This work is distributed under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. Contents 1 Background and objectives 3 Operations 7 Lithostratigraphy 21 Biostratigraphy 24 Paleomagnetism 27 Structural geology 33 Geochemistry 37 Physical properties 40 Downhole measurements 42 Logging while drilling 49 Core-log-seismic integration 56 Observatory 61 References

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This work is distributed under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. Contents 1 Background and objectives 3 Operations 6 Lithostratigraphy 12 Biostratigraphy 14 Paleomagnetism 16 Structural geology 19 Geochemistry 22 Physical properties 24 Downhole measurements 26 Logging while drilling 31 Core-log-seismic integration 38 Observatory 42 References

This work is distributed under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. Contents 1 Background and objectives 3 Operations 4 Lithostratigraphy 9 Biostratigraphy 11 Paleomagnetism 14 Structural geology 16 Geochemistry 18 Physical properties 20 Core-seismic integration 25 References L.M. Wallace et al. Site U1526 IODP Proceedings 3 Volume 372B/375 Operations Transit to Site U1526 The vessel arrived at Site U1526 at 1700 h (UTC + 12 h) on 22 April 2018. Upon arrival, the thrusters were lowered, and the dynamic positioning (DP) system was engaged. Hole U1526A A rotary core barrel (RCB) bottom-hole assembly (BHA) was assembled, and the drill string was lowered to the seafloor. Hole U1526A (39°1.3203ʹS, 179°14.7594ʹE; 2890.1 meters below sea level [mbsl]; Table T1) was spudded at 0150 h on 23 April 2018. Cores 1R-14R advanced from 0 to 83.6 meters below seafloor (mbsf) and recovered 29.26 m (35% recovery). Nonmagnetic core barrels were used for all cores. At the completion of coring, the drill bit was partially raised from the seafloor, and the ship returned to nearby Site U1520. Hole U1526B On 2 May 2018, we returned to Site U1526 following an 8 h transit from Site U1520 in DP mode. The ship was offset 20 m westnorthwest from Hole U1526A, and Hole U1526B (39°1.3146ʹS, 179°14.7481ʹE; 2888.4 mbsl) was spudded at 2015 h on 2 May. Advanced piston corer (APC) Cores 1H-4H and extended core barrel (XCB) Core 5X advanced from 0 to 33.5 mbsf and recovered 31.56 m (94% recovery). Nonmagnetic core barrels were used for all APC cores. The total time spent at Site U1526 was 2.88 days.

The empirical relationship between the porosity and P‐wave velocity is a useful tool for probing large‐scale underground physical properties and stress states based on P‐wave velocity structures acquired by seismic surveys. In this study, the porosity‐velocity curves were examined using local core sample and logging datasets acquired along the Kumano transect in the Nankai Trough, Southwest Japan. We tested Hashimoto et al.’s (2010) hypothesis that slope apron and accreted sediments have different relationships. Our advantage is using a large amount of logging and core data obtained at multiple sites in the Nankai Trough under various geological conditions, from the incoming oceanic plate to the inner wedge. We identified multiple types of porosity – P‐wave velocity relationships: 1) The first type agrees with the low‐velocity model in the global empirical relation of Erickson and Jarrard 1998), which is observed at lithostatic stress state condition (incoming upper Shikoku basin and slope sediments), 2) The second type consistent with high‐velocity models of the global empirical relation observed at compressive stress state condition (accreted sediments), which have a higher velocity and larger dependence on the porosity than the first type with the same porosity, and 3) The third type for incoming sediments with high smectite content has lower velocity than the first type. Based on our results, the transition between the first and second porosity – velocity relationships occurs in the prism toe, implying that compressive stress due to subduction controls acoustic properties and the lithification process of these sediments.