Estimation of in-situ rock strength of input materials and prism sediments at the Hikurangi Margin

At the Hikurangi Margin offshore New Zealand, where the Pacific plate subducts underneath the Indo-Australian plate, recurring slow slip events (SSEs) have been observed. The International Ocean Discovery Program (IODP) Expeditions 372 and 375 explored the region where SSEs have occurred through drilling, coring, logging, and the installation of observatories. We have analyzed drilling parameters to investigate the rock strength at two sites located in the frontal thrust region (Site U1518) and on the incoming Pacific plate (Site U1520). Site U1518 drilled through one of the frontal thrust faults, the Papaku Fault at ~300 meters below the seafloor (mbsf) and recovered samples from both hanging wall and footwall as well as the fault zone. For Site U1520 located ~25 km seaward from Site U1518, we focus on the shallower portion (above 500 mbsf) composed of hemipelagic sediments, similar to the lithology at Site U1518. The drilling parameters including bit depth, weight on bit, torque on bit, and rotation speed were recorded every 1 second during drilling operation. We first extracted data where the weight on bit was applied, calculated the averaged values using a time-based moving average over 10 s (U1518) and 100 s (U1520), and then extracted the data only when the depth was advancing. From this dataset, we calculated the rate of penetration over a 1-m interval. We plotted the rate of penetration as a function of torque on bit (performance curve) for each 10-m interval to investigate how the formation strength evolves with depth at each site.The drilling performance analyses indicate that the prism sediments found in the hanging wall of the Papaku Fault at Site U1518 have greater strength than its footwall sediments and also the incoming sediments at Site U1520. The contrast in the rock strength across the fault as well as two sites reflects differences in loading and compaction histories; the prism sediments particularly in the hanging wall of the Papaku Fault have undergone more compaction through tectonic loading. Although the drilling performance analyses can reveal the relative strength change, quantification of the in-situ rock strength requires some calibration. We plan to conduct triaxial deformation experiments on hemipelagic materials.