Toshikatsu Yoshii

A detailed cross-section of the deep seismic zone beneath northeastern Honshu, Japan

Abstract A cross-section of earthquakes located in northeastern Japan is presented by using pPdepths reported by the International Seismological Centre. Travel-time corrections for the water layer were used to recompute pP-depths of earthquakes located below the sea regions. Seven new focal-mechanism solutions, based on teleseismic and Japanese data, were determined for this region. The reconstructed cross-section shows a double seismic zone at intermediate depths of 80–150 km. Earthquakes located within the upper seismic plane are characterized by down-dip compression while those in the lower plane, located about 35 km below the other seismic plane, are characterized by down-dip extension. These observations suggest that, at these depths, stresses attributable to a simple “unbending” of a plate may contribute to the generation of earthquakes in addition to stresses generated by the gravitational sinking of the lithosphere. A detailed cross-section of shallow earthquakes in the same area between the trench and eastern coast of northeastern Honshu is presented along with focal-mechanism solutions. This cross-section delineates more clearly the seismic zones characterized by normal and low-angle thrust faulting.

Regionality of group velocities of Rayleigh waves in the Pacific and thickening of the plate

Abstract The relationship between group velocities of Rayleigh waves and the ocean-bottom age in the Pacific is examined. The Pacific basin is divided into four regions by isochrons determined from geomagnetic lineations. A significant change in group velocities of Rayleigh waves is obtained for these four regions by the use of the least-squares method from data for 27 paths in a period range 40–90 s. The present result and other geophysical observations strongly suggest the “thickening of the oceanic plate”, and are well explained by a simple plate-thickness/age relation l(km) = 7.49 t (m.y.) 1/2 inferred from the “mantle gravity anomaly”.

Multifractal Analysis of Earthquakes

Multifractal properties of the epicenter and hypocenter distribution and also of the energy distribution of earthquakes are studied for California, Japan, and Greece. The calculatedDq-q curves (the generalized dimension) indicate that the earthquake process is multifractal or heterogeneous in the fractal dimension. Japanese earthquakes are the most heterogeneous and Californian earthquakes are the least. Since the earthquake process is multifractal, a single value of the so-called fractal dimension is not sufficient to characterize the earthquake process. Studies of multifractal models of earthquakes are recommended. Temporal changes of theDq-q curve are also obtained for Californian and Japanese earthquakes. TheDq-q curve shows two distinctly different types in each region; the gentle type and the steep type. The steeptype corresponds to a strongly heterogeneous multifractal, which appears during seismically active periods when large earthquakes occur.Dq for smallq or negativeq is considerably more sensitive to the change in fractal structure of earthquakes thanDq forq≥2. We recommend use ofDq at smallq to detect the seismicity change in a local area.

Precise P and S wave velocity structures in the Kitakami Massif, Northern Honshu, Japan, from a seismic refraction experiment

The Kitakami massif, which is located in the eastern part of Northern Honshu, Japan, is composed of two geological units. The northern Kitakami terrane is characterized as a Jurassic accretionary complex, while the southern Kitakami terrane consists of pre-Silurian basement and Silurian-lower Cretaceous marine sediments. The boundary region of these two units, called the Hayachine tectonic belt (HTB), is composed of mafic to ultramafic rocks. The Kitakami massif experienced intense granitic intrusions in the Cretaceous. We present a detailed crustal structure model for the eastern part of the massif derived from an extensive seismic refraction experiment conducted on a 194-km N-S line. The uppermost crust is covered with a very thin (0.5–1 km) surface layer with a velocity of 3.1–5.4 km/s. The velocity structure below this layer shows remarkable lateral variation. In the northern Kitakami terrane the P wave velocity and Vp/Vs at the top of the basement are 5.85–5.95 km/s and 1.68–1.70, respectively. The seismic attenuation in this region is high (Qp = 150–200 and Qs = 70–100). In contrast, the uppermost crust in the southern Kitakami terrane is characterized by a high P wave velocity (6.05–6.15 km/s) and Vp/Vs (1.74–1.77). The Qp and Qs also show high values of 300–400 and 150–200, respectively. Such a structural difference persists to 14-to 16-km depth, at which the P wave velocity increases to 6.45 km/s. The low velocity and high attenuation in the northern Kitakami terrane represent a highly deformed structure of the accretionary complex. The high P wave velocity and Vp/Vs in the southern Kitakami terrane indicate the relatively mafic crustal composition, which may result from the fragment of the oceanic crust incorporated by the accretion process or the uplifting in the latest Jurassic-early Cretaceous. A midcrustal interface determined from wide-angle reflections shows an abrupt southward depth decrease from 25 to 20 km under the HTB. The P wave velocity and Vp/Vs between 14- and 16-km depth and the midcrustal interface are 6.45–6.55 km/s and 1.74–1.78, respectively. The Moho depth under the northern Kitakami terrane decreases southward from 34 to 32 km. In the southern Kitakami terrane the Moho dips slightly southward. The P wave velocity and the Vp/Vs ratio in the lower crust are 6.9–7.0 km/s and 1.75–1.76, respectively. The P wave velocity in the uppermost mantle is not well resolved but is probably less than 7.7 km/s. The S wave velocity derived from relatively clear Sn is 4.35–4.40 km/s. Our results show that the HTB is a prominent structural boundary extending to the Moho. The crust of Kitakami massif was not homogenized by the Cretaceous granitic intrusions, and the original structural difference remains in the upper crust.

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.

Seismological features of island arc crust as inferred from recent seismic expeditions in Japan

Abstract Crustal studies within the Japanese islands have provided important constraints on the physical properties and deformation styles of the island arc crust. The upper crust in the Japanese islands has a significant heterogeneity characterized by large velocity variation (5.5–6.1 km/s) and high seismic attenuation (Qp=100–400 for 5–15 Hz). The lateral velocity change sometimes occurs at major tectonic lines. In many cases of recent refraction/wide-angle reflection profiles, a “middle crust” with a velocity of 6.2–6.5 km/s is found in a depth range of 5–15 km. Most shallow microearthquakes are concentrated in the upper/middle crust. The velocity in the lower crust is estimated to be 6.6–7.0 km/s. The lower crust often involves a highly reflective zone with less seismicity, indicating its ductile rheology. The uppermost mantle is characterized by a low Pn velocity of 7.5–7.9 km/s. Several observations on PmP phase indicate that the Moho is not a sharp boundary with a distinct velocity contrast, but forms a transition zone from the upper mantle to the lower crust. Recent seismic reflection experiments revealed ongoing crustal deformations within the Japanese islands. A clear image of crustal delamination obtained for an arc–arc collision zone in central Hokkaido provides an important key for the evolution process from island arc to more felsic continental crust. In northern Honshu, a major fault system with listric geometry, which was formed by Miocene back arc spreading, was successfully mapped down to 12–15 km.

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.

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.