School Seminar: Volcano-tectonic earthquakes as indicators of magma viscosity
Dr Diana Roman, Bristol Benjamin Meaker Distinguished Visiting Professor
Volcanic microearthquakes have long provided a key indication of eruption likelihood, but to date have not been understood to provide any information on eruption style. Magma viscosity strongly controls the style (for example, explosive versus effusive) of a volcanic eruption and thus its hazard potential, but can only be measured during or after an eruption. Therefore, the identification of precursors indicative of magma viscosity would enable forecasting of the eruption style and the scale of associated hazards. A review of case studies of high-frequency volcanic microearthquakes (volcano-tectonic earthquakes, or ‘VTs’) indicates an apparent link between magma viscosity and the occurrence of an approximately 90° rotation of accompanying VT earthquake fault-plane solutions (FPS), which may reflect stresses produced in the walls of an inflating or pressurizing dike. Such stress-field rotations of around 90° have been documented only in cases where the erupted magma is highly evolved and/or partially crystalline, and thus may reflect the magnitude of stress that is normal to the flow direction during magma emplacement. However, numerical modeling demonstrates that a strongly deviatoric ambient stress field can suppress a local stress-field rotation during dike emplacement. A recently-published study of VT FPS orientations preceding and accompanying the 2018 eruption of Kilauea, Hawai’i, conclusively shows, for the first time, that rotations of the local stress field are uniquely linked to the flow of relatively high-bulk-viscosity magma, thus their identification provides constraints on magma viscosity and anticipated eruption style. Furthermore, analysis of the bulk viscosity of Kilauea 2018 and earlier lavas provides a tight constraint on the minimum bulk viscosity associated with stress field reorientation, a key variable for probablistic hazard assessments.
Speaker website: Diana Roman.
This will be hybrid meeting taking place in G27 Wills Memorial Building and online via https://bristol-ac-uk.zoom.us/j/95638888104.