‘SLIPPING’ INTO THE SEA: CAN UPPER PLATE FAULTS PRODUCE COASTAL SUBSIDENCE AT SUBDUCTION MARGINS?

Earthquakes near the coast have the potential to create cascading hazards as land level changes interact with sea level. Coseismic subsidence, in particular, will exacerbate the effects of ongoing sea level rise, storm surges, tsunamis, and tides. Understanding the character and likelihood of earthquake-driven coastal deformation is therefore an important consideration in hazard planning and forecasting how sea level changes will impact coastal communities over time. Near Napier, New Zealand, rapid subsidence in the paleoseismic record is typically attributed to subduction zone earthquakes while uplift is attributed to upper plate faults. If upper plate events also produce rapid subsidence events, however, then seismic hazard, tsunami, and sea level rise models currently mischaracterize hazards at the coast. We investigate coastal coseismic deformation near Napier with elastic dislocation models of upper plate fault earthquakes. Using slip constraints from seismic surveys and historical earthquakes, we capture a suite of plausible earthquakes to compare to paleoseismic uplift and subsidence records preserved at Ahuriri Lagoon. The results indicate that while faults near Napier likely only produce coastal uplift, offshore fault sources are capable of producing subsidence up to c. 0.5–1 m at Ahuriri Lagoon at recurrence intervals of c. 1–4 kyr. This suggests that both upper plate faults and megathrust earthquakes may have contributed to the eight subsidence events recorded at Ahuriri Lagoon over the last c. 7 kyr. These results challenge the current paradigm that coastal subsidence in near Napier only records megathrust earthquakes, and merits reconsideration of the role of upper plate faults in characterizing coastal earthquake hazards.