Tracking Submarine Volcanic Activity at Monowai: Constraints From Long-Range Hydroacoustic Measurements

Abstract

Monowai is a submarine volcanic center in the Kermadec Arc, Southwest Pacific Ocean. In the past, activity at the volcano had been intermittently observed in the form of fallout at the sea surface, discolored water, changes in seafloor topography, and T phase seismicity, but there is no continuous record for more recent years. In this study, we investigated 3.5 years of recordings at a hydrophone array of the International Monitoring System, located near Juan Fernández Islands, for long-range underwater sound waves from Monowai. Results from direction-of-arrival calculations and density-based spatial clustering indicate that 82 discrete episodes of activity occurred between July 2003 and March 2004 and from April 2014 to January 2017. Volcanic episodes are typically spaced days to weeks apart, range from hours to days in length, and amount to a cumulative sum of 137 days of arrivals in total, making Monowai one of the most active submarine arc volcanoes on Earth. The resolution of the hydrophone recordings surpasses broadband network data by at least 1 order of magnitude, identifying seismic events as low as 2.2 mb in the Kermadec Arc region. Further observations suggest volcanic activity at a location approximately 400 km north of Monowai in the Tonga Arc and at Healy or Brothers volcano in the southern Kermadec Arc. Our findings are consistent with previous studies and highlight the exceptional capabilities of the International Monitoring System network for the scientific study of active volcanism in the global ocean.