National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2013

Underwater acoustic records from the March 2009 eruption of Hunga Ha’apai–Hunga Tonga volcano in the Kingdom of Tonga

Bohnenstiehl, D.R., R.P. Dziak, H. Matsumoto, and T.-K. Lau

J. Volcanol. Geotherm. Res., 249, 12–24, doi: 10.1016/j.jvolgeores.2012.08.014 (2013)

A network of autonomous underwater hydrophones is used to monitor acoustic activity associated with Hunga Ha'apai-Hunga Tonga volcano during a period of 15 months. The data provide a continuous record spanning a surtseyan eruption (VEI 2) in March of 2009, which input ~ 1013 J of acoustic energy into the ocean soundscape. In the months before the eruption, the volcano can be identified as an intermittent source of ambient noise. The period of seismic unrest that precedes the eruption begins at 15:11 UTC on 16 March (04:11 LT on 17 March), approximately 7 h before the first satellite confirmation of eruptive activity and 14 h before the first eyewitness reports. The initial seismic activity, which includes a single 4.8 mb event at 15:25, evolves as a typical foreshock–mainshock–aftershock sequence. By 15:38, however, the rate of small earthquakes begins to increase, marking the onset of the seismic swarm. The period of highest-amplitude acoustic energy release between 16:40 and ~ 17:10 is interpreted to mark the opening of the volcanic conduit. By 19:00 on 16 March, the acoustic signature of the volcano is marked by a continuous wide-band (1–20 Hz) noise and a set of transient very-broadband (1–125 Hz) explosion signals. This activity is characteristic of the main surtseyan phase of the eruption. Both the intensity of explosions and the amplitude of the lower frequency wide-band noise decay through time, and eruptive activity likely ends at ~ 09:00 on 19 March, ~ 2.7 days after the initiation of seismic activity. At this time the continuous low frequency noise decays to near background levels and signal coherence drops suddenly. Low-level acoustic unrest persists through June of 2009, after which the volcano becomes acoustically dormant during the remaining ten months of monitoring. The analysis of volcano-acoustic signals associated with Hunga Ha'apai-Hunga Tonga volcano highlights the potential role of regional hydroacoustic monitoring in assessing volcanic hazards in arc settings.

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