Abstract
Deformation caused by processes within a volcanic conduit are localised, transient, and therefore challenging to measure. However, observations of such deformation are important because they provide insight into conditions preceding explosive activity, and are important for hazard assessment.Here, we present measurements of low magnitude, transient deformation covering an area of ∼4km2 at Masaya volcano spanning a period of explosive eruptions (30th April–17th May 2012). Radial uplift of duration 24days and peak displacements of a few millimeters occurred in the month before the eruption, but switched to subsidence ∼27days before the onset of the explosive eruption on 30th of April. Uplift resumed during, and continued for ∼16days after the end of the explosive eruption period. We use a finite element modelling approach to investigate a range of possible source geometries for this deformation, and find that the changes in pressurisation of a conduit 450m below the surface vent (radius 160m and length 700m), surrounded by a halo of brecciated material with a Young's modulus of 15GPa, gave a good fit to the InSAR displacements. We propose that the pre-eruptive deformation sequence at Masaya is likely to have been caused by the movement of magma through a constriction within the shallow conduit system.Although measuring displacements associated with conduit processes remains challenging, new high resolution InSAR datasets will increasingly allow the measurement of transient and lower magnitude deformation signals, improving the method's applicability for observing transitions between volcanic activity characterised by an open and a closed conduit system.
Highlights
Deformation caused by processes within a volcanic conduit are localised, transient, and challenging to measure
Characteristic eruptive behaviour at a vol- prevents the free flow of gas and magma between the magma resercano depends on the magma ascent rate, which is in turn determined voir and the surface are known as closed conduit systems (Worster by subsurface geometries, physical properties of the magma and et al, 1993; Tazieff, 1994)
Shear stresses have only been shown to cause deformation at high viscosity systems (e.g. Beauducel et al, 2000; Chadwick et al, 1988; Dzurisin et al, 1983; Green et al, 2006), and most examples of conduit pressurisation attributed to abrupt decreases in shear stress have occurred rapidly, while subsidence at Masaya was observed over ∼27 days
Summary
Displacements caused by transient processes in volcanic conduits are localised and of short duration, and are rarely captured by Interferometric Synthetic Aperture Radar (InSAR) (e.g. Wadge et al, 2006). Cycles of pressurisation-depressurisation prior to explosive activity have been observed at Soufrière Hills Volcano, at Mount St. Helens and at Fuego (Anderson et al, 2010; Voight et al, 2010; Lyons and Waite, 2011). Other studies have found that shear stresses provide a better explanation for within the conduit radial displacements at Mount St. Helens (observation period 1981–1982), Mount Merapi and Soufrière Hills Volcano (Dzurisin et al, 1983; Chadwick et al, 1988; Beauducel et al, 2000; Green et al, 2006)
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