Sadaomi Suzuki

Variations of fluid pressure within the subducting oceanic crust and slow earthquakes

[1] We show fine-scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high-pressure fluids released by progressive metamorphic dehydration reactions in the subducting Philippine Sea plate in Tokai district, Japan. These zones have a strong correlation with the distribution of slow earthquakes, including long-term slow slip (LTSS) and low-frequency earthquakes (LFEs). Overpressured fluids in the LTSS region appear to be trapped within the oceanic crust by an impermeable cap rock in the fore-arc, and impede intraslab earthquakes therein. In contrast, fluid pressures are reduced in the LFE zone, which is deeper than the centroid of the LTSS, because there fluids are able to infiltrate into the narrow corner of the mantle wedge, leading to mantle serpentinization. The combination of fluids released from the subducting oceanic crust with heterogeneous fluid transport properties in the hanging wall generates variations of fluid pressures along the downgoing plate boundary, which in turn control the occurrence of slow earthquakes.

Double seismic zone beneath the middle of Hokkaido, Japan, in the southwestern side of the Kurile Arc

Abstract The vertical section of microearthquakes, determined accurately by using the Hokkaido University network, shows two dipping zones (the double seismic zone) 25–30 km apart in the depth range of 80–150 km beneath the middle of Hokkaido in the southwestern side of the Kurile arc. Hypocentral distribution of large earthquakes ( m b > 4) based on the ISC (International Seismological Centre) bulletin also shows the double seismic zone beneath the same region. The hypocentral distribution indicates that the frequency of events occurring in the lower zone is four times greater than that in the upper zone. The difference in seismic activity between the two zones beneath Hokkaido is in contrast with the region beneath northeastern Honshu in the northeastern Japan arc. Composite focal mechanisms of microearthquakes and individual mechanisms of large events mainly characterize the down-dip extension for the lower zone as is observed beneath northeastern Honshu. For the upper zone, however, the stress field is rather complex and not necessarily similar to that beneath northeastern Honshu. This may be considered to indicate the influence of slab contortion or transformation in the Hokkaido corner between the Kurile and the northeastern Japan arcs.

Tomographic low‐velocity anomalies in the uppermost mantle around the northeastern edge of Okinawa Trough, the backarc of Kyushu

A simultaneous inversion for three-dimensional velocity structure and hypocentral parameters is used to determine the tomographic images of the backarc of Kyushu, southwestern Japan. The data used in the imaging are the first arrival times of 2736 local earthquakes recorded at 305 stations in a wide region from western Japan to eastern China. Additional data are provided by including arrival times of 357 teleseismic earthquakes. We got the high resolution of the uppermost mantle images for P waves at least in the depth range to 150 km, the latitude range of 30°–34°N and the longitude range of 128°–132°E. Results of inversion reveal a major zone of strong low velocity under west off Kyushu Island in the depth from 40 km to 100 km. This feature may result from partially melting in relation to the upwelling of hot mantle around the northeastern edge of the extending Okinawa trough.

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.

Unbending and horizontal fracture of the subducting Pacific plate, as evidenced by the 1993 Kushiro-oki and the 1981 and 1987 intermediate-depth earthquakes in Hokkaido

Abstract A large number of aftershocks of the 1993 Kushiro-oki earthquake ( M JMA = 7.8), 1987 Hidaka earthquake ( M JMA = 7.0), and 1981 Western Hidaka earthquake ( M JMA = 7.1) have been observed by the seismic network of Hokkaido University. These three earthquakes are the largest events that have occurred in the depth range from 100 km to 200 km under the Pacific side of Hokkaido for 109 years. They migrated from west to east in the last 12 years. Their focal mechanism solutions represent typical down-dip extension in the lower zone of the double seismic zone in the descending Pacific plate. We investigated aftershock distributions of the three intermediate-depth earthquakes to study physical properties of the descending slab. Aftershocks of the Kushiro-oki earthquake spread horizontally over an area of 50 km × 40 km. A vertical cross-section of this aftershock distribution shows that it extends from the lower seismic zone almost to the upper seismic zone. It shows a slightly concave shape of about 35 km in length. Aftershocks of the Hidaka and Western Hidaka earthquakes also extend horizontally over areas of 10 km × 10 km and 8 km × 12 km, respectively. These main shocks occurred in the lower seismic zone. The horizontal aftershock distributions of all three large earthquakes indicate that the horizontal nodal planes of the focal mechanisms, rather than the vertical nodal planes, are the fault planes. Such a characteristic suggests that the descending lithosphere under Hokkaido has been unbent macroscopically over the last 12 years. Unbending and the long horizontal faults of the intermediate-depth earthquakes may be caused by mantle flow in the asthenosphere beneath the Pacific plate. We suggest that the descending slab of about 100 km thickness constitutes two layers: a seismic, brittle upper layer and an aseismic, ductile lower layer. Small ruptures may tend to occur easily along both boundary surfaces of the upper layer under the action of forces, especially unbending, and form the double seismic zone.

