Seismo-acoustic evidence for an avalanche driven phreatic eruption through a beheaded hydrothermal system: An example from the 2012 Tongariro eruption

The 6 August 2012 Te Maari eruption comprises a complex eruption sequence including multiple eruption pulses, a debris avalanche that propagated ~. 2. km from the vent, and the formation of a 500. m long, arcuate chasm, located ~. 300. m from the main eruption vent.The eruption included 6 distinct impulses that were coherent across a local infrasound network marking the eruption onset at 11:52:18 (all times UTC). An eruption energy release of ~3×1012J was calculated using a body wave equation for radiated seismic energy. A similar calculation based on the infrasound record, shows that ~90% of the acoustic energy was released from three impulses at onset times 11:52:20 (~20% of total eruption energy), 11:52:27 (~50%), and 11:52:31 (~20%). These energy impulses may coincide with eyewitness accounts describing an initial eastward directed blast, followed by a westward directed blast, and a final vertical blast.Pre-eruption seismic activity includes numerous small unlocatable micro-earthquakes that began at 11:46:50. Two larger high frequency earthquakes were recorded at 11:49:06 and 11:49:21 followed directly by a third earthquake at 11:50:17. The first event was located within the scarp based on an arrival time location from good first P arrival times and probably represents the onset of the debris avalanche. The third event was a tornillo, characterised by a 0.8. Hz single frequency resonance, and has a resonator attenuation factor of Q~. 40, consistent with a bubbly fluid filled resonator. This contrasts with a similar tornillo event occurring 2.5. weeks earlier having Q~. 250-1000, consistent with a dusty gas charged resonator. We surmise from pre-eruption seismicity, and the observed attenuation change, that the debris avalanche resulted from the influx of fluids into the hydrothermal system, causing destabilisation and failure. The beheaded hydrothermal system may have then caused depressurisation frothing of the remaining gas charged system leading to the onset of explosive activity