Spatial, Temporal and Size-Frequency Characteristics of Microearthquake Sequences Leading up to the 2015 Eruption of Axial Seamount

Microearthquakes beneath the summit of Axial Seamount on the Juan de Fuca Ridge were located using three-component ocean-bottom seismometers (OBS) deployed as part of the Ocean Observatories Initiative (OOI) Cabled Array Network. A microearthquake catalog of more than 100,000 events spans the period before after and during the eruption of the volcano in April and May of 2015. A hierarchical clustering approach, with a combined space-time metric, is used to define 95 earthquakes sequences over a three month period leading up to eruption. Their time-magnitude characteristics represent a mixture of mainshock-aftershock, foreshock-mainshock and swarm activity. Earthquakes within these sequences are distributed primarily along the shallow ring fault system, with a significant migration of hypocenters observed for roughly half of the sequences. The sequences are temporally clustered. They increase in frequency and size, but show no systematic change in size-frequency distribution, approaching the eruption. Sequences are typically initiated under conditions of favorable ocean tidal loading, approaching low ocean tide, and terminate as the water level rises. The number of sequences initiating during, or extending into, times of discouraging tidal stress increases approaching the eruption.