[S-CG46] Uncovering stress accumulation and fault strengthening of megathrust earthquakes

[SCG46-P06] Depth-Temporal Strength profile of the shallow part of the Japan Trench plate boundary through analytical processing of drilling parameters

*濱田 洋平¹、Conin Marianne²、氏家 恒太郎³、Kirkpatrick Jamie⁴、小平 秀一¹、Regalla Christine⁵、Fulton Patrick⁶、江口 暢久¹、前田 玲奈¹、奥津 なつみ¹ (1.国立研究開発法人海洋研究開発機構、2.University of Lorraine、3.筑波大学、4.University of Nevada Reno、5.Nothern Arizona University、6.Cornell University)

キーワード：IODP、JTRACK、掘削パラメータ、掘削等価強度、プレート境界断層

Focusing on the objective of elucidating the particulars of the 2011 Tohoku earthquake, Integrated Ocean Drilling Program Expedition 343 “Japan Trench Fast Drilling Project (JFAST)”, conducted in 2012, successfully penetrated the shallow segment of the plate boundary fault, where a substantial slip of over 50 meters had occurred. This expedition accomplished the acquisition of LWD data, the recovery of core samples, and the installation of thermometers along the fault. The International Ocean Discovery Program (IODP) Expedition 405: Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK) was carried out in 2024 with the aim of investigating changes in the physical properties of the plate boundary fault and its vicinity, as well as fluid flow, at the site drilled by JFAST. The JTRACK project involved LWD, coring, observatory installation drilling, and the total 6 boreholes penetrated through the plate boundary fault. Additionally, two boreholes were drilled through the seafloor to the top of the chert layer of the input site on the Pacific Plate.
Drilling parameters were obtained for each hole and recorded at one-second intervals. These parameters are acquired on the rig floor and used to optimise drilling. In the case of JTRACK, these parameters include the number of rotations and torque when the drill pipe is rotated using the Top Drive system, as well as the position of the drill bit and the weight on the bit, which is calculated from the number of drill pipes connected. This data can be used as a proxy for the physical properties of the rock formations through classical data processing. In our presentation, the conversion of the drilling parameters obtained by JTRACK into depth-based data will be introduced, with the calculation of drilling-equivalent strength (EST) and the strength profiles of the Japan Trench Frontal Prism, plate boundary faults, and the subducting cherts and basalts. In addition to comparing the data with the structural and physical properties obtained on board, we will also compare the data with the drilling parameters obtained at JFAST, and discuss the changes in the strength of the shallow plate boundary between one and thirteen years after the earthquake.