Eruption source processes derived from seismic and acoustic observations of the 25 September 2007 Ruapehu eruption—North Island, New Zealand

Abstract : Mt. Ruapehu erupted on 25 September 2007 at 20:26 PM NZDT (8:26 UT) generating a steam column to about 15,000 ft (4600 m), a directed ballistic and surge deposit of coarse blocks and ash to the north of the Crater Lake, and initiated lahars in the Whangaehu catchment and Whakapapa ski field. The eruption was recorded on three broadband seismometers and two acoustic pressure sensors which indicated broadly coherent waveform characteristics across the network. The eruption had a duration of less than 1 min as indicated by strong seismic and acoustic pulses. The acoustic wave coincided with observed very long-period (VLP) seismic signal (2–25 s period). The acoustic pulse had a positive pressure (indicating an explosion) and travelled at a velocity of ∼320 m/s. The syn-eruptive VLP was composed of surface waves having strong radial and transverse components. The immediate aftermath of the eruption produced about 4 min of continued high amplitude spasmodic tremor signals that probably resulted from post-explosion phase associated vent backfill/stabilisation. The eruption was preceded by minor volcano-tectonic earthquakes and tremor bursts which began 10 min before the main eruption. Two smaller VLP signals were associated with these tremor pulses at 20:17 and 20:25. These pre-eruptive VLP signals had particle motions indicating body and surface waves from a sub-surface source region. Modelling of the VLP sources using a 3D finite-difference method shows that that the pre-eruption VLP signals are consistent with a volume fluctuation at 3–7 km depth from a radially symmetric source. The main eruption VLP was consistent with a south and downward directed single force in response to a northward directed jet and northward directed ballistic deposits.