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

地球上最大、最具破坏性的地震发生在俯冲带，在那里，沿两个汇聚的构造板块之间的板块边界断层积聚了巨大的应力。板块边界断层定义了上覆板块（增生棱镜）上的沉积物和随下行板块俯冲的沉积物之间的划分。断层将优先在相对较弱的沉积物中形成，这些沉积物起到润滑滑动面的作用，使滑动不致发生地震。再往下，这个断层产生地震的潜力就会发生变化，从地震带（无地震）变为地震带或发震带（地震）。因此，俯冲带进入的沉积物的物理性质对于控制俯冲断层的形成以及随后地震的位置和破裂模式非常重要。在俯冲带的大多数海洋钻探都集中在活跃变形的增生棱镜沉积物上，因为海底较浅，而且有许多有趣的构造目标。然而，进入的沉积物是俯冲系统演化的主要控制因素之一，控制着初始板块边界断层的位置和强度。由于进入的沉积物（通常在深水中）难以接近，很难获得样品和测量原位物理性质。此外，钻探提供了数据点，我们知道沉积物性质可能存在相当大的变化。地震反射数据允许通过代理测量沉积物的物理性质，我们的解释远远超出了钻孔地点。然而，这种方法需要在地震数据特征与样品测量和测井数据确定的物理性质之间进行良好的校准。C0012H孔位于海底小丘顶部，水深相对较浅。该站点的岩心-测井-地震对比（使用338考察期间获得的数据）与邻近的C0011站点可以提供一种了解南开俯冲带输入沉积物物理性质变化的方法。该项目将重点研究位于日本南部的南开俯冲带，菲律宾海板块在这里被俯冲到欧亚板块之下。自2007年以来，Kii半岛附近的南开俯冲带一直是NanTroSEIZE项目的重点，沿着NanTroSEIZE样带有15个钻探点和两个专用的3D地震体。上个世纪发生过两次大的板块边界地震（1944年的托南海和1946年的南海道）。在俯冲带的这一段进入的板块有中等起伏，沉积物主要是半深海的（缓慢地从水柱中沉积的沉积物），有火山沉积层和陆源沉积层（浊积岩）。在最近的IODP 338考察中，在位于小丘上的C0012H孔收集了测井数据。所收集的声波测井数据（沉积物中的地震波速度）是特别有趣的，因为地震波速度是沉积物和下伏基底岩石物理性质的函数。我们将利用这些数据推导和比较岩心、测井和地震数据之间的沉积物物理性质。将沉积物的物理性质与地震反射率联系起来，可以在没有直接从钻井现场采样的情况下绘制变化图。这将提供关于在变形前沿进入的沉积物的物理性质的关键信息，并导致对板块边界断层开始的解释。

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.