Live from the Seafloor: Seismic Signals associated with the 2015 Eruption of Axial Seamount

Abstract:

Axial seamount is the most active volcanic feature on the Juan de Fuca Ridge having previously erupted in 1998 and 2011. In 2014, the Ocean Observatories Initiative installed a multidisciplinary, cabled observatory on Axial seamount that includes a compact network of 7 seismometers spanning the southern half of the summit caldera with another seismometer located at the base of the volcano. Real-time data from the seismic network has been archived at the IRIS Data Management Center from mid-November 2014. We have implemented an automated algorithm to detect earthquakes, pick P and S wave phases, determine hypocenters and estimate magnitudes. Over 5 months to mid-April earthquake rates increased from an average of ~100 per day to >500 per day. Most of the earthquakes were small (M_W ≤ 1.5) and occurred in swarms beneath the east and west walls of the caldera at depths clustering around 1.5 km. On April 23, 2015 at 1700-2100 GMT there was a sizeable swarm of earthquakes near the east wall of the caldera with most activity north of the network. The main seismic crisis, which coincided with ~2 m of deflation at the center of the caldera (Nooner et al., this meeting), commenced at 0500 GMT on April 24. For the first 2 hours epicenters were confined near the east wall north of the network. The southern limit of seismicity then migrated ~3 km south over one hour to beneath the network where very high levels of seismicity (~500 per hour) persisted until 1500 GMT. During the seismic crisis, the caldera seismometers were swamped by nearby earthquakes, including ~100 per hour with M_W ≥ 2; additional analysis is required to determine if seismicity also migrated outside the caldera. In the following weeks, earthquake rates beneath the caldera decreased to <10 per day. An exciting discovery during this period was the detection of a large number of impulsive water borne signals, each comprising a train of water column multiples with arrival times consistent with sound sources on the seafloor 15-20 km to the north of the network. These events commenced at the end of the seismic crisis, peaked at a rate of >2000 per day on April 26 and continued at decreasing levels until mid-May. Recent seafloor mapping and observations have confirmed that these impulsive events mark the site of an eruption on the north rift of the volcano (Kelley et al., this meeting).