Geophysical Constraints on the Relationship Between Seamount Subduction, Slow Slip, and Tremor at the North Hikurangi Subduction Zone, New Zealand

We use a prestack depth migration reflection image and magnetic anomaly data across the northern Hikurangi subduction zone, New Zealand, to constrain plate boundary structure and geometry of a subducting seamount in a region of shallow slow slip and recent International Ocean Discovery Program drilling. Our 3-D model reveals the subducting seamount as a SW-NE striking, lozenge-shaped ridge approximately 40 km long and 15 km wide, with relief up to 2.5 km. This seamount broadly correlates with a 20-km-wide gap separating two patches of large (> 10 cm) slow slip and the locus of tectonic tremor associated with the September-October 2014 Gisborne slow slip event. Largest slow slip magnitudes occurred where the decollement is underlain by a 3.0-km-thick zone of highly reflective subducting sediments. Wave speeds within this zone are 7% lower than adjacent and overlying strata, supporting the view that high fluid pressures within subducting sediments may facilitate shallow slow slip along the north Hikurangi margin. Plain Language Summary Using a suite of geophysical data from the northern Hikurangi margin, New Zealand, we determine the location and geometry of a subducting seamount on the subducting Pacific Plate and establish its spatial relationship with slow slip and tremor that occurred on the plate boundary in September-October 2014. We infer that slow slip appears to occur preferentially where there are sediments with high fluid pressure in pore fluids subducting adjacent to the seamount but is reduced above the seamount itself. This observation has implications for understanding what physical conditions contribute to spatial variation in frictional properties of the plate interface that may control fault slip behavior on large, plate boundary subduction thrusts.