Hydroacoustic detection of volcanic ocean-island earthquakes.

Real-time monitoring of volcanic earthquakes forms a critical component of assessing volcanic hazard but requires a widely dispersed land-based instrument network and analysis infrastructure to be effective. Here we show how a sparse network of hydrophones can be used for volcano monitoring as well. Detection of acoustic events by the Mid-Atlantic Ridge hydrophone array includes many events in the vicinity of the Cape Verde Islands, upon which a volcano monitoring network operates. Correlating the hydroacoustic events with earthquake locations determined by the network, we find that earthquakes of magnitudes as low as ML 2.0 can be hydroacoustically located to ±25 km at ranges up to 2000 km. All correlatable events are volcano-tectonic in character. Acoustic propagation of small-magnitude earthquake signals is more efficient if their sources are perched in the island's volcanic edifice in the ocean sound transmission channel, which contributes to volcanic event detectability. The location accuracy at this low magnitude range, which includes magnitudes typical of seismic crises precursory to eruptions, provides an additional volcanic hazard monitoring capability. Seismicity and hydroacoustic activity in the northern Cape Verdes suggest that this monitoring element may be of use in assessing future risk there. [ABSTRACT FROM AUTHOR]