9:30 AM - 9:45 AM
[SVC28-03] Unravelling seismic precursors and common eruption mechanisms in New Zealand and worldwide volcanoes
★Invited Papers
Keywords:Volcano seismology, Eruption precursors, Phreatic eruptions
Volcanic eruptions can be dangerous when occurred without warning in touristic and populated areas, as recently tragically proved in Ontake 2014 (Japan) and Whakaari 2019 (New Zealand) eruptions. Forecasting sudden eruptions has been elusive because their precursors are difficult to recognize with real-time monitoring and can vary between volcanoes. Here, we present a series of methods that allow us to locate a general seismic precursor for gas-driven eruptions, identified through correlation analysis of 18 well-recorded eruptions in New Zealand, Alaska and Kamchatka. The precursor exhibits a characteristic cycle that peaks around a week prior to several eruptions. We associate this precursor to the formation of a hydrothermal seal that enables rapid pressurization of shallow groundwater, a model that we applied to the 2019 Whakaari eruption in New Zealand. For this eruption, we describe five pre-eruptive phases that lead to the pressurization of the system in the week before the eruption, and cascading seal failure in the 16 hours prior to the explosion. Our work is relevant because identifying and proving generalizable eruption precursors can improve short term forecasting systems, potentially saving lives.
Hydrothermal seals
Hydrothermal seals and their failure can play an important role in phreatic eruptions because weakened permeability in the vent area increase pressurization, and a rapid increase can drive sudden decompression. Little is known about the timescales of seal formation. They are suggested to vary from weeks to months for mineral precipitation (sulphur, sulphates, silica); and from seconds to hours due to mineralized zones (alunite, anhydrite) that open/close in response to pressure variations. Our results indicate the rapid formation of a hydrothermal seal prior to the 2019 Whakaari eruption that consolidates around 3-4 days before the volcanic event.
Volcanoes and data streams studied
We analysed tremor time-series at different frequency bands prior to 18 eruptions at six volcanoes: Whakaari, Ruapehu and Tongariro (New Zealand), Veniaminof and Pavlof (Alaska, USA), and Bezymianny (Russia). We used feature engineering to extract latent patterns from the tremor and then systematically mined these for statistically differentiable correlations across all volcanoes in the weeks prior to eruptions. A noticeable correlation identified is a characteristic peak in the 2-day median of normalized DSAR tremor (displacement seismic amplitude ratio, DSAR, between medium, 4-8 Hz, and high, 8-16 Hz, frequency tremor) that recurred prior to eruptions at several volcanoes.
Major findings
Two precursors named ‘DSAR median’ and ‘DSAR rate variance’ are especially similar across five Whakaari eruptions, and also recognised prior to Ruapehu, and Tongariro eruptions. For the Whakaari 2019 eruption, we relate these precursors with hydrothermal reservoir sealing and pressurization that prepared the system for a phreatic eruption. Five distinct phases track the onset to such shallow gas and steam-driven eruption: (1) deep source fluid input into a geothermal reservoir; (2) pulsatory gas fluxing with weak temporary sealing over one or more weeks; (3) strong seal formation over the hydrothermal system several days before the event (4) leading to a critically pressured shallow aquifer (in near-equilibrium with deep gas pressures); before (5) external perturbation, and/or cascading material failure and crack formation over ~10-20 hours, leading to final seal breach and explosive decompression.
We found several common feature patterns, but just the ‘DSAR median’ and ‘DSAR rate variance’ can be classed as phreatic eruption precursors because they are recurrent, differentiable from non-eruptive repose, and somewhat transferable to other volcanoes. Monitoring these observables could help to improve real-time eruption forecasting, and provide new tools and warning products, ultimately saving lives, especially at regularly visited touristic volcanoes.
Hydrothermal seals
Hydrothermal seals and their failure can play an important role in phreatic eruptions because weakened permeability in the vent area increase pressurization, and a rapid increase can drive sudden decompression. Little is known about the timescales of seal formation. They are suggested to vary from weeks to months for mineral precipitation (sulphur, sulphates, silica); and from seconds to hours due to mineralized zones (alunite, anhydrite) that open/close in response to pressure variations. Our results indicate the rapid formation of a hydrothermal seal prior to the 2019 Whakaari eruption that consolidates around 3-4 days before the volcanic event.
Volcanoes and data streams studied
We analysed tremor time-series at different frequency bands prior to 18 eruptions at six volcanoes: Whakaari, Ruapehu and Tongariro (New Zealand), Veniaminof and Pavlof (Alaska, USA), and Bezymianny (Russia). We used feature engineering to extract latent patterns from the tremor and then systematically mined these for statistically differentiable correlations across all volcanoes in the weeks prior to eruptions. A noticeable correlation identified is a characteristic peak in the 2-day median of normalized DSAR tremor (displacement seismic amplitude ratio, DSAR, between medium, 4-8 Hz, and high, 8-16 Hz, frequency tremor) that recurred prior to eruptions at several volcanoes.
Major findings
Two precursors named ‘DSAR median’ and ‘DSAR rate variance’ are especially similar across five Whakaari eruptions, and also recognised prior to Ruapehu, and Tongariro eruptions. For the Whakaari 2019 eruption, we relate these precursors with hydrothermal reservoir sealing and pressurization that prepared the system for a phreatic eruption. Five distinct phases track the onset to such shallow gas and steam-driven eruption: (1) deep source fluid input into a geothermal reservoir; (2) pulsatory gas fluxing with weak temporary sealing over one or more weeks; (3) strong seal formation over the hydrothermal system several days before the event (4) leading to a critically pressured shallow aquifer (in near-equilibrium with deep gas pressures); before (5) external perturbation, and/or cascading material failure and crack formation over ~10-20 hours, leading to final seal breach and explosive decompression.
We found several common feature patterns, but just the ‘DSAR median’ and ‘DSAR rate variance’ can be classed as phreatic eruption precursors because they are recurrent, differentiable from non-eruptive repose, and somewhat transferable to other volcanoes. Monitoring these observables could help to improve real-time eruption forecasting, and provide new tools and warning products, ultimately saving lives, especially at regularly visited touristic volcanoes.