Shuzo Asano

Simultaneous determination of the three-dimensional crustal structure and hypocenters beneath the Kanto—Tokai District, Japan

Abstract A new method is proposed for determining the two-dimensional depth-distributions to the Conrad and the Moho together with station corrections and source parameters from first arrival time data. In the present study, the velocity structure is modelled by a hyperbolic function (tanh) and the configuration of each interface is expressed in a series of Chebyshev functions with coefficients which are unknown parameters. A non-linear inversion method by Tarantola and Valette is applied for inversion. An approximate ray tracing method with sufficient accuracy for a complex region is adopted for computation of seismic ray paths and travel times. The method is applied to P-wave first arrival time data from local earthquakes and explosions gathered in the seismological network by the National Research Center for Disaster Prevention to reveal the crustal structure beneath the Kanto-Tokai District, Japan. The results obtained in this study are very interesting and show the usefulness of the present method for the study of the crustal structure. The distribution of station corrections is reasonable where there is information on the shallow structure. Low-velocity zones appear around Tokyo Bay and the Cape of Omae and high-velocity zones appear in the mountainous regions. The depth to the Conrad is about 12–16 km and its lateral change is gentle. While the lateral change in the depth to the Moho is comparatively large and there is a narrow region where the depth to the Moho exceeds 40 km. Except for this region, the central-mountainous region, the agreement of the crustal structure obtained in the present study with those by the previous refraction studies is fairly good. In comparing our results to fractional velocity perturbation in Layer 1 (0–32 km) derived with Aki and Lees method by Ishida and Hasemi, our work provides good information on the features causing the crustal velocity anomalies which have been unable to be constrained in Ishida and Hasemis work.

Collision structure in the upper crust beneath the southwestern foot of the Hidaka Mountains, Hokkaido, Japan as derived from explosion seismic observations

The P-wave velocity structure of the upper crust beneath a profile ranging from Niikappu to Samani in the southwestern foot of the Hidaka Mountains, Hokkaido, Japan was obtained through analysis of refraction and wide-angle reflection data. The mountains are characterized by high seismicity and a large gravity anomaly. The present profile crosses the source region of the 1982 Urakawa-oki earthquake (Ms 6.8). The length of the profile is 66 km striking northwest and southeast. Along the profile, 64 vertical geophones were set up and 5 shot points were chosen. For each shot, a 400–600 kg charge of dynamite was detonated. The studied area is composed of four major geological belts: Neogene sedimentary rocks, the Kamuikotan belt, the Yezo Group, and the Hidaka belt. The measurement line crosses these geological trend at an oblique angle. The structure obtained is characterized by remarkable velocity variations in the lateral direction and reflects the surface geological characteristics. A thin, high-velocity layer (HVL) was found between low-velocity materials in the central part of the profile, beneath the Kamuikotan Metamorphic Belt, at a depth ranging from 0.5 to 6 km, overthrusting toward the west on the low-velocity materials consisting of Neogene sedimentary rocks, and forming gentle folds. Outlines of the velocity structure of the Hidaka Mountains yielded by other studies have shown a large-scale overthrust structure associated with the collision of the Outer Kurile and the Outer Northern Honshu Arcs. The shallow velocity structure inferred by the present study showed a similar (although small scale) overthrust structure. The obtained structure shows that the composite tectonic force, comprising westward movement of the Outer Kurile Arc and northward movement of the Outer Northern Honshu Arc, plays an important role in the evolution of the tectonic features of the crust and upper mantle in a wide depth range beneath the Hidaka Mountains.

Anomalous crustal uplift and crustal structure in the Matsushiro and the Izu Peninsula earthquake swarm regions

Abstract A close relation between the anomalous crustal uplift and the crustal structure has been revealed by a comparison between the precise crustal structure deduced from explosion seismology and the anomalous crustal uplift observed by the precise leveling survey in the Matsushiro and the Izu Peninsula earthquake swarm regions. That is, the anomalous crustal uplift has occurred around the place where change in the crustal structure is present within a narrow zone. These two examples imply the importance of information on crustal structure towards understanding the real cause of crustal movement.

Crustal Structure in the Matsushiro Earthquake Swarm Area

The crustal structure of the Matsushiro area, Central Japan, was studied in two profiles, A and B, with the explosion seismic method to obtain a better understanding of the physical processes of the Matsushiro Swarm Earthquakes. The layer with a velocity of 6.0 km/sec is extremely shallow and becomes deeper west of Chikuma River and around the southeastern end of profile B; there exists a faultlike structure in the most active area. The comparison of hypocenter distributions with the crustal structure shows that almost all swarm earthquakes have their hypocenters below the 6.0 km/sec layer and are confined to the region where this 6.0 km/sec layer is shallow. The velocity gradient in the 6.0 km/ sec layer is determined with certainty by the time-term analysis. In the seismically most active region the anomalous structure is derived not only from the traveltime analysis but also from the amplitude studies; that is, the velocity and the Q-value are smaller than in other regions.