with the slip distribution of the 2004 Mw 9.0 Sumatra-Andaman earthquake. The slip directions on the megathrust plate boundary, which are necessary for predicting the slip rate, were determined using the fault plane solutions of medium-sized earthquakes occurring for the last 30 years in the 2004 rupture zone. At long wavelengths the predicted slip rate correlates well with the latitudinal variation of the coseismic slip, suggesting that kinematics described by the model of oblique convergence operated over a long interseismic period and that the seismic coupling coefficient has been generally uniform along the strike of the rupture zone. Using the data for the same period of time, we estimated the total seismic moment released by the earthquakes on the Sumatra fault and its submarine continuation located on the backarc side of the Nicobar and Andaman islands. The ratio of the observed seismic moment to the seismic moment predicted from the model of oblique convergence is small. The residual may either be stored for generation of future earthquakes or was taken up by the fault creep at shallow depths.

The geodynamic context of the Latur (India) earthquake, 30 September 1993

Abstract In the past 30 years, six disastrous earthquakes have occurred in the Indian shield. These earthquakes were believed to be associated either with reservoir induced seismicity or with well known geological features. In spite of these earthquakes, the Indian shield was considered to be a stable shield. The Latur earthquake of 30 September 1993 (29 September 1993 UMT) was far from any geological features of reservoir that might explain its presence. The present analysis of recent seismicity in the Indian continent shows that the Latur earthquake is consistent with release of stress accumulated by interaction of the Indian plate with the Asian continent. The observed seismicity in the Indian shield appears to draw its energy from continuing interaction between the two plates and it might be triggered by subsurface hydrological fluctuations.

A feasibility test of CMT inversion using regional network of broad-band strong-motion seismographs for near-distance large earthquakes

When a large earthquake occurs in and around the Japanese Islands, we should immediately determine the size and the fault geometry of the earthquake to mitigate earthquake disasters. Broad-band, strong-motion data of near-distance large earthquakes are used to investigate the possibility of CMT inversion. We perform a moment tensor inversion using the data sets of the 1993 Hokkaido Nansei-oki earthquake (MJMA 7.8), the 1994 Hokkaido Toho-oki earthquake (MJMA 8.1) and the 1994 Sanriku Haruka-oki earthquake (MJMA 7.5). The accuracy of the solutions is examined by comparison with the solutions derived from global data sets, which reveals that the mechanism and moment magnitude are quite adequate for the purpose. Numerical experiments are made to analyze the influence of the source finiteness on the solutions. The numerical experiments suggest that the influence of the source finiteness on the mechanism retrieval is smaller than that on the moment retrieval. We suggest the installation of a network system of broad-band, strong-motion seismographs in the Japanese Islands.

A Source Model for Explaining the Predominant Directions of the Ground Motion Inferred from the Damages to Gravestones and Houses: ―1976年7月5日鬼首地震―

Modes and degrees of the damages caused by the Onikobe earthquake (M=4.9) of July 5, 1976 were investigated in special reference to the predominant direction of the ground motion. The directions of falling and slip of gravestones near the focal region were restricted to a narrow range of directions and these data were found useful for estimating the predominant direction of the ground motion. On the basis of the seismological data such as the hypocenter location, the focal mechanism and aftershock distribution, various dynamicsource models were tested if they could explain the predominant direction near the focal region. As a result, precise determination of the hypocenter location became possible, and the finally selected model explained the systematic pattern of the predominant directions observed near and around the focal region.