Precursory changes in well water level prior to the March, 2000 eruption of Usu Volcano, Japan

Shibata, Tomo; Akita, Fujio

doi:10.1029/2000gl012467

Cited by 31 publications

(28 citation statements)

References 15 publications

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“…Although intrusion‐induced pore pressure increases in the simulations are much larger at the “core” location (<2.5 MPa), even the relatively small pore pressure increases on the slopes (Figures c and e) and water‐table increases of tens of meters (Figures b and d) can reduce the factor of safety and the stability of the edifice (Table ). The large simulated water level changes are consistent with observations of large water level fluctuations (<100 m) in wells, increased flow at hot springs, and large influxes of thermal water into drainages as a result of inferred magma intrusion (either within the edifice itself or at greater depth; Matsumoto et al, ; Newhall et al, ; Shibata & Akita, ). Mechanical processes (strain) tend to be invoked to explain changes in water levels (Hurwitz & Johnston, ; Newhall et al, ; Shibata & Akita, ; Strehlow et al, ).…”

Section: Discussionsupporting

confidence: 81%

Combining Multiphase Groundwater Flow and Slope Stability Models to Assess Stratovolcano Flank Collapse in the Cascade Range

et al. 2018

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Hydrothermal alteration can create low‐permeability zones, potentially resulting in elevated pore‐fluid pressures, within a volcanic edifice. Strength reduction by rock alteration and high pore‐fluid pressures have been suggested as a mechanism for edifice flank instability. Here we combine numerical models of multiphase heat transport and groundwater flow with a slope‐stability code that incorporates three‐dimensional distributions of strength and pore‐water pressure to address the following questions: (1) What permeability distributions and contrasts produce elevated pore‐fluid pressures in a stratovolcano? (2) What are the effects of these elevated pressures on flank stability? (3) Finally, what are the effects of magma intrusion on potential flank failure in an edifice? Simulation results show that under a range of plausible parameters, water tables in a stratovolcano can be elevated or perched. These elevated water tables result in universally lower stability (lower factor of safety) compared with equivalent dry edifices, indicating a higher likelihood of flank collapse. Low‐permeability (<1 × 10−17 m2) layers such as altered pyroclastic deposits or breccias can result in locally saturated regions (perched water) and lower factors of safety near the ground surface but may actually reduce liquid water saturation and pore pressures in the core of the edifice and thus may favor small, shallow collapses over larger, deeper collapses. Magma intrusion into the base of the edifice increases pore‐fluid pressures and decreases the factor of safety. However, the shear strength of edifice rocks also exerts a significant control on stability, so both mechanical properties and pore‐fluid pressures are important for stability assessments.

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Section: Discussionsupporting

confidence: 81%

Combining Multiphase Groundwater Flow and Slope Stability Models to Assess Stratovolcano Flank Collapse in the Cascade Range

et al. 2018

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“…These water‐level rises are caused by crustal compression as magma neared the surface. No variations in temperature and chemical composition of thermal waters and fumaroles were evident prior to the eruption [ Shibata and Akita , ; Matsumoto et al , ; Shibata et al , ]. During the early phase of the eruption, uplift of the Usu edifice was geodetically detected.…”

Section: Types Of Caldera Unrestmentioning

confidence: 99%

An overview of recent (1988 to 2014) caldera unrest: Knowledge and perspectives

Acocella

Lorenzo

Newhall³

et al. 2015

Reviews of Geophysics

154

165

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Calderas are among the most active and dangerous volcanoes. Caldera unrest is defined by enhanced seismicity, gravity changes, surface deformation, and degassing. Although much caldera unrest does not lead to an eruption, every eruption is preceded by an unrest episode. Therefore, the proper description of unrest and the forecast of its possible outcome is a timely and challenging task. Here we review the best known unrest at calderas from 1988 to 2014, building on previous work and proposing an updated database. Where established, the root cause for unrest is always magmatic; none was purely hydrothermal or tectonic. An interpretive classification of unrest invokes two spectra-compositional (mafic to felsic) and the state of magma conduits feeding from the magma reservoir(s) to the surface (from fully plugged, through semiplugged, to open). Magma and gas in open conduits can rise and erupt freely; magma in semiplugged conduits erupts less frequently yet still allows some gas to escape; plugged conduits allow neither magma nor gas to escape. Unrest in mafic calderas is subtler, less pronounced, and repeated, especially with open systems, ensuring the continuous, aseismic, and moderate release of magma. Plugged felsic calderas erupt infrequently, anticipated by isolated, short and seismically active unrest. Semiplugged felsic calderas also erupt infrequently and are restless over decades or centuries, with uplift, seismicity, and degassing and, on the longer-term, resurgence, suggesting repeated stalled intrusions. Finally, the expected advances in better understanding caldera unrest are discussed.

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“…Shibata and Akita, 2001;Takahashi et al, 2012). A method of assessing the strain sensitivity of an aquifer is to track water level changes as a result of predictable excitations such as Earth tides or measured barometric variations.…”

mentioning

confidence: 99%

Poroelastic responses of confined aquifers to subsurface strain and their use for volcano monitoring

2015

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Abstract. Well water level changes associated with magmatic unrest can be interpreted as a result of pore pressure changes in the aquifer due to crustal deformation, and so could provide constraints on the subsurface processes causing this strain. We use finite element analysis to demonstrate the response of aquifers to volumetric strain induced by pressurized magma reservoirs. Two different aquifers are invoked -an unconsolidated pyroclastic deposit and a vesicular lava flow -and embedded in an impermeable crust, overlying a magma chamber. The time-dependent, fully coupled models simulate crustal deformation accompanying chamber pressurization and the resulting hydraulic head changes as well as flow through the porous aquifer, i.e. porous flow. The simulated strain leads to centimetres (pyroclastic aquifer) to metres (lava flow aquifer) of hydraulic head changes; both strain and hydraulic head change with time due to substantial porous flow in the hydrological system.Well level changes are particularly sensitive to chamber volume, shape and pressurization strength, followed by aquifer permeability and the phase of the pore fluid. The depths of chamber and aquifer, as well as the aquifer's Young's modulus also have significant influence on the hydraulic head signal. While source characteristics, the distance between chamber and aquifer and the elastic stratigraphy determine the strain field and its partitioning, flow and coupling parameters define how the aquifer responds to this strain and how signals change with time.We find that generic analytical models can fail to capture the complex pre-eruptive subsurface mechanics leading to strain-induced well level changes, due to aquifer pressure changes being sensitive to chamber shape and lithological heterogeneities. In addition, the presence of a pore fluid and its flow have a significant influence on the strain signal in the aquifer and are commonly neglected in analytical models. These findings highlight the need for numerical models for the interpretation of observed well level signals. However, simulated water table changes do indeed mirror volumetric strain, and wells are therefore a valuable addition to monitoring systems that could provide important insights into preeruptive dynamics.

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Precursory changes in well water level prior to the March, 2000 eruption of Usu Volcano, Japan

Cited by 31 publications

References 15 publications

Combining Multiphase Groundwater Flow and Slope Stability Models to Assess Stratovolcano Flank Collapse in the Cascade Range

Combining Multiphase Groundwater Flow and Slope Stability Models to Assess Stratovolcano Flank Collapse in the Cascade Range

An overview of recent (1988 to 2014) caldera unrest: Knowledge and perspectives

Poroelastic responses of confined aquifers to subsurface strain and their use for volcano monitoring

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