[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. Citation: Kato, A., et al. (2010), Variations of fluid pressure within the subducting oceanic crust and slow earthquakes, Geophys.

W e have applied an inverse method to P and S wave arrival time data observed at 52 seismic stations for 349 local earthquakes in order to estimate a three-dimensional (3-D) velocity structure beneath northern Japan. The method is characterized by a simultaneous determination of the two-dimensional depth distributions of velocity boundaries, 3-D velocity distribution and station corrections as well as hypocenter parameters of earthquakes. The Moho discontinuity under eastern Hokkaido is inclined from a depth of 32 km beneath the Hidaka Mountains to 20 km beneath the Konsen Plateau, while the thick crust with a thickness of 30-36 km distributes widely in western Hokkaido and the Tohoku region. The dip direction of the Moho in eastern Hokkaido is approximately perpendicular to the range of the Hidaka Mountains. The upper boundary of the subducting Pacific plate distributes in the depth range from 50 to 170 km in the surveyed area and the dip angle of the plate boundary in eastern Hokkaido (the Kurile arc) is slightly larger than that in the Tohoku region (the northeastern Japan arc).Estimated P (S) wave velocities are 5.8-7.0 km/s (3.4-4.1 km/s) in the crust, 7.4-8.0 km/s (4.2-4.5 km/s) in the upper mantle, and 8.1-8.6 km/s (4.6-4.9 km/s) in the plate, respectively. The velocities in the crust beneath northern Hokkaido are lower than those beneath eastern Hokkaido and the Tohoku region. The upper mantle has velocities which gradually increase with depth, and the subducted plate contains a high velocity zone with a P wave velocity faster than 8.3 km/s. VP/VS ratios derived from P and S wave velocities generally range from 1.75 to 1.80 in the crust and upper mantle, while the ratios in the high velocity zone within the plate are less than 1.75. These results suggest that the plate is composed of two layers: the first layer is a low-velocity and high-VP/VS zone with a thickness less than 20 km, and the second layer is a thick high-velocity and low VP/VS zone. The estimated plate thickness is about 90 km in the Kurile arc and about 110 km in the northeastern Japan arc.The relocated hypocenter distribution beneath Hokkaido clearly shows the double seismic plane in the subducting plate. The upper seismic plane is located in the first Received April 27, 1994; Accepted October 5, 1994 * To whom correspondence should be addressed . ** Present address: Faculty of Science , Kyushu University, Fukuoka 812, Japan. layer with a low velocity and the lower seismic plane is in the second layer with a high velocity. The seismic activity in the upper seismic plane is high in the depth range from 60 to 90 km, while that in the lower seismic plane is high at depths deeper than 100 km. The double seismic plane joins at depths from 90 to 130 km.

In 1999, a marine seismic structure survey was conducted in the northernmost Okinawa Trough. Outcrops of the acoustic basement through the sedimentary layers are seen in the obtained structures. Velocities of lower layer in upper crust are varying laterally from 5.7 to 6.2 km/s and thickness of 6.0‐km/s layer ranges from 3 to 9 km. The lower crust has uniform thickness of about 15 km. The Pn velocity is 7.7–7.8 km/s and the total crustal thickness is about 25 km including sedimentary layers. The structure is in a first approximation similar to that of island arcs. However, the large heterogeneity of upper crust can be interpreted as a result of crustal thinning. The low Pn velocity and the outcrops of the acoustic basement relate to igneous activity in this area. These results suggest that the northernmost part of the Okinawa Trough is in the incipient stage of a back arc rifting.

The crustal evolution in the central part of Hokkaido Island, Japan, has been dominated by a series of accretion and collision processes from the late Jurassic to the present. This region is divided into three main tectonic belts (the Sorachi‐Yezo, Hidaka and Tokoro belts from west to east) formed in different Cretaceous subduction systems. Since the Miocene, the Sorachi‐Yezo Belt and the western part of the Hidaka Belt (the Hidaka Metamorphic Belt, HMB) have suffered severe deformation during two tectonic events: the dextral oblique collision between the North American (Okhotsk) and Eurasian plates and the arc–arc collision of the Kuril forearc sliver with the western part of Hokkaido (the NE Japan Arc). A thrust belt along the western margin of the Sorachi‐Yezo Belt is considered to represent the Miocene to Pliocene eastward subduction of the NE Japan Arc due to the compressional stress regime. This subduction jumped to the eastern margin of the Japan Sea in Pleistocene time.  The reanalysis of wide‐angle seismic data collected in central Hokkaido has provided a new image of crustal‐scale structural variation and deformation in the tectonic environments mentioned above. The western part of the Sorachi‐Yezo Belt is characterized by a 2–4.5 km thick forearc sedimentary layer with a velocity of 3.0–3.9 km s−1, beneath which a 3.9–5.8 km s−1 body forms an eastward‐dipping boundary to the upper unit. This boundary may be the eastward extension of the thrust belt, representing the Miocene to Pliocene plate boundary. A 5.3–5.7 km s−1 body almost crops out beneath the eastern part of the Sorachi‐Yezo Belt where ophiolite sequences and high‐P metamorphic rocks are exposed. Our rather low velocity of 5.3–5.7 km s−1 may suggest that the original oceanic crustal rocks or the metamorphics were fragmented and merged with low‐velocity accretionary prism rocks. Plausible tectonic events responsible for this process are fast uplift of a large amount of underplated materials in the Cretaceous subduction system of this region and/or severe deformation by collision tectonics since the Miocene. The HMB is characterized by high‐T  metamorphic rocks almost equivalent to those of continental or arc crust. The seismic velocity in the upper 5–7 km of crust in this part increases westwards from 5.6 to 5.85 km s−1, showing a good correlation with the change in metamorphic grade of the exposed rocks. The velocity structure in the eastern part of the Hidaka Belt is composed of 15–20 km thick upper crust overlying 6.6–6.7 km s−1 lower crust. The crystalline part of the upper crust is divided into a 6–7 km thick 5.6–6.0 km s−1 layer and an 8.5–11 km thick 6.2–6.35 km s−1 layer. In the depth range 25–45 km, there exist crustal laminations, probably forming a ductile and sheared zone. Good agreement of this velocity structure with physical properties of the metamorphics exposed in the HMB indicates that a 25–26 km thick brittle part of the Hidaka crust was obducted towards the west to form the HMB under the collision framework in this region. M...

Abstract.A simultaneous inversion for threedimensional 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 regicn 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 30ø-34øN and the longitude range of 128ø-132øE. 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.