Anatomy of phreatic eruptions

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 ∼ 15 min later as high-frequency bursts superimposed to tilt signals. At White Island, the hydrothermal system later (∼ 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.

Pressure Controlled Permeability in a Conduit Filled with Fractured Hydrothermal Breccia Reconstructed from Ballistics from Whakaari (White Island), New Zealand

“…Similar timescales for pressurisation as a result of alteration-induced reductions to permeability were proposed by [3]. Similar processes have been envisaged at other volcanoes [73]. However, here we offer an explanation for geophysical, geochemical, and visible changes that can occur in seconds to hours.…”

Relating gas ascent to eruption triggering for the April 27, 2016, White Island (Whakaari), New Zealand eruption sequence

“…7). In this case, the perturbation and failure of a shallow hydrothermal seal (by either advective pressurization or a stress perturbation) could induce localized failure and explosive ejection of the shallow seal materials (e.g., Caudron et al 2018). In these cases, the transfer of stress could propagate at elastic wave velocities and time lags might be very short, while an advective transfer mechanism may allow much longer eruption lag times (Fig.…”

Origin of Shallow Volcanic Tremor: The Dynamics of Gas Pockets Trapped Beneath Thin Permeable Media

“…One of the most enigmatic signals is shallow tremor, which is typically sourced less than ~1 km beneath active volcanic craters, is long lasting (from minutes to years) and often precedes eruptions (Chouet et al, ; Konstantinou & Schlindwein, ; Métaxian et al, ; Métaxian et al, ; Neuberg, ; Steinberg & Steinberg, ; Figure ). Shallow tremor has been extensively studied at many volcanoes over the last decades, including Etna and Stromboli, Italy (Chouet et al, ; De Martino et al, ; Di Lieto et al, ; Ripepe et al, ; Ripepe et al, ); Langila, Papua New Guinea (Mori et al, ); Ruapehu, New Zealand (Buurman et al, ; Girona et al, ; Hurst, ; Hurst & Sherburn, ; Sherburn et al, ); Kawah Ijen, Indonesia (Caudron, Lecocq, et al, , Caudron, Syahbana, et al, ; Caudron et al, ); Redoubt, Kilauea, and Mt. St. Helens, USA (Felher, ; Goldstein & Chouet, ; Hofstetter & Malone, ; Hotovec et al, ; Koyanagi et al, ); Galeras, Colombia (Gil‐Cruz, ); Masaya, Nicaragua (Métaxian et al, ); Arenal and Turrialba, Costa Rica (Benoit & McNutt, ; Conde et al, ; Hagerty et al, ; Lesage et al, ; Métaxian et al, ); Fuego, Guatemala (Nadeau et al, ); Sakurajima and Oshima, Japan (Ishihara, ; Maryanto et al, ; Yamaoka et al, ); and Soufrière Hills, UK Caribbean (Neuberg, )).…”