A Comparative Study of the 2011 and 2015 Eruptions of Axial Volcano, Juan de Fuca Ridge, From Seafloor Hydrophones

Abstract:

Axial volcano, on the Juan de Fuca ridge, erupted in April 2011, and again in April 2015. Both eruptions were captured on ocean bottom hydrophones (OBHs). The 2011 eruption was recorded by three OBHs deployed near or within the caldera, although one instrument was overrun by lava and its data could not be recovered. The 2015 eruption was captured in real time by a network of ocean bottom seismometers and hydrophones deployed as part of the Ocean Observatories Initiative Cabled Array. Intense seismicity preceded and accompanied both eruptions. Earthquakes are recognizable in the hydroacoustic record by the timing of surface reflections: secondary and tertiary pulses are consistent with arrivals coming from below the hydrophone and reflecting off of the sea surface. The 2015 eruption also produced signals that initiate in the water column, interpreted as water-lava interactions. Surface reflections for these water column events are consistent with a more distant source, and the relative polarity of arrivals on the co-located OBH and OBSs are consistent with a signal traveling through the water rather than the subsurface. Comparisons of arrivals on the hydroacoustic and seismic channels show that there is little energy on the EW seismic channel, consistent with an impulsive signal arriving from the north. These water column events are absent from the 2011 eruption, despite the fact that preliminary estimates suggest that the eruptions were of comparable volume. Also observed during the 2015 event were a number of a diffuse broadband (10-100 Hz) signals that last from several minutes to over an hour. These events appear similar to hydroacoustic signals recorded in association with explosive degassing at other submarine volcanoes including NW Rota-1 (Marianas) and West Mata (Lau Basin). A comparison of hydroacoustic data recorded in the two events indicates that the 2015 eruption was more vigorous than the 2011 event and resulted in more explosive lava-water interactions. This may reflect a larger gas content or higher effusion rate in 2015. The combination of hydroacoustic and seismic data provides an excellent means of investigating eruption styles in the submarine environment.