Seismic and Acoustic Constraints on the Dynamics of the Submarine Eruption Cycle at Axial Seamount

Abstract:

In January of 2015, as part of the Ocean Observatories Initiative (OOI), a small network of cabled ocean-bottom seismometers and hydrophones began operating continuously within the caldera of Axial Seamount. The subsequent eruption of Axial Seamount during April and May of 2015 presents a unique opportunity to study the spatial and temporal evolution of a mid-ocean ridge volcanic system. During the period of magmatic inflation leading up to the eruption, microearthquake hypocenters delineate outward-dipping ring faults with a normal sense of slip. These ring faults are critically stressed, with high event rates and a temporal pattern that is strongly modulated by tidal forcing. At ~04:00Z on 24 April, an increase in earthquake rate and magnitude mark the onset of the seismic crisis. Over the next few hours, the caldera floor begins to subside as magma is emplaced in a dike extending to the north. Seismicity patterns and focal mechanisms show that this subsidence is accommodated by ring-fault reactivation with a reverse sense of slip. Critical to our understanding of this eruption is the identification of distinct explosive acoustic signals associated with the eruption of lava on the seafloor. Beginning at ~08:00Z on 24 April and continuing through 21 May these water-borne arrivals track the eruption of lava within the caldera and subsequent emplacement of dike-fed flows down rift. After the eruption, stresses within the volcanic edifice are lower and the stress field is more heterogeneous, as evidenced by a reduced rate of seismicity, elevated b-values, insensitivity to tidal forcing, and focal mechanisms that show a more diverse sense of slip. The planned 25-year longevity of the OOI seismic network is expected to support continuous observations of the volcano throughout multiple eruption cycles.