Permeability of the Pāpaku Fault Within the Hikurangi Subduction Zone Determined From the Thermal Response to a Free Flowing Well Test

Abstract The paucity of in situ meso‐scale permeability measurements within subduction zones makes it difficult to fully characterize hydrologic processes and conditions that influence fault zone behavior. In the Hikurangi subduction zone, where large shallow slow slips are observed, there are outstanding questions about the hydrologic influence splay faults have on fluid pressures along the megathrust. Here, by utilizing the borehole thermal response to a free‐flowing well test, we estimate the permeability of the Pāpaku thrust fault, which extends from the Hikurangi décollement to the seafloor of the frontal accretionary wedge. The experiment is made possible by using a high‐resolution temperature sensor array within the Te‐Matakite subseafloor observatory near the northern Hikurangi Trough which is open to the Pāpaku fault ∼323 m below seafloor. The observatory was installed during International Ocean Discovery Program (IODP) Expedition 375 in 2018. After 5 years of recording, the wellhead was unsealed resulting in outflow indicative of overpressures within the fault. The observed temperature response to the outflow reflects flow velocities that constrain the Pāpaku fault zone permeability to k ≥ 1.9 × 10 −14 m 2 . The temporal evolution of flow, however, reveals that the connectivity of this permeable zone is bounded to several tens of meters of extent and not to the seafloor. These results suggest the Pāpaku fault and other splay faults within the Hikurangi margin may serve as conduits for localized fluid flow and pressure redistribution but may not have sufficient hydrologic connection to transmit fluid or sufficiently relieve fluid pressure from the décollement.