Late Pleistocene crustal uplift and gravity anomaly in the eastern part of Kyushu, Japan, and its geophysical implications

The Miyazaki Plain, eastern part of Kyushu, Japan, is characterized by both significant negative gravity anomalies and aseismic crustal uplifting (∼1 mm/year) in the Late Pleistocene and Holocene. We examine the relationship between these two phenomena, which may provide important constraints on the interaction between the collision and/or subduction of the Kyushu-Palau Ridge and the forearc. We estimate the mass deficiency below 11-km depth by using the gravity anomalies and P-wave velocity structure of the upper crust. The onset of the load accumulation, 0.5–0.4 Ma, is inferred from the movement of the fluvial terraces considering the tephrochronology. The loading history is assumed to be a linear function of time. We evaluate the crustal rebound by assuming a viscoelastic plate deformation with an underplating load existing at 20- or 30-km depth. The predicted crustal movement for models with a lithospheric (crustal) viscosity of 1023–1024 Pa s can explain the observed altitudes of the shoreline of the marine terraces formed at the Last Interglacial of about 125 kyr BP and the middle Holocene of 5–6 kyr BP. Although we cannot restrict the origin of the buoyant body, the subduction of the Kyushu-Palau Ridge, remnant arc associated with back-arc opening of the Shikoku Basin, may be related to the buoyancy for the uplifting region examined here. On the other hand, the buoyant body off the Miyazaki Plain probably plays an important role in the interaction between the subducting oceanic slab and the overriding forearc crust. Thus, the observed lateral variation of the interplate coupling on the convergent boundary along the Nankai Trough may be attributed to the existence of the buoyant body.

A two-point, three-dimensional seismic ray tracing using genetic algorithms

Abstract Ray tracing between two fixed points is a boundary value problem. An initial path connecting two points, the source and the receiver, is algorithmically perturbed until it converges to a solution giving a minimum travel-time path. In multi-pathing cases, it is quite possible for an algorithm to converge to a ray path with local minimum travel time. Furthermore, when the velocity structure has discontinuities, there is another serious problem, i.e., how to determine the correct intersection between the ray and the surface of discontinuity. In this paper, a new approach for two-point ray tracing is presented, which uses genetic algorithms (GAs) to overcome these problems. Since GAs are efficient global search techniques, the proposed method guarantees to find a global minimum travel-time path and also the correct intersections. A micro-GA implementation is adopted to further enhance computational efficiency. This approach is suitable for tracing the first arriving seismic waves through a 3-D heterogeneous medium with discontinuities. The method can also find the minimum time paths of later arrivals, such as reflected and refracted waves, by constraining the ray to hit a prescribed interface. The accuracy of the method is demonstrated on the numerical examples.

Strong S-wave anisotropy in the aftershock region of the 2000 Tottori-ken Seibu, Japan, earthquake (Mw6.6)

The 2000 Tottori-ken Seibu earthquake (Mw6.6) occurred in Tottori prefecture, western Japan on October 6, 2000. We conducted aftershock observation and analyzed S-wave anisotropy using the aftershocks Mjma2.0 to 3.5 observed at two stations near the aftershock region. The fast S-waves are polarized to almost E-W direction, which corresponds to the P-axis direction of the mainshock. The delay time of the split S-waves ranges between 20 and 100 ms. The crack density inside and outside the aftershock area is estimated from the delay time. The distribution of crack density shows a strong spatial variation and depends on the ratio of the path length inside the aftershock region against the whole path length. Assuming a uniform distribution of anisotropy inside and outside the aftershock region, the crack densities inside and outside are 0.017 and 0.007, respectively. It thus seems probable that in the aftershock region the distribution of cracks is intensive and cracks are opened due to the presence of fluids in seismogenic layers. This considerable spatial variation of anisotropy between inside and outside the aftershock region suggests that the aftershock region may have different mechanical properties from the surrounding area.

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.