Seismic reflection profiles reveal steeply landward-dipping splay faults in the rupture area of the magnitude (M) 8.1 Tonankai earthquake in the Nankai subduction zone. These splay faults branch upward from the plate-boundary interface (that is, the subduction zone) at a depth of approximately 10 kilometers, approximately 50 to 55 kilometers landward of the trough axis, breaking through the upper crustal plate. Slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion.

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.

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[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.

[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.

Abstract. Near the Japan Trench convergent plate margin the seaward edge of the continental plate is deformed by subduction of the oceanic plate. We report the results of a multichannel seismic survey in the northern Japan Trench in which this deformed zone is demarcated from the rigid continental framework by a pronounced landward dipping reflector. The oceanic plate also undergoes deformation as the two plates interact in the subduction processes, resulting in a progressive deformation or destruction of a horst structure along the top of the subducting oceanic crust. This may eventually lead to the formation of a smooth plate boundary at the greater depth. More than 45 km landward from the trench axis, a smooth reflector suggesting a stable slip plane is visible along and above the oceanic crust. Our data indicate that the deformed zone pinches out landward --•60 km from the axis at 13 km depth and the slip plane becomes less obvious there. Seismicity of interplate earthquakes rapidly increases landward from this location, leading us to speculate that this is where coupling at the plate boundary becomes strong enough for earthquakes to occur. We conclude that the updip limit of the seismogenic zone of interplate earthquakes in the study area is characterized by the tectonic feature of a pinchout of the deformed sediments which overlie the subducting oceanic crust.

Abstract. The Nankai Trough, southwestern Japan, is recognized as a vigorous seismogenic zone with well-studied historic earthquakes. This paper presents results of a wide-angle ocean bottom seismographs (OBS) study at the western Nankai Trough seismogenic zone. The OBS data used were acquired on a profile (250 km long) across the presumed coseismic slip zone of the 1946 Nankaido earthquake (Ms=8.2). The main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake. The crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge. P wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities (Vp=5.0-5.6 km/s and 6.6-6.8 km/s in oceanic layers 2 and 3, respectively). The maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with Vp=3.4-4.6 km/s in the deeper part. The subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward: 3.2øand 7.2 ø at 0-50 km and 50-100 km from the trough axis, respectively. The oceanic crust is smooth to the hypocenter zone, down to 40 km depth beneath Shikoku Island. Our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust-island arc crust contact zone. Even though there is large uncertainty about the seaward limit of the coseismic slip zone, the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism.

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.