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

Abstract

AbstractDiverse 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.