Ultra‐long‐range hydroacoustic observations of submarine volcanic activity at Monowai, Kermadec Arc

Abstract

Monowai is an active submarine volcanic center in the Kermadec Arc, Southwest Pacific Ocean. During May 2011, it erupted over a period of 5 days, with explosive activity directly linked to the generation of seismoacoustic T phases. We show, using cross‐correlation and time‐difference‐of‐arrival techniques, that the eruption is detected as far as Ascension Island, equatorial South Atlantic Ocean, where a bottom moored hydrophone array is operated as part of the International Monitoring System of the Comprehensive Nuclear‐Test‐Ban Treaty Organization. Hydroacoustic phases from the volcanic center must therefore have propagated through the Sound Fixing and Ranging channel in the South Pacific and South Atlantic Oceans, a source‐receiver distance of ~15,800 km. We believe this to be the furthest documented range of a naturally occurring underwater signal above 1 Hz. Our findings, which are consistent with observations at regional broadband stations and long‐range, acoustic parabolic equation modeling, have implications for submarine volcano monitoring.

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

“…Figure b) by up to 20 dB (Figures g and h). We observe maximum values of up to 128 dB for individual detections, which is on the same order as long‐range acoustic measurements of the May 2011 eruption (Metz et al, ). Peak RMS amplitudes are typically scattered across individual episodes.…”

“…Acoustic resolution of 97 dB corresponds to a magnitude of completeness of 2.2 m b , which is an order of magnitude lower than the smallest tectonic event previously detected by IMS seismometers in the central Kermadec Arc region (3.3 m b ). Following this first‐order approximation, mean magnitudes of volcanic episodes at Monowai range from 1.6 (88 dB) to 3.1 m b (110 dB) and are consistent with presumed levels of activity at the volcano during its 2011 eruption (~2 m b ; Metz et al, ). Since only a small number of individual arrivals reach peak amplitudes greater than 120 dB (>3.5 m b ), overall levels of activity at the volcano most likely are too low to be resolved by a sparse network of land‐based seismometers over teleseismic distances.…”

“…For this study, the minimum number of points m , that is, 1‐min detections, is set to 60, which corresponds to the shortest period of activity previously observed at Monowai, that is, an hour‐long eruptive collapse event in May 2002 (Wright et al, ). We define the search radius ε as 12 hr along the x axis and 0.5° along the y axis, which reflects the estimated accuracy of the plane wave fitting routine and is twice the duration of the longest known intraeruptive pause in activity at the volcano (Metz et al, ). As DBSCAN is computationally expensive and low‐frequency arrivals from Monowai are not expected to vary by more than a few degrees (Bohnenstiehl et al, ), the spatial domain of the data set is limited to a 10° range centered around the geodesic back azimuth to Monowai (243.8 ± 5°).…”

“…During the most recent documented eruption in May 2011, a five‐day‐long burst of T phases, recorded at broadband seismometers at Rarotonga (Cook Islands), Papeete (Tahiti), and Nuku Hiva (Marquesas Islands), could be linked to the growth of a 72‐m summit cone and a flanking sector collapse of 18 m (Watts et al, ). Furthermore, long‐range underwater sound waves associated with the same volcanic episode have been remotely detected by a hydrophone array near Ascension Island in the southern Equatorial Atlantic Ocean, over a geodesic range of 15,800 km (Metz et al, ). Activity at Monowai may have occurred as recently as October 2014 and May 2016, when seismic amplitudes at Rarotonga station rose for multiple days and discolored water was reported during flyovers conducted by the Royal New Zealand Air Force (Global Volcanism Program, ).…”

Hydroacoustic, Seismic, and Bathymetric Observations of the 2014 Submarine Eruption at Ahyi Seamount, Mariana Arc

“…Two IMS hydrophone triads in the Juan Fernandez Islands, Chile (H03 north and south), also recorded a small percentage of the Ahyi events nearly 15,500 km across the Pacific Ocean, but those observations are not described here in detail. Metz et al () found similar long‐distance detections of an eruption of Monowai Seamount. For our study, the H11S2 and H11N1 hydrophones were chosen as the best stations for single‐station analyses.…”