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

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.

This project will make significant scientific advancement towards understanding how the properties of subducting sediments control the localisation of the plate boundary fault (décollement). This project will benefit the expansive research community engaged in researching the Nankai subduction zone and the subduction zone research community more generally. However, the results are also of importance for our understanding of seismic slip in subduction zones and hence for seismic and tsunami hazard assessment. This subject is therefore of great public interest, and engagement with the general public, and schools in particular, will emphasise the significance of our results and how they will be used. Benefits are outlined below.

Benefits to: IODP and related Hazard Assessment
The proposed research will contribute to addressing a major objective in the new science plan for IODP; 'What mechanisms control the occurrence of destructive earthquakes, landslides, and tsunami?'. A key objective of the IODP NanTroSEIZE project is to understand 'how water and rock interact in subduction zones to influence earthquake occurrence' and this includes characterising the properties of incoming sediment that ultimately influences fault slip in the seismogenic zone. Our results will emphasize IODP's position at the forefront of pioneering high impact, hazard assessment science as well as directly contributing to hazard assessment itself.

Benefits to: Geohazard Assessment
The research undertaken in this grant will be of significance to natural hazard assessment and mitigation agencies in Japan. More generally, the results are relevant to economies and governments worldwide due to the potential global impact of large (M8-9) magnitude earthquakes, particularly those close to major economic centres, e.g., Wellington. Refinement of the core-log-seismic integration method will also be of relevance to hazard assessment along the Lesser Antilles, Cascadia, Hikurangi and other convergent margins, because the synthesis of our results will generate new techniques for determination of seismic and tsunami potential.

Benefits to: Public
The technical abilities of scientific ocean drilling, sub-seafloor geology and the geophysical investigation of deep-sea sediments make this Earth science subject of great interest to the general public, especially school children. This research will help to address important questions relating to the spatial variability of sediment properties and the parameters that control the location of large and great subduction zone earthquakes, and hence directly impact how we assess seismic and tsunami hazard at subduction zones.

How does the proposed research generate impact?
The NanTroSEIZE drilling transect is the most intensively studied subduction zone system because it's recent and historical seismic potential is well documented and large earthquakes are known to occur frequently along the margin. Understanding how the physical properties of incoming sediments control subduction zone seismogenesis in this region and how these properties can be tied to seismic data and mapped spatially on a regional scale will allow for a greater understanding of the global occurrence of large earthquakes and tsunami.

What will be done (pathway to impact)?
To ensure that our research benefits other academic researchers, and to meet the publication obligations to IODP, our proposed research will be published in peer reviewed journals within five years of sailing on IODP Expedition 338. We will engage, inform, and inspire the wider public with our research results and their implications through participation in University Open Day events, the NOCS Geology and Geophysics Group webpages, IODP and University of Southampton press releases, and publishing results in articles accessible to the wider public. In addition we will engage with local schools in developing resources for educating the younger generation.