IAP-24-110

Giant megathrust earthquakes: structural and sedimentary controls on tsunamigenic shallow slip

Subduction zones host Earth’s largest and most destructive ‘megathrust’ earthquakes, which can exceed Mw 9. In some cases, these earthquakes are associated with significant shallow slip, where large displacement extends from the deep seismogenic zone to the seafloor near the trench, resulting in large tsunamis. During the 2011 Tohoku-Oki earthquake, shallow slip below the frontal prism exceeded 50 m (Fujiwara et al., 2011), causing a devastating, >40 m high tsunami that resulted in over 19,000 fatalities in Japan, and economic damage of hundreds of billions of dollars.

Investigating the reasons for shallow slip is one of the key goals of International Ocean Discovery Program (IODP) Expedition 405 (Kodaira et al., 2023), on which supervisors UN, AG and YN recently participated. The reasons for shallow slip are poorly understood, but it is likely facilitated by the presence of weak, smectite-rich clays that form the probable plate boundary in this area (Chester et al., 2013; Nakamura et al., 2013). These clays exhibit velocity-weakening behaviour that results in rapid fault propagation during the earthquake cycle (Ikari et al., 2015). New results from IODP expedition 405 will allow the spatial distribution of these clays to be mapped using seismic reflection data, enabling assessment of their lateral facies and thickness variability, and the likely consequences for large-segment earthquake failure. Earthquake dynamics are also likely impacted by the architecture of the subducting plate at the Japan Trench, which is characterised by large horst-and-graben structures inferred to cause net subduction erosion in this location. Structural variation may also affect slip behaviour, by segmenting the margin, or changing the local stress field in the prism. Structural changes, and their influence on the geometry of the plate boundary fault zone and frontal prism, can now be mapped on new multichannel seismic data in the region.

The clay-rich weak layer that is thought to control slip behaviour in the Japan Trench is not ubiquitous across the subduction zones of Japan. In the Nankai Trough, where a large magnitude earthquake (>8 Mw) is expected in the next ~30 years (Earthquake Research Committee, 2020), the subducting plate is much younger than in the Japan Trench, and subducting sediments are highly heterogeneous (Tilley et al., 2011). Despite extensive ocean drilling campaigns as part of Nankai Trough Seismogenic Experiment (NanTroSEIZE; https://www.jamstec.go.jp/chikyu/e/nantroseize/), the frictional properties and plate boundary architecture of the Nankai Trough subduction zone are not fully understood. The Ryukyu and Kuril trenches are yet more poorly understood, because of the greater sparsity of acquired data associated with such low population density settings. Island Arc settings produce much lower sediment flux than the Japan Trench and Nankai Trough, which likely impacts subduction dynamics (Heuret et al., 2012). However, large fault segments could still allow for tsunamigenic Mw >8.5 earthquakes in these settings, and the sedimentary controls, structural architecture and degree of erosion vs accretion in these areas are poorly understood.

This project will use new and legacy seismic reflection data and scientific ocean drilling data to address the fundamental controls on subduction dynamics in these settings, addressing the following specific questions:

1. How does the lateral change in facies and thickness of subducting weak layers correspond to distinct slip segments (Japan Trench, Kuril Trench)?
2. How does subducting plate structure and topography impact subduction erosion (including the thickness of the subduction channel) and strain in the accretionary prism?
3. How do sediment starved subduction zones (Kuril, Ryukyu) compare to sediment-rich areas (Nankai, Japan Trench), in terms of prism architecture and stress at the plate boundary?
4. What are the likely weak layers in the more poorly studied margins (Kuril, Ryukyu) and how do they influence shallow slip?

This project will use an extensive seismic reflection dataset collected by the Japanese Agency for Marine Science and Technology (JAMSTEC) over recent years, to constrain the structural architecture and sedimentary properties of the subducting plate, accretionary prisms and subduction channel/plate boundary faults, across four separate plate boundary segments in Japan – the Ryukyu trench, Nankai Trough, Japan Trench and Kuril Trench (Fig. X). These different plate boundary segments have contrasting sediment thickness and composition in the trench, which impacts subduction erosion vs accretion, prism architecture, and plate boundary frictional properties.

Seismic interpretation will include full stack 2D and 3D seismic data and seismic velocity data from JAMSTEC, as well as a rich database of ocean drilling boreholes recovered from across the region over recent decades, including Expedition 405 (drilling September-December 2024).

Year 1

Year 1 will include a detailed literature review of subduction dynamics and shallow slip, and previous studies offshore Japan; compilation of seismic and core data into a Petrel project; a research exchange visit with JAMSTEC in Japan; and a case study of the Japan Trench system integrating IODP Expedition 405 results. The timeline for this is as follows:

• September-December 2025: literature review
• January-February 2026: Build regional seismic and core database, initial interpretation
• March-April: visit Japan for training with JAMSTEC, attend Expedition 405 post-cruise meeting, and visit outcrops of exhumed plate-boundary faults.
• May-August: regional interpretation and core-log-seismic integration for the Japan Trench
• August: first year report (major review)

Year 2

Year 2 will include the integration of work on the Japan Trench and work on the second main case study in the Nankai Trough.

• September-December: draft chapter/paper on Japan Trench
• January-June: seismic interpretation in the Nankai Trough
• ~July: conference
• August: consolidate material and second year report

Year 3

Year 3 will include integration of work on the Nankai Trough, and work on the 3rd and 4th case studies in the Ryukyu and Kuril trenches.

• September-December: draft chapter on Nankai Trough
• January-June: seismic interpretation of sparse 2D data in Ryukyu and Kuril trenches
• ~July: conference
• August: consolidate material and second year report

Year 3.5

Year 3.5 will involve full integration of all the case studies in the final thesis to systematically address the research questions set out above.

The student will be trained in seismic interpretation, core-log-seismic integration, and velocity modelling, particularly using Petrel SLB software at HWU. They will receive additional geophysical training during an extended visit to project partners in JAMSTEC during the first year of the expedition. They will also benefit from in-house training in writing, coding and machine learning. The student will be part of a dynamic research group, with frequent opportunities to present their research to peers and supervisors, and receive constructive feedback to improve public speaking and writing.