The physical properties of an active subduction megathrust

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.

This work aims to constrain fundamental aspects of earthquake dynamics on an active megathrust fault and determine how this might affect seismic hazard assessment. These aims have the potential to inform and benefit a wide range of end users. Potential beneficiaries can be divided into different groups:

1. Earthquake hazard scientists/policy advisors. The results from this research will be of direct interest to scientists and policymakers who want to understand the seismic hazard associated with megathrust faulting. Information on the conditions that could potentially allow seismic rupture will allow a re-assessment of risk maps in seismically active area. As this project is part of the wider NanTroSEIZE project, there is already an infrastructure in place for disseminating results in forums where scientists are actively incorporating new findings into probabalistic seismic hazard assessments that will be used to inform future policy regarding seismic hazard mapping and planning and building regulations.

2. Industry. The generation of seismicity on active faults is not only of interest to scientists working on hazards in subduction zones and other tectonic plate boundaries. Many industries have concerns over induced seismicity. These include gas production from unconventional resources, the injection of carbon dioxide for storage in oil and gas reservoirs, and the production of geothermal energy. Many of the lithologies that are exploited by these industries have similarities with accretionary wedge sediments that will be sampled during the expedition. Enhancing our understanding of rupture propagation through these rock-types will allow better planning of safe injection rates for shale gas, geothermal and carbon dioxide storage, likely magnitude of induced events, and long-term management of storage facilities. We will seek to apply the results of this research in the context of induced seismicity, addressing issues highlighted through cooperation with industry and academics working in this field. The PI already has links to oil companies (Cuadrilla, iGas) in the UK that are investigating hydraulic fracturing, and with geothermal (Geo Energie Suisse) in Switzerland.

3. General public. Large-scale earth processes such as earthquakes and tsunamis regularly appear in the news as they capture the public intrigue and imagination. They feed an interest in science and can encourage the next generation into careers that are essential for the future economic and environmental development of the UK and beyond. This research is part of the larger NanTroSEIZE project which has received regular media coverage and has been ongoing for the last 15 years. It already has an active website for the dissemination of findings and progress reports to the public (https://www.jamstec.go.jp/chikyu/e/nantroseize/index.html). This website will continue to be updated with results from the current expedition and provides the ideal platform for public engagement. There are also social media updates from the Chikyu and ECORD twitter pages.