The physical properties of an active subduction megathrust

俯冲带大型逆冲地震是地球上最大、最具破坏性的地震，许多地震会导致海底破裂，并引发毁灭性的海啸，比如2011年日本东北大地震（M9.0）后产生的海啸。俯冲带也是最难研究的地方之一，因此我们对其结构和物理性质的了解非常有限。为了解决这一知识空白，IODP 358 NanTroSEIZE（南开海槽地震带实验）计划于2019年春季钻探至~5200 mbsf，并穿越1944年Tonankai M8.1地震的俯冲大逆冲带，沿途收集数据和岩心样本。该项目将通过进行巡航后研究，利用从探险中收集的样本和船上数据，阐明可能导致大型逆冲地震破裂的条件，以表征断层带材料的物理特性。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逆冲断层的原位条件，包括一个新的高压旋转剪切设备，可以应用从微米/秒到米/秒的滑动速度（即地震滑动速度）。增生楔主要由富含粘土的海底沉积物组成，这些沉积物在俯冲过程中从下沉的板块上刮下来。我们从缓慢滑动的实验室实验中得知，地震不会在富含粘土的断层上形成核，因为随着滑动开始加速，它们会加强，从而阻止任何潜在的破裂。这可以从在许多俯冲带的增生前带中看到的地震活动的缺乏来说明。然而，据观察，地震确实周期性地在这些地区传播，包括在南开海槽，IODP Exp. 358的主题（例如1944年的Tonankai 8.1级地震），以及最近在日本海沟附近，2011年发生了9.0级东北- oki地震，产生了有史以来最大的同震滑动（约50米），并产生了毁灭性的海啸。因此，通过直接从地震发震深度的活动俯冲大逆冲断层中取样，重要的是表征断裂带材料的物理性质，以便阐明地震可能发生在通常预期会抑制破裂传播的材料中的条件。这项研究将使用利物浦最近开发的独特实验室设备，该设备可以复制地震期间的条件，并允许我们测量大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。它将允许在完全受限的条件下模拟摩擦滑动，接近高达15公里的深度，从而获得与大型逆冲断层的原位条件相关的压力。在另一组实验中，将确定远离主断层核心的吸积楔材料的物理性质。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破坏带的行为提供见解，这些破坏带可以在地震破裂期间充当能量汇。

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.