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1. Expedition 386 Preliminary Report: Japan Trench Paleoseismology

   Ikehara, K.; Strasser, M.; Everest, J.; Maeda, L.; Hochmuth, K. (September 2023, Preliminary report)

   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. 

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2. Expedition 386 summary

   Strasser, M.; Ikehara, K.; Everest, J.; Maeda, L.; Hochmuth, K.; Grant, H.; Stewart, M.; Okutsu, N.; Sakurai, N.; Yokoyama, T.; et al (November 2023, Proceedings of the International Ocean Discovery Program Expedition reports)

   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 entire 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 sub-marine 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 nonsteady-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. 

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3. Japan Trench Paleoseismology

   https://doi.org/10.14379/iodp.proc.386.2023  

   Strasser, M; Ikehara, K; Everest, J (November 2023, Proceedings of the International Ocean Discovery Program Expedition reports)

   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 entire 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 sub-marine 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 nonsteady-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. 

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4. Expedition 386 Scientific Prospectus: Japan Trench Paleoseismology

   https://doi.org/10.14379/iodp.sp.386.2019  

   Strasser, M.; Ikehara, K.; Cotterill, C. (October 2019, Scientific prospectus)

   null (Ed.)

   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. Examining prehistoric events preserved in the geological record is essential to reconstruct the long-term history of earthquakes and to deliver observational data that help to reduce epistemic uncertainties in seismic hazard assessment for long return periods. “Submarine paleoseismology” is a promising approach to investigate deposits from the deep sea, where earthquakes leave traces preserved in the stratigraphic succession. However, at present we lack the comprehensive data sets and long-term records that allow for conclusive distinctions between quality and completeness of the paleoseismic archives. Motivated by the mission to fill the gap in long-term records of giant (Mw 9 class) earthquakes, International Ocean Discovery Program (IODP) Expedition 386, Japan Trench Paleoseismology, aims at testing and developing submarine paleoseismology in the Japan Trench. We will implement a multicoring approach by Mission Specific Platform shallow subsurface (40 m) giant piston coring to recover the continuous upper Pleistocene to Holocene stratigraphic successions of trench-fill basins along an axis-parallel transect of the 7–8 km deep trench. The cores from 18 proposed primary (and/or 13 alternate) sites will be used for multimethod applications to characterize event deposits for which the detailed stratigraphic expressions and spatiotemporal distribution will be analyzed for proxy evidence of earthquakes. Sediment remobilization related to the 2011 Mw 9.0 Tohoku-Oki earthquake and the respective deposits are preserved in trench basins formed by flexural bending of the subducting Pacific plate. These basins are ideal study areas for testing event deposits for earthquake triggering because they are poorly connected for sediment transport from the shelf and experience high sedimentation rates and low benthos activity (and thus high preservation potential) in the hadal environment. Results from conventional coring covering the last ~1,500 y reveal good agreement between the sedimentary record and historical documents. Subbottom profile images are consistent with basin-fill successions of episodic muddy turbidite deposition and thus define clear targets for paleoseismologic investigations on longer timescales accessible only by IODP coring. We will apply, further refine, and implement new methods for establishing event stratigraphy in the deep sea and for recognizing giant versus smaller earthquakes versus other driving mechanisms. Our results can potentially produce a fascinating record that unravels an earthquake history that is 10–100 times longer than currently available information. This would contribute to a tremendous advance in the understanding of the recurrence pattern of giant earthquakes and earthquake-induced geohazards globally and provide new constraints on sediment and carbon flux of event-triggered sediment mobilization to a deep-sea trench and its influence on the hadal environment. 

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5. Surfi cial Sediment Entrainment AboveLarge Megathrust Ruptures:Experimental Study

   Seibert, C; McHugh, C M; Paola, C; Seeber, L; Tucker, J (December 2024, age.confex.com)

   The 2011 Mw9.0 Tohoku-Oki earthquake may be representative of “maximum”earthquakes: it ruptured the entire seismogenic depth range of the Japan megathrust, including the shallowest segment that reaches the trench where the displacement grew to 60 m and spawned a catastrophic tsunami. Models and direct seafloor measurements imply a comparably large initial relative motion and sustained long-period oscillations between sediment and water at the seafloor above the shallowest megathrust segment. This motion may develop enough shear to re-suspend sediment, but exclusively for the maximum earthquakes. This new co-seismic sediment-entrainment process should leave a recognizable sedimentary fingerprint of these earthquakes. Our physical experiments are testing effects of this shear between sediment and water and its interaction with high-frequency vertical shaking. We also investigate the impact of sediment properties and slope on the entrainment. We worked on several synthetic mixtures, defined according to the grain size distribution, clay mineralogy and water content with either freshwater or sea water. The grain size distribution is simplified but matches those of sediment cores from different subduction zones. For each mixture, we built matrices of the erosion rates according to the flow velocities, which shows the role of water content and vertical shaking. We have also identifi ed different mechanism during the runs:grain-by-grain or clasts entrainment, stripping, motion of the sediment interface, and formation of a dense sediment layer above the surface. These observations maybe recorded in the associated deposit, suggesting different fingerprinting by the tsunamigenic earthquakes depending on the characteristics of each subduction zone. 

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