The physical properties of an active subduction megathrust

俯冲带大地震是地球上最大和最具破坏性的地震，许多地震使海底破裂，并产生毁灭性的海啸，如2011年东北冲绳9.0级地震后产生的海啸。俯冲带也是一些最难进入的研究场所，因此我们对它们的结构和物理性质的了解非常有限。为了填补我们知识上的这一空白，IODP Expesion358 NantroSEIZE(南开海槽地震带实验)的目标是在2019年春季钻探至~5200Msf，并与1944年托南凯8.1级地震所在的俯冲巨型金字塔相交，沿途收集数据和岩心样本。该项目将通过进行巡航后的研究，使用从探险中收集的样本和船上数据，来表征断裂带材料的物理性质，从而阐明可能导致沿巨型逆冲断裂的地震破裂的条件。这项工作将利用利物浦大学岩石变形实验室的一系列高压变形设备来复制巨型断裂的现场条件，包括一种新的高压旋转剪切仪，它可以应用从微米/S到m/S的滑动速度(即地震滑动速度)。吸积楔主要由富含粘土的海底沉积物组成，这些沉积物是在俯冲过程中从下行板块上刮下来的。我们从缓慢滑动的实验室实验中了解到，地震预计不会在粘土丰富的断层上孕育，因为随着滑动开始加速，地震会加强，从而阻止任何潜在的破裂。在许多俯冲带的吸积前弧中，缺乏地震活动就说明了这一点。然而，据观察，地震确实周期性地在这些地区传播，包括在IODP Exp的主题--南开海槽。358级地震(例如1944年东南海8.1级地震)，以及最近发生在2011年东北冲9.0级地震附近的日本海沟，产生了有记录以来最大的同震滑动(约50米)，并引发了毁灭性的海啸。因此，通过直接从孕震深度的活动俯冲巨型逆冲中取样，重要的是描述断裂带物质的物理性质，以便阐明在通常预计会抑制破裂传播的物质中可能发生地震的条件。这项研究将使用利物浦最近开发的独特的实验室设备，这些设备可以复制地震期间的条件，使我们能够测量巨型逆冲断层材料在不同应力条件和滑动速度下的摩擦强度如何发展。它将允许在完全受限的条件下模拟摩擦滑动，大约深度可达15公里，从而实现与自然界中巨型金字塔原地条件相关的压力。在一组不同的实验中，将确定远离主要断层核心的吸积楔形物质的物理性质。这将包括强度、渗透率和地震速度。这些实验的结果将为更广泛的损伤区的行为提供洞察，在地震破裂期间，损伤区可以充当能量汇。

Subduction zone megathrust earthquakes are the largest and most destructive on Earth, with many rupturing the seafloor and generating devastating tsunamis such as those produced after the M9.0 Tohoku-Oki earthquake in 2011. Subduction zones are also some of the most inaccessible places to study and consequently our understanding of their structure and physical properties is very limited. To address this gap in our knowledge, IODP Expedition 358 NanTroSEIZE (the Nankai Trough SEIsmic Zone Experiment) aims to drill to ~5200 mbsf in Spring 2019 and intersect the subduction megathrust, which hosted the 1944 Tonankai M8.1 earthquake, collecting data and core samples along the way. This project will elucidate the conditions that could lead to earthquake rupture along megathrusts by conducting post-cruise research, using samples and shipboard data collected from the expedition, to characterize the physical properties of the fault zone materials. The work will utilize a range of high-pressure deformation apparatus at the Rock Deformation Laboratory, University of Liverpool, to replicate the in situ conditions of the megathrust, including a new high-pressure rotary shear apparatus that can apply slip velocities from microns/s to m/s (i.e. earthquake slip speeds). Accretionary wedges are primarily comprised of clay-rich seafloor sediments that are scraped off the down-going slab during subduction. We know from slow-slip laboratory experiments that earthquakes are not expected to nucleate on clay-rich faults as they strengthen as slip starts to accelerate, thereby arresting any potential rupture. This is illustrated by a lack of seismicity seen in accretionary forearcs of many subduction zones. However it is observed that earthquakes do periodically propagate through these regions including at the Nankai Trough, the subject of the IODP Exp. 358 (e.g. the 1944 Tonankai M8.1 earthquake), and more recently on the nearby Japan Trench where the M9.0 Tohoku-Oki earthquake in 2011 occurred, producing the largest co-seismic slip ever recorded (~50m) and generating a devastating tsunami. Therefore, by sampling directly from an active subduction megathrust at seismogenic depths, it is important to characterize the physical properties of the fault zone materials in order to elucidate the conditions where an earthquake might occur in materials that would typically be expected to inhibit rupture propagation. This research will use unique laboratory equipment recently developed at Liverpool that can replicate the conditions during earthquakes and allow us to measure how the frictional strength of the megathrust fault material develops under different stress conditions and sliding velocities. It will allow for frictional sliding to be simulated under fully confined conditions, approximating to up to 15km depth, thus achieving pressures associated with the in situ conditions of the megathrust in nature. In a different set of experiments the physical properties of the accretionary wedge material away from main fault core will be determined. This will include strength, permeability and seismic velocities. The results from these experiments will provide insights into behaviour of the wider damage zone which can act as energy sink during earthquake rupture.