Physical properties and reflection character of incoming sediments, Nankai, Japan

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The largest and most destructive earthquakes on Earth occur at subduction zones, where enormous stresses build up along the plate boundary fault between two convergent tectonic plates. The plate boundary fault defines the partition between sediment that is added to the overriding plate (the accretionary prism) and sediment that is subducted along with the down-going plate. The fault will form preferentially in relatively weak sediments that act to lubricate the sliding plane allowing slip without earthquakes. Further down-dip, the earthquake generating potential of this fault changes, moving from an aseismic (no earthquakes) zone to a seismic or seismogenic (earthquake) zone. The physical properties of the incoming sediments at subduction zones are therefore important in controlling subduction fault formation and the subsequent location and rupture pattern of earthquakes.The majority of ocean drilling at subduction zones focuses on sites over the actively deforming accretionary prism sediments as the seabed is shallower and there are many interesting structural targets. However, the incoming sediments represent one of the major controls on the evolution of the subduction system, controlling the location and strength of the initial plate boundary fault. Due to the inaccessibility of the incoming sediments (often in deep water), it is difficult to obtain samples and measure in situ physical properties. In addition drilling provides points of data where we know there may be considerable variation in sediment properties. Seismic reflection data allow the physical properties of the sediments to be measured by proxy and our interpretations extended well beyond the borehole sites. However this method requires a good calibration between seismic data characteristics and the physical properties determined from sample measurements and logging data. Hole C0012H is located on top of a knoll on the seafloor with relatively shallow water depths. The core-log-seismic correlation at this Site (using data acquired during Expedition 338) together with neighbouring Site C0011 can provide a way to understand variations in the physical properties of the input sediments at the Nankai subduction zone.This project will focus on the Nankai subduction zone, located south of Japan, where the Philippine Sea plate is being subducted beneath the Eurasian plate. The section of the Nankai subduction zone off the Kii Peninsula has been the focus of the NanTroSEIZE project since 2007 with 15 drill sites along the NanTroSEIZE transect and two dedicated 3D seismic volumes. There have been two large plate boundary earthquakes in the last century (Tonankai, 1944 and Nankaido, 1946). The incoming plate at this section of the subduction zone has intermediate relief and the sediments are predominantly hemipelagic (sediments deposited slowly out of the water column) with layers of volcanic sediments and terrestrially-derived sediment layers (turbidites). During the recent IODP Expedition 338, log data were collected at Hole C0012H, located on the knoll. The sonic log data that were collected (seismic wave speed within the sediments) are of particular interest as the wave speed is a function of the physical properties of the sediments and underlying basement rocks. We will use these data to derive and compare the sediment physical properties between the cores, logs and seismic data. Tying the sediment physical properties to seismic reflectivity will allow changes to be mapped where there is no direct sampling from drilling sites. This will provide key information about the physical properties of the incoming sediments at the deformation front and lead to an explanation of the inception of the plate boundary fault.