Seismo-acoustic characterisation of the 2018 Ambae (Manaro Voui) eruption, Vanuatu

Iseul Park,Esline Garaebiti,Robin S Matoza,Arthur Jolly,Ben Kennedy,Sandrine Cevuard,Richard Johnson,Geoff Kilgour

doi:10.1007/s00445-021-01474-z

Abstract

A new episode of unrest and phreatic/phreatomagmatic/magmatic eruptions occurred at Ambae volcano, Vanuatu, in 2017–2018. We installed a multi-station seismo-acoustic network consisting of seven 3-component broadband seismic stations and four 3-element (26–62 m maximum inter-element separation) infrasound arrays during the last phase of the 2018 eruption episode, capturing at least six reported major explosions towards the end of the eruption episode. The observed volcanic seismic signals are generally in the passband 0.5–10 Hz during the eruptive activity, but the corresponding acoustic signals have relatively low frequencies (< 1 Hz). Apparent very-long-period (< 0.2 Hz) seismic signals are also observed during the eruptive episode, but we show that they are generated as ground-coupled airwaves and propagate with atmospheric acoustic velocity. We observe strongly coherent infrasound waves at all acoustic arrays during the eruptions. Using waveform similarity of the acoustic signals, we detect previously unreported volcanic explosions at the summit vent region based on constant-celerity reverse-time-migration (RTM) analysis. The detected acoustic bursts are temporally related to shallow seismic volcanic tremor (frequency content of 5–10 Hz), which we characterise using a simplified amplitude ratio method at a seismic station pair with different distances from the vent. The amplitude ratio increased at the onset of large explosions and then decreased, which is interpreted as the seismic source ascent and descent. The ratio change is potentially useful to recognise volcanic unrest using only two seismic stations quickly. This study reiterates the value of joint seismo-acoustic data for improving interpretation of volcanic activity and reducing ambiguity in geophysical monitoring.

Highlights

Basaltic eruptions are a dominant volcanism on Earth the volcanic activity is mostly related to submarine volcanoes (e.g., Parfitt 2004; Siebert et al 2010, 2015)
The active vent at Ambae is centred on Lake Voui, and lake water may be involved in the explosion and acoustic source mechanism
Based on the seismo-acoustic features, we suggest three key observations in this study: 1) Ground-coupled airwaves are observed as seismic VLP signals during large explosions at Ambae, and the apparent VLP are unrelated to subsurface volcanic fluid processes

Summary

Introduction

Basaltic eruptions are a dominant volcanism on Earth the volcanic activity is mostly related to submarine volcanoes (e.g., Parfitt 2004; Siebert et al 2010, 2015). Manaro Voui ( called Aoba or Ambae) is a basaltic shield volcano on Ambae Island of the Vanuatu archipelago (Fig. 1a). It has been active since the late Pliocene or early Pleistocene (Warden 1970). The latest eruption episode began on 6 September 2017, with phreatic explosions that transitioned into Strombolian eruptions and lava effusion Due to this increased activity, Vanuatu Meteorology and Geohazards Department (VMGD) raised the Volcanic Alert Level (VAL) to 3 indicating minor eruption. This episode ceased in late October 2018 and was divided into four phases. During the 2017–2018 episode, the volcano displayed phreatic, phreatomagmatic, and magmatic eruptions with ash, steam, gas, and lava from Lake Voui, which led to an emergency evacuation of all residents to neighbouring islands in August 2018

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Discussion

Conclusion