Pore Fluid Pressures and Strength Contrasts Maintain Frontal Fault Activity, Northern Hikurangi Margin, New Zealand

Diverse modes of fault slip occur within shallow subduction zones, but their relationships to upper plate deformation are poorly understood. IODP Expedition 375 collected samples and data across the northern Hikurangi margin, where tsunamigenic earthquakes and slow slip events occur. Site U1518 intersected the Pāpaku fault, a splay fault within the frontal accretionary wedge. To constrain in situ physical conditions and strength of the system, we conducted isotropic loading and triaxial compression experiments on hanging wall and foot wall cores and interpreted the results using critical state soil mechanics theory. The hanging wall is relatively strong, reflecting its tectonic history, whereas the foot wall and fault are anomalously weak owing to elevated pore pressure (λ∗ ≥ 0.3). Critical state in both samples corresponds to a friction coefficient of 0.6, constraining the strength of the Pāpaku fault. Low fault strength, maintained by excess pore pressures, suggests that the Pāpaku fault may still be active. Plain Language Summary The greatest earthquake and tsunami hazards occur where one tectonic plate descends beneath another, known as a subduction zone. In some cases, the plates slip past one another at a slow continuous pace, and in others, they slip in punctuated bursts that produce earthquakes and sometimes tsunamis. In northern New Zealand, a segment of the subduction zone known as the Hikurangi margin experiences a combination of these slow and earthquake‐producing slip modes. However, subduction plate boundaries may never reach Earth's surface, making the effects of their slip processes challenging to study. The Integrated Ocean Drilling Program (IODP) conducted two research cruises to drill boreholes, collect samples, and install instruments along the Hikurangi margin to better understand how and why the boundary slips. In a suite of laboratory experiments, we measured the strength of the rock recovered from one of the boreholes. Our results show that the strength is low due to the presence of high pressure fluids that fill the pore space of the rocks, which enables plate boundary deformation to propagate to the seafloor, potentially contributing to tsunami‐generating earthquakes. Key Points The hanging wall of the Pāpaku fault is relatively strong due to tectonic stress history and lithification The foot wall and Pāpaku fault are relatively weak due to persistently elevated pore fluid pressures not inherent material properties High fluid pres