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] At the Japan Trench convergent margin, many large interplate earthquakes of greater than M7.5 frequently occur. Their epicenters have uneven distribution, mostly located in the northern area. To investigate the relationship between this distribution and tectonic structures, we have conducted multichannel seismic surveys since 1996. Our data show two kinds of interplate sedimentary units: a wedge-shaped unit and a channel-like unit. Both units have a lower P wave velocity than the basal part of the overriding island arc crust. The wedge-shaped unit having a velocity of 2-3 km/s is widely distributed over the forearc region in the northern area. Its thickness decreases with depth, becoming several hundred meters at a depth of $12 km. The channel-like unit having a velocity of 3-4 km/s is observed in the southern area, extending in the downdip direction. Its thickness reaches $2 km at a depth of $12 km. If the low velocity of these units results from the existence of fluid, as many authors assume, the units being thick implies higher fluid content assuming constant porosity. Considering that fluid reduces basal friction and with an assumption that fluid available at a specific interface is proportional to the total fluid content in the sediment, the thickness variation of the units would cause different degrees of coupling at the plate boundary along the arc. This may provide one explanation for the regional disparity in the interplate earthquake occurrence in the margin. Furthermore, we attempt to call attention to the possibility that the channel-like sediment works as a shear stress releaser.

A new high-resolution velocity model of the Mariana arc-backarc system obtained from active-source seismic profi ling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafi c compositions. The characteristics of the oceanic-island-arc crust are a middle crust with velocity of ~6 km/s, laterally heterogeneous lower crust with velocities of ~7 km/s, and unusually low mantle velocities. Petrologic modeling suggests that the volume of the lower crust, composed of restites and olivine cumulates after the extraction of the middle crust, should be signifi cantly larger than is observed, suggesting that a part of the lower crust, especially the cumulates, is seismically a part of the mantle.

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] Recent seismic structural studies in trench-outer rise regions have shown that V p within the incoming oceanic plate systematically decreases toward the trench, probably owing to bending and fracturing of the plate. To understand the mechanisms acting to reduce V p , V s is critical because the V p /V s ratio is a sensitive indicator of lithology, porosity, and the presence of fluid. In the outer rise region of the Kuril trench, we conducted an extensive seismic refraction and reflection survey that revealed systematic changes in V p , V s , and V p /V s . Our results suggest that water content within the incoming oceanic plate increases toward the trench accompanied by the development of bending-related fractures at the top of the oceanic crust, consistent with the seawater percolation. Our results support the idea that plate bending and fracturing during the bending in the outer rise of the trench play an important role in the water cycle of subduction zones. Citation: Fujie, G.,

An extensive seismic survey using ocean‐bottom seismographs (OBS) was performed in the area across the Jan Mayen Basin, North Atlantic, from the Jan Mayen Ridge to the Iceland Plateau. The Jan Mayen Ridge and surrounding area are considered to be a fragment of a continent which was separated from Greenland just prior to magnetic anomaly 6. This study presents the crustal structure of the Jan Mayen microcontinent and the ocean/continent transition to the west of the Jan Mayen Ridge. The crustal structures from the centre of the Jan Mayen Ridge to the Jan Mayen Basin are characterized by a deep sedimentary basin, a thin basaltic layer within the sedimentary section and extreme thinning of the continental crust towards the Iceland Plateau. The OBS data indicate that a continental upper crust (V p=5.8–6.1 km s−1) and lower crust (V p=6.7–6.8 km s−1) underlie the deep sedimentary basin. The thickness of the continental lower crust varies significantly from 12 km beneath the Jan Mayen Ridge to almost zero thickness beneath the northwestern part of the Jan Mayen Basin. An ocean/continent transition zone is found at the western edge of the Jan Mayen Basin. Within the 10 km wide transition zone, crustal velocities increase towards the Iceland Plateau, and approach the velocities of the oceanic crust obtained at the Iceland Plateau, that is 3.8–5.1 km s−1 (oceanic layer 2A), 5.9–6.5 km s−1 (oceanic layer 2B) and 6.8–7.3 km s−1 (oceanic layer 3). The crustal model indicates very thin oceanic crust (5 km) immediately oceanwards of the ocean/continent transition zone. Beneath the Iceland Plateau, the oceanic crust is thicker (9 km) than the typical thickness of normal oceanic crust. This might imply that the oceanic crust at the Iceland Plateau has been generated by asthenospheric material slightly hotter than normal. From the crustal structure obtained by the present study, it is proposed that the western part of the Jan Mayen Ridge may be referred to as a non‐volcanic continental margin, generated by a long duration of rifting. Even if the asthenospheric material upwelling along the margin were hotter than normal, only small amounts of magmatic intrusions and extrusions would have been generated because of significant conductive cooling under the long duration of rifting.

A giant earthquake occasionally occurs in a subduction zone owing to a simultaneous rupture in adjacent segments which have been previously ruptured by large earthquakes. However, it is still unknown if a giant earthquake coincidentally occurs, or if there is a causal factor to control its generation. In this study we show a cause and a growth process of a giant earthquake which may occur along southwestern Japan, on the basis of seismic images obtained from wide‐angle seismic data and a numerical simulation incorporating the structural images. The wide‐angle seismic data were acquired along three trough parallel profiles crossing the rupture segmentation boundary between the 1944 Tonankai (moment magnitude Mw = 8.1) and the 1946 Nankai (Mw = 8.4) earthquakes. The seismic imaging detected a high seismic velocity body forming a strongly coupled patch at the segmentation boundary. The numerical simulation explained the historic rupture patterns and shows the occurrence of a giant earthquake along the entire Nankai trough, a distance of over 600 km long (Mw = 8.7). The growth process revealed from the simulated slip history in and around the strongly coupled patch is: (1) Prior to the giant earthquake, a small slow event (or earthquake) occurs near the segmentation boundary; (2) this accelerates a very slow slip (slower than the plate convergent rate), at the strong patch, which reduces a degree of coupling; and (3) then a rupture easily propagates through the strong patch when the next earthquake is nucleated near the segmentation boundary, consequently growing into a giant earthquake.

The 2011 moment magnitude 9.0 Tohoku-Oki earthquake produced a maximum coseismic slip of more than 50 meters near the Japan trench, which could result in a completely reduced stress state in the region. We tested this hypothesis by determining the in situ stress state of the frontal prism from boreholes drilled by the Integrated Ocean Drilling Program approximately 1 year after the earthquake and by inferring the pre-earthquake stress state. On the basis of the horizontal stress orientations and magnitudes estimated from borehole breakouts and the increase in coseismic displacement during propagation of the rupture to the trench axis, in situ horizontal stress decreased during the earthquake. The stress change suggests an active slip of the frontal plate interface, which is consistent with coseismic fault weakening and a nearly total stress drop.