Dr. Evan Solomon, Associate Professor, College of the Environment, School of Oceanography, University of Washington, Unraveling the hydrogeologic habitat of slow slip events at the Hikurangi subduction zone, offshore New Zealand

Slow slip events (SSEs) involve millimeters to decimeters of fault slip lasting days to months, thus occur at a rate intermediate between long-term plate motion and the slip velocity required to generate seismic waves. Recent geodetic studies have shown that SSEs can occur on shallow subduction faults (<15 km depth) where tsunamis are also generated. Although observations of SSEs at subduction zones are now widespread, the physical conditions promoting shallow SSEs remain poorly understood. There are several mechanisms that may promote slow slip behavior including low effective stress linked to elevated pore fluid pressure. Considering pore fluid pressure and fault behavior are tightly interlinked, the characterization of fluid generation, fluid flow, and the pore fluid pressure in the source region of SSEs has been an area of active research over the past decade. However, there are only a few direct measurements of pore fluid pressure and fluid flow along subduction zone faults and most inferences of the pore fluid pressure distribution are indirect, either from numerical models or geophysical remote sensing techniques.

Here I present the results from several offshore field programs over the past 5 years that aim to assess the interrelationships between pore fluid pressure, fault slip, and fluid flow at the Hikurangi subduction zone. I will mainly focus on hydrogeochemical data collected along the northern Hikurangi margin during the SAFFRONZ expeditions in 2019 and 2021 and IODP Expedition 375 in 2018. During the SAFFRONZ expeditions, we collected piston and gravity cores at structurally controlled seep sites and non-seep locations from the deformation front to the shelf-break and deployed 16 benthic fluid flow meters to generate a 2-year, continuous record of fluid flow rates and composition. During IODP Expedition 375, we cored the incoming sediment section and an active splay fault landward of the trench, and installed a CORK borehole observatory to continuously measure pore fluid pressure, fluid flow rates, fluid composition, and temperature within the fault zone over a 5-year period. This comprehensive dataset provides (1) direct information on the background state of fluid flow and how it relates to the pore pressure distribution along the plate boundary, and (2) a continuous 2 to 5-year record of fluid flow rates and composition spanning the source region of slow slip that characterizes the hydrgeochemical response to a large slow slip event that occurred in 2019.