[M-GI28] 地球掘削科学

[MGI28-01] IODP Expedition 405, JTRACK: Tracking Tsunamigenic Slips Across and Along the Japan Trench

*小平 秀一¹、Sample James²、Strasser Michael³、氏家 恒太郎⁴、Kirkpatrick James⁵、Fulton Patrick⁶、Japanese proponent group (1.海洋研究開発機構、2.Northern Arizona University、3.University of Innsbruck、4.筑波大学、5.McGill University、6.Cornell University)

キーワード：2011 Tohoku-oki earthquake、Megathrust fault、IODP

IODP exp. 405, JTRACK, is scheduled for 2024. JTRACK was originally proposed as two 2-holes transects crossing regions of a large and a small fault slip by the 2011 Tohoku-oki earthquake, and is designed to be carried out by several short duration expeditions. Due to a logistical constraint, the CIB decided to carry out one drilling transect during the current IODP phase. A main objective of the project is to investigate spatial and temporal aspects of physical, hydrological, and chemical properties of the fault zone to elucidate key factors that can control large slip on the megathrust. These results may be used to understand the 2011 earthquake, healing process of the megathrust fault, and possibly other great subduction earthquakes world-wide. Planned drilling includes a variety of investigations targeting the plate boundary fault zone and associated structures. Geologic studies will focus on structure and physical properties, especially frictional characteristics for components of the input pelagic sediments, such as the abundant smectite recognized by JFAST. Hydrological and chemical effects in and around the fault zone are largely unknown, but likely contribute to earthquake processes. We plan analyses of interstitial water to evaluate the role of fluids during faulting, along with investigations of the local permeability structure. Near-fault measurements provided by geophysical logging and an observatory have the potential to provide unique insight into fault-zone healing/re-strengthening processes, hydrologically-controlled stress transients, and the hydrogeology of a fault that has hosted large slip to the trench. The strategy for the expedition consists of two holes across the Japan Trench in the large slip zone: an inner trench slope site targeting the plate boundary fault zone at the same site of JFAST, and an ‘input’ site seaward of the trench as a reference site. A borehole observatory is planned for deployment at the inner trench slope site.