Infrasound from Giant Bubbles During Shallow, Explosive Submarine Eruptions

Abstract:

Shallow submarine volcanoes pose unique scientific and monitoring challenges. Magma-water interactions can create violent explosions just below the surface, but the inaccessibility of submerged volcanoes means they are typically not instrumented. This both increases the risk to marine and aviation traffic and leaves the underlying eruption physics poorly understood. The 2016-2017 shallow submarine eruption of Bogoslof volcano, Alaska produced 70 explosive events over nine months that were well-recorded by regional infrasound stations operated by the Alaska Volcano Observatory. These data provide new insights into how submarine eruptions produce infrasound, thus aiding future monitoring efforts.

The Bogoslof eruption produced primarily low-frequency (0.1-1 Hz) infrasound that we attribute to the vent being flooded by shallow (~10-100 m) seawater. We show that discrete, low-frequency infrasound signals originate from the oscillation and rupture of gas bubbles that initially formed at submerged vents, but that grew and burst above sea level. We model the infrasound signals as overpressurized gas bubbles that grow near the water–air interface. Our analysis indicates that bubbles with radii of 50–220 m likely formed during the Bogoslof eruption. Bubbles of this size and larger have been described in explosive subaqueous eruptions for more than a century, but we present a unique geophysical record of this phenomenon. We propose that the dominant role of seawater during the effusion of gas-rich magma into shallow water is to repeatedly produce a gas-tight seal near the vent rather than explosive magma-water interactions. This resealing mechanism leads to sequences of violent explosions and the release of large, bubble-forming volumes of gas—activity we describe as hydrovulcanian.