Recent seismic reflection and ocean bottom seismometer (OBS) studies reveal broad regions of low seismic velocity along the Nankai subduction plate boundary megathrust offshore SW Japan. These low velocity zones (LVZ's) extend ∼55 km landward from the trench, corresponding to depths of >∼10 km below sea floor. Here, we estimate the in‐situ pore pressure and stress state within these LVZ's by combining P‐wave velocities obtained from the geophysical surveys with new well‐constrained empirical relations between P‐wave velocity, porosity, and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment. We document excess pore pressures of 17–87 MPa that extend ∼55 km into the subduction zone, indicating that trapped pore fluids support ∼45–91% of the overburden stress along the base of the upper plate and surrounding major fault zones. The resulting effective stresses in the LVZ are limited to ∼1/3 of the values expected for non‐overpressured conditions. These low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary. The region of lowest effective stress coincides with precisely located very low frequency earthquakes, providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure.

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.

[1] To understand the frictional behavior of natural faults at seismic slip rates, high-speed rotary shear experiments were conducted on disaggregated ultracataclasite from the Punchbowl fault. The experimental gouge layers were sheared at normal stresses of 0.2-1.3 MPa and velocities of 0.1-1.3 m/s to total displacements of 1.3-84 m. We employ thermomechanical FEM models and microstructural observations to consider spatial and temporal variation of normal stress and temperature in the samples and understand microprocesses. Four distinct gouge units form during shear. A slightly sheared starting material (Unit 1) and a strongly sheared and foliated gouge (Unit 2) are produced when frictional heating is insignificant and the coefficient of sliding friction is 0.4-0.6. A random fabric gouge with rounded prophyroclasts (Unit 3) and an extremely fine, microfoliated layer (Unit 4) develop when significant frictional heating occurs at greater velocity and normal stress, and the coefficient of sliding friction drops to approximately 0.2. Unit 3 forms at the critical temperature for vaporization of water and is associated with localization of slip to Unit 4 and elevation of temperature. The critical displacement for dynamic weakening in the rotary configuration can be understood as a consequence of the progressive inward migration of the friction-generated thermal front and the weaker localized slip surface and associated fluidized zone.Citation: Kitajima, H., J. S. Chester, F. M. Chester, and T. Shimamoto (2010), High-speed friction of disaggregated ultracataclasite in rotary shear: Characterization of frictional heating, mechanical behavior, and microstructure evolution,

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

Geophysical observations show spatial and temporal variations in fault slip style on shallow subduction thrust faults, but geological signatures and underlying deformation processes remain poorly understood. International Ocean Discovery Program (IODP) Expeditions 372 and 375 investigated New Zealand’s Hikurangi margin in a region that has experienced both tsunami earthquakes and repeated slow-slip events. We report direct observations from cores that sampled the active Pāpaku splay fault at 304 m below the seafloor. This fault roots into the plate interface and comprises an 18-m-thick main fault underlain by ∼30 m of less intensely deformed footwall and an ∼10-m-thick subsidiary fault above undeformed footwall. Fault zone structures include breccias, folds, and asymmetric clasts within transposed and/or dismembered, relatively homogeneous, silty hemipelagic sediments. The data demonstrate that the fault has experienced both ductile and brittle deformation. This structural variation indicates that a range of local slip speeds can occur along shallow faults, and they are controlled by temporal, potentially far-field, changes in strain rate or effective stress.