Silent-slip events have been detected at several subduction zones, but the cause of these events is unknown. Using seismic imaging, we detected a cause of the Tokai silent slip, which occurred at a presumed fault zone of a great earthquake. The seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down-dip of a subducted ridge. We propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip.

[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.

[1] Differences in the coseismic rupture process between the 1944 Tonankai and the 1946 Nankai earthquakes have been studied by many fault models. To understand what factors control coseismic rupture zones, it is important to investigate differences in deep crustal structures of the rupture zones between the 1944 and 1946 earthquakes. The previously published crustal structure of the rupture zone of the 1946 earthquake shows that the coseismic rupture extends to the Neogene-Quaternary accretionary prism. However, little is known about the structure of the rupture zone of the 1944 earthquake. To obtain a complete image of the seismogenic zone of the 1944 earthquake, a wide-angle seismic survey was performed across the presumed coseismic rupture zone of the 1944 earthquake from ocean to land. Our model for the crustal structure is based on ocean bottom seismographic data. The crustal structure appears characteristic for subducting oceanic crust and a Neogene-Quaternary accretionary prism bounded by an island arc crust. The Neogene-Quaternary accretionary prism reaches a maximum thickness of 7 km at 50 km distance landward from the deformation front. The subducting oceanic crust can be traced down to 35 km. The subduction angle becomes steeper landward, reaching up to 11°beneath the island arc crust. The depth of the top of subducting oceanic crust at the downdip limit of the rupture zone is 23 km, while the updip limit is located beneath the island arc upper crust. Similar structures of the updip and downdip limits are also published for several other subduction zones.

Summary The Nankai Trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100–200 yr. The 1946 Nankaido earthquake is well known as an unusual event among these earthquakes, because the rupture zone estimated from long‐period geodetic data is more than twice as large as that derived from seismic wave data. In the summer of 1999, an onshore–offshore deep seismic survey was performed along a 355 km long profile in the western Nankai Trough seismogenic zone. Seismic signals both from an airgun array (207 l) and land explosions (maximum of 500 kg) were recorded simultaneously by 98 ocean‐bottom seismographs and 93 land seismic stations. Conventional 2‐D seismic reflection data were also acquired along part of the offshore profile. From the wide‐angle seismic data, we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth. The seismic velocity image also shows that the seamount is now colliding with the Japanese island arc crust. From this significant structure, this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake, i.e. the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount, and then the rupture was deflected around the subducted seamount at the down‐dip end of the locked zone between Cape Muroto and Cape Ashizuri. Another significant structure, a highly reflective layer, is obtained beneath Shikoku Island. A very slow P‐wave velocity (3 km s−1) is necessary in a thin layer at the base of the island arc crust in order to explain the observed high‐amplitude reflection phases. An area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer. This suggests a possible water layer at the base of the island arc crust. The water may be generated by dehydration of the downgoing probably partially serpentinized mantle, which is implied by a low P‐wave velocity (7.5 km s−1) beneath the subducted seamount. A locally observed non‐slip region during the 1946 earthquake at the eastern part of Shikoku Island is interpreted as a result of weak coupling at the possible water layer.

[1] The Seismic Array Hikurangi Experiment (SAHKE) investigated the structure of the forearc and subduction plate boundary beneath the southern North Island along a 350 km transect. Tomographic inversion of first-arrival travel times was used to derive a well-resolved 15-20 km deep P wave image of the crust. The refracted phases and migrated reflection events image subducting slab geometry and crustal structure. In the west, Australian Plate Moho depth decreases westward across the Taranaki Fault system from 35 to $28-30 km. In the east, subducted Pacific Plate oceanic crust is recognized to have a positive velocity gradient, but becomes less distinct beneath the Tararua Ranges, where the interface increases in dip at about 15 km depth from <5 to >15 . This bend in the subducted plate is associated with vertical clusters in seismicity, splay fault branching, and low-velocity high-attenuation material that we interpret to be an underplated subduction sedimentary channel. We infer that a step down in the decollement transfers slip on the plate interface at the top of a subduction channel to the oceanic crust and drives local uplift of the Tararua Ranges. Reflections from the Wairarapa Fault show that it is listric and soles into the top of underplated sediments, which in turn abut the Moho of the overriding plate at $32 km depth, near the downdip end of the strongly locked

Devastating earthquakes occur on a megathrust fault that underlies the Tokyo metropolitan region. We identify this fault with use of deep seismic reflection profiling to be the upper surface of the Philippine Sea plate. The depth to the top of this plate, 4 to 26 kilometers, is much shallower than previous estimates based on the distribution of seismicity. This shallower plate geometry changes the location of maximum finite slip of the 1923 Kanto earthquake and will affect estimations of strong ground motion for seismic hazards analysis within the Tokyo region.

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[1] We deployed a dense temporal seismic network in the source region of the 2004 mid-Niigata prefecture earthquake (thrust fault), Japan. A detailed velocity structure and accurate aftershock distributions were elucidated by inverting aftershock arrival times using double-difference tomography. A stress tensor inversion using the first-motion data was also conducted in order to investigate the stress field. The seismic velocities in the hanging wall above the main shock fault are lower than those in the footwall, with the velocity contrast extending to a depth of approximately 10 km. The aftershocks along the main shock rupture zone are distributed around the sharp boundary between the low-and high-velocity bodies. Furthermore, aftershocks associated with the largest aftershock appear to be aligned on a boundary between low-and high-velocity zones, in the footwall. The orientation of maximum principal stress (s 1 ) is consistent with the regional compressional strain rate axis inferred from GPS data, except in the southwestern side of the main shock hypocenter where the azimuth of s 1 rotates approximately 20°c ounterclockwise. The main shock hypocenter was located roughly at the transition zone where the structure of the hanging wall changes laterally and the azimuth of s 1 rotates. Heterogeneous structures of the seismic velocity and the stress field, combined with the ductile deformation of the upper crust, may have concentrated seismogenic stress around the hypocenter area to cause the complex distributions of aftershock sequence on structural boundaries.