Ocean-bottom seismic and seafloor pressure data from the Ocean Observatories Initiative’s Cabled Array were used to study fault reactivation within Axial Seamount (offshore Oregon, USA). Microearthquakes that occurred during 2015–2016 were located on portions of an outward-dipping ring fault system that was reactivated in response to the inflation and deflation of the underlying magma chamber. Prior to an eruption in April 2015, focal mechanisms showed a pattern of normal slip consistent with the differential vertical uplift of the caldera floor relative to the rim. During the eruption, seismic activity remained localized along these outward-dipping structures; however, the slip direction was reversed as the caldera floor subsided. After the eruption, as the volcano reinflated and the caldera floor uplifted, these faults exhibited sparser seismicity with a more heterogeneous pattern of slip. Monitoring the evolution of ring fault behavior through time may have utility as a metric in future eruption forecasts.

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