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A new component to the drilling operations has been added to the International Ocean Discovery Program (IODP) Expedition 405 schedule following approval of an Ancillary Project Letter (1013-APL). The new operations involve deploying a borehole observatory into Hole C0019D with a temperature sensor string. This hole and observatory infrastructure (i.e., casing) was previously drilled as part of Integrated Ocean Drilling Program Expedition 343/343T in 2012 (Expedition 343/343T Scientists, 2013). Site C0019 is the same site as Site JTCT-01A, described as part of Expedition 405 in Kodaira et al. (2023). Installation of a new instrument string in Hole C0019D at the beginning of Expedition 405 operations will allow the passive observation of anticipated subsurface hydrologic effects caused by nearby drilling (e.g., Kinoshita and Saffer, 2018). Together, the new drilling around Site C0019/JTCT-01A and the resulting observatory temperature observations in Hole C0019D will constitute a series of cross-borehole experiments that enable the determination of large-scale hydrogeologic properties around the plate boundary fault and overlying damage zone. The site priorities and drilling and coring strategy at the primary sites for Expedition 405 (Sites JTCT-01A and JTCT-02A) remain unchanged from the original Expedition 405 Scientific Prospectus (Kodaira et al., 2023). 

Two decades of onshore-offshore, ocean bottom seismometer and marine multi-channel seismic data are integrated to constrain the crustal structure of the entire Hikurangi subduction zone. Our method provides refined 3-D constraints on the width and properties of the frontal prism, the thickness and geological architecture of the forearc crust, and the crustal structure and geometry of the subducting Hikurangi Plateau to 40 km depth. Our results reveal significant along-strike changes in the distribution of rigid crustal rocks in the overthrusting plate and along-strike changes in the crustal thickness of the subducting Hikurangi Plateau. We also provide regional constraints on seismic structure in the vicinity of the subduction interface. In this presentation, we will describe our observations and integrate our tomographic model with residual gravity anomalies, onshore geology, and geodetic observations to describe the relationship between crustal structure and fault-slip behavior along the Hikurangi margin. 

The 11 March 2011 M 9.0 Tohoku-oki earthquake was one of the largest earthquakes ever recorded and was accompanied by a devastating tsunami. Slip during the earthquake was exceptionally large at shallow depth on the plate boundary fault, which was one of the primary factors that contributed to the extreme tsunami amplitudes that inundated the coast of Japan. International Ocean Discovery Program Expedition 405 aims to investigate the conditions and processes that facilitated the extremely shallow slip on the subduction interface in the 2011 Tohoku-oki earthquake. Proposed work includes coring and logging operations at two sites in a transect across the trench. The first site, located within the overriding plate, will access the fault zone in the region of large shallow slip, targeting the plate boundary décollement, overlying frontal prism, and subducted units cut by the décollement. The second site, located on the Pacific plate, will access the undisturbed sedimentary and volcanic inputs to the subduction zone. A borehole observatory will be installed into the décollement and surrounding rocks to provide measurements of the temperature in and around the fault over the following several years. Sampling, geophysical logs, and the observatory temperature time series will document the compositional, structural, mechanical, and frictional properties of the rocks in the décollement and adjacent country rock, as well as the hydrogeologic structure and pore fluid pressure of the fault zone and frontal prism—key properties that influence the effective stress to facilitate earthquake slip and potential for large slip. Results from Expedition 405 will address fundamental questions about earthquake slip on subduction zones that may directly inform earthquake and tsunami hazard assessments around the world.