Provenance of sediments from IODP Expedition 358, (NanTroSEIZE Deep Riser Drilling: Nankai Seismogenic / Slow Slip Megathrust)

Subduction zones, where one plate is forced beneath another, are the locations of the largest-magnitude earthquakes. Typically, these earthquakes occur on faults that are subsea, therefore there is also a hazard due to tsunamis. Recent examples include the Tohoku earthquake in 2011 offshore Japan, in 2004 offshore Sumatra, and in 2016 in southern Chile. Such tsunamigenic earthquakes are caused by slip events along the upper part of the subducting plate in the so-called "seismogenic zone". The material properties of the incoming plate and its sediment cover, referred to as "subduction inputs", play a critical role in the generation of tsunamigenic earthquakes. Therefore, drilling into and instrumenting an active interplate seismogenic zone is a high priority for understanding seismogenic earthquakes.

Along SW Japan, the Philippine Sea Plate subducts NW beneath the Eurasian Plate, forming the Nankai subduction zone - one of the best convergent-plate margins to study such zones. The 1944 Tonankai earthquake, the presence of a cluster of a low-frequency seismic events, and the first tectonic tremor recorded in the accretionary prism, show that the area is very active seismically. The NanTroSEIZE is a multi-expedition drilling project built to improve understanding and characterisation of fault architecture, and fault and wall rock composition throughout an active subduction megathrust zone. These expeditions undertaken in the last 10 years, have resulted in >45 holes being drilled in 13 sites, with direct sampling, in-situ measurements, and long-term borehole monitoring data.

The IODP Expedition 358 focusses on the deeper part of Site C0002 (Kumano forearc basin) and targets the penetration of the high-amplitude seismic reflector believed to be the main plate boundary fault at ~ 5,000 metres below sea floor (mbsf). Previous expeditions 338 and 348 have already drilled at Site C0002 to 3,058.5 mbsf with casing installed in that hole to 2,922.5 mbsf. These expeditions show that the Kumano forearc basin is mainly infilled by Quaternary deep-marine sediments (e.g., turbidites and hemipelagic muds 1), and that the upper part of the accretionary-prism sediments (~2,000- 3,000 mbsf) are bedded and well-lithified mudstones and sandstones, variably deformed and cemented.
Results from previous IODP expeditions (322 & 333) in the Shikoku Basin suggest very oblique subduction (possibly associated with sinistral strike slip) took place during the late Miocene, such that terrigeneous sands may have been supplied to what is now part of the Nankai accretionary prism offshore Honshu-Shikoku from a sediment source in the East China Sea.

Lithostratigraphy exerts a first-order influence over the material properties and tectonic behaviour of subduction zones. The 3-D location and proportion of coarse- versus fine-grained sedimentary environments, e.g., submarine channels or lobes, are appreciated among geotechnical engineers and geologists to affect coefficients of friction and permeability of the subduction inputs. This proposed research will contribute to the analysis of the sediment and their provenance in the lower-part of the accretionary prism (>3,000 mbsf), and help in understanding the plate-tectonic evolution of SW Japan, e.g., helping to constrain the hypothesis that oblique subduction associated with sinistral strike slip may have been important the western parts of the late Miocene Nankai subduction system such that early sediment routing to the Nankai system was from the East China Sea.

See document uploaded "Pathways To Impact"