The physical properties of an active subduction megathrust

点击翻译按钮获取中文摘要

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.