Ambrym, Vanuatu (July–August 2000): spectral and dynamical transitions on the hours-to-days timescale

Ambrym is a large basaltic volcano with a 12-km-wide, 1900-year-old caldera, located in the Vanuatu archipelago, formerly known as New Hebrides. Two cones are currently active in the caldera, Marum and Benbow, each with several active vents that hosted visible lava lakes until 1999; on 26 November 1999 an earthquake (Mw=7.5), located near the northern part of the island, caused important changes to the vent morphology. During July 2000, we installed a short-period seismic station on the eastern rim of Benbow crater in the western side of the caldera. The station acquired continuous data for about 1 month. During this period volcanic tremor was permanently recorded, together with discrete events. In this paper first results from these data are discussed, focusing attention on the existence of significant transitions in the long-term volcano dynamics, studied both with spectrally derived (dominant and average frequencies) and more properly dynamical (embedding dimensions and delay time) parameters. During the observation period, on 9 August 2000, another significant earthquake was recorded (mb=6.3, about 65 km north of the volcano). The influence of this and other regional tectonic events on the volcano dynamics are discussed.