International Ocean Discovery Program Expedition 386 Preliminary Report

Japan Trench Paleoseismology¹

Offshore expedition 13 April–1 June 2021

Onshore Science Party 14 February–15 March 2022

Personal Sampling Party 13 November–30 November 2022

Ken Ikehara, Michael Strasser, Jeremy Everest, Lena Maeda, Katharina Hochmuth, and the Expedition 386 Scientists

¹Ikehara, K., Strasser, M., Everest, J., Maeda, L., Hochmuth, K., and the Expedition 386 Scientists, 2023. Expedition 386 Preliminary Report: Japan Trench Paleoseismology. International Ocean Discovery Program. https://doi.org/10.14379/iodp.pr.386.2023

Abstract

Short historical and even shorter instrumental records limit our perspective of earthquake maximum magnitude and recurrence and thus are inadequate to fully characterize Earth’s complex and multiscale seismic behavior and its consequences. Motivated by the mission to fill the gap in long-term paleoseismic records of giant (Mw 9 class) subduction zone earthquakes, such as the Tohoku-Oki earthquake in 2011, International Ocean Discovery Program Expedition 386 successfully collected 29 giant piston cores at 15 sites (total core recovery = 831.19 m), recovering up to 37.82 m long, continuous, upper Pleistocene to Holocene stratigraphic successions of 11 individual trench-fill basins that are expected to have recorded past earthquakes. Preliminary expedition results document event-stratigraphic successions comprising numerous event deposits and initially characterize their different types, facies, properties, composition, and frequency of occurrence, which show spatial variations across the southern, central, and northern Japan Trench.

The occurrence of several tephra beds, radiolarian biostratigraphic events, and characteristic variations of paleomagnetic declination and inclination that probably represent paleomagnetic secular variation reveal high potential for establishing robust age models in all parts of the Japan Trench. The central Japan Trench models are most likely to cover the longest timescales, with expected age ranges reaching back to ~24 ka. Together, these preliminary initial results indicate that the applied concept and strategy of multisite coring will likely be successful to test and further develop submarine paleoseismology to extract megathrust earthquake signals from event-stratigraphic sequences preserved in the sedimentary record. Obtained data and samples will now be examined using postexpedition multimethod applications to comprehensively characterize and date event deposits. Detailed work will include detailed characterization of the sedimentologic, physical, and (bio-)geochemical features; stratigraphic expressions of relationships; and spatiotemporal distribution of event beds. These will be analyzed as foundational proxy evidence for distinguishing giant earthquakes from smaller earthquakes and aseismic processes driving mechanisms to ultimately develop a long-term record of giant earthquakes. Furthermore, Expedition 386 achievements comprise the first ever high temporal and high spatial resolution subsurface investigation and sampling in a hadal oceanic trench, which are the deepest and least explored environments on our planet. Preliminary initial results show high total organic carbon content and downcore pore water and headspace gas profiles with characteristic changes related to organic matter degradation. In combination, these are suggestive of the occurrence of intensive remineralization and reveal evidence of non-steady state behavior. Together with the successful offshore sampling for microbiology postexpedition analyses and research, this provides exciting new perspectives to advance our understanding of deep-sea elemental cycles and their influence on hadal environments.