John Browning, BSc, MSc, PhD
Caldera ring-fault mechanics
By combining mapping of a well-exposed caldera ring-fault at Hafnarfjall caldera, Iceland with FEM numerical models we were able to provide a mechanical model for the control of magma deflection and capture at caldera ring-faults.
The conditions which lead to caldera collapse are still poorly constrained. As there have only been four, possibly ﬁve, well-documented caldera forming events in the past century, the geodetic signals produced during chamber roof subsidence, or chamber volume reduction (shrinkage) in general, are not well documented or understood. In particular, when two or more geodetic sources are operating and providing signals at the same time, it is important to be able to estimate the likely contribution of each. Simultaneous activities of different geodetic sources are common and include pressure changes in magma chambers/reservoirs occurring at the same time as dyke emplacement. Here we present results from numerical models designed to simulate the subsidence of a magma-chamber roof, either directly (chamber shrinkage) or through ring-fault displacement, and the induced surface deformation and crustal stresses. We consider chamber depths at 3 km, 5 km, and 7 km below the crustal surface, using both non-layered (isotropic) and layered (anisotropic) crustal models. We also model the effects of a caldera lake and of a thick ice cover (ice sheet) on top of the caldera. The results suggest that magma-chamber roof subsidences between 20 m and 100 m generate large (tens of centimetres) vertical and, in particular, hori-zontal displacements at the surfaces of the ice and the crust out to distances of up to tens of kilometres from the caldera/chamber centre. Crustal layering tends to reduce, but increasing chamber depth to enlarge, the horizontal and vertical surface displacements. Applying the results to the ice subsidence in the Bardarbunga Caldera during the 2014–2015 Bardarbunga–Holuhraun volcanotectonic episode indicates that the modelled ice displacements are less than those geodetically measured. Also, the geodetically measured crustal displacements are less than expected for a 60 m chamber-roof subsidence. The modelling results thus suggest that only part of the ice subsidence is due to chamber-roof subsidence, the other part being related to ