Remote Hydroacoustic and Infrasonic Detection and Characterization of Anak Krakatau Eruptive Activity Leading To, During, and Following the December 2018 Major Flank Collapse and Tsunami

Eruptions at submarine and partially submerged volcanoes are often difficult to detect, yet pose a significant hazard to coastal populations, ships, and aircraft. Hydroacoustic and infrasonic waves generated by explosive eruptions can propagate thousands of kilometers in atmospheric and oceanic waveguides. We employ a selection of remote moored hydroacoustic (H08S, 3,307 km; H01W, 3,720 km) and infrasonic (IS06, 1,156 km; IS07, 3,475 km; IS52, 3,638 km) stations of the International Monitoring System (IMS) to investigate the new eruptive phase of Anak Krakatau volcano in the Sunda Strait of Indonesia which began in June 2018. A climactic eruption phase on 22 December 2018 triggered the collapse of the southwest flank and summit, generating a large tsunami which struck the coastlines of Sumatra and Java [Global Volcanism Pro- gram, 2019]. We perform array processing for a new eruptive phase from June 2018 through January 2019, using the Progressive Multi-Channel Correlation (PMCC) algorithm. Coherent explosive eruption (co-eruptive) signals arriving from the back-azimuth of Anak Krakatau were recorded at two infrasound stations (IS06, 1,156 km and IS52, 3,638 km) of the IMS network during the main eruption phase and flank collapse, but were not detected hydroacoustically. We also investigate intermittent infrasound signal from smaller eruptive activity prior to the flank collapse and tsunami. The signals of interest are associated between the two stations and correspond to eruption reports from the Indonesian Center for Volcanology and Geological Disaster Mitigation (PVMBG). We detect a 12-day swarm of hydroacoustic signals starting 24 days before the main collapse event that were part of the sustained eruptive sequence at Anak Krakatau, recorded at IMS hydroacoustic station H08S (3,307 km) and partially recorded at station H01W (3,720 km). We interpret an evolution in signal content in the weeks directly preceding the climactic eruption phase, as resulting from intermittent subaerial (infrasonic) and submarine (hydroacoustic) detections, to entirely subaerial content at the start of the climactic phase. We perform waveform cross-correlation and clustering analysis to assess the similarity of infrasound events and the similarity of hydroacoustic events, grouping similar events into families for each signal type. Infrasonic events recorded during the main infrasound-generating eruption phase are similar (with 61.8% of events assigned to the largest 4 event families using a threshold ρ of 0.65), indicating repetitive explosions at Anak Krakatau from the same source mechanism and similar long-range propagation conditions. Hydroacoustic event families are less similar, indicating a possible range of submarine eruption processes and signal types occurring at Anak Krakatau including submarine explosions, bursting magmatic gas bubbles, or shallow volcanic earthquakes. This study underscores the potential of using remote acoustic technology to detect and characterize eruptions at submarine or partially submerged volcanoes