Anatomy of phreatic eruptions

Suparjan Suparjan,Thomas Lecocq,Devy Syahbana,Benoit Taisne,Bruce Christenson,Gede Suantika,Jurgen Neuberg,Corentin Caudron,Arthur D. Jolly

doi:10.1186/s40623-018-0938-x

Abstract

This study investigates phreatic eruptions at two similar volcanoes, Kawah Ijen (Indonesia) and White Island (New Zealand). By carefully processing broadband seismic signals, we reveal seismic signatures and characteristics of these eruptions. At both volcanoes, the phreatic eruptions are initiated by a very-long-period (VLP) seismic event located at shallow depths between 700 and 900 m below the crater region, and may be triggered by excitation of gas trapped behind a ductile magma carapace. The shallow hydrothermal systems respond in different ways. At Kawah Ijen, the stress change induced by VLPs directly triggers an eigenoscillation of the hyperacidic lake. This so-called seiche is characterized by long-lasting, long-period oscillations with frequencies governed by the dimensions of the crater lake. A progressive lateral rupture of a seal below the crater lake and/or fluids migrating toward the surface is seismically recorded sim 15 min later as high-frequency bursts superimposed to tilt signals. At White Island, the hydrothermal system later (sim 25 min) responds by radiating harmonic tremor at a fixed location that could be generated through eddy-shedding. These seismic signals shed light on several aspects of phreatic eruptions, their generation and timeline. They are mostly recorded at periods longer than tens of seconds further emphasizing the need to deploy broadband seismic equipment close to active volcanic activity.

Highlights

To date, significant progress has been made in forecasting volcanic eruptions involving substantial magma movement by using seismicity, deformation and gas emission monitoring (e.g., 2010 Merapi eruption, Surono et al 2012)
In this study, we explore with mainly seismological means phreatic eruptions and the hydrothermal systems responses at two similar volcanoes, Kawah Ijen, Indonesia and White Island, New Zealand
We focus on periods of phreatic eruptions which are currently not fully understood, but pose, a considerable volcanic hazard claiming time and again some human lives

Summary

Introduction

Significant progress has been made in forecasting volcanic eruptions involving substantial magma movement by using seismicity, deformation and gas emission monitoring (e.g., 2010 Merapi eruption, Surono et al 2012). Instrumentation and processing At Kawah Ijen volcano, four seismic stations installed in the crater area recorded the March 2013 volcanic activity. Three VLP signals with dominant periods around 8 s and superimposed low-amplitude shorter oscillations (down to 1 s, Fig. 4c, d) have been recorded during the eruption, with the first VLP initiating this seismic sequence The data are high-pass filtered above the corner frequency of the sensors (i.e., 0.0833 Hz or 120 s period) to avoid any contamination from the ULP tilt previously described These oscillations are detected on all seismic components; VLPs and these induced long-lasting oscillations seem to occur independently from the tilt and HF bursts. The classic waveform semblance (Kawakatsu et al 2000), using the same parameters as

Findings

Discussion

Conclusions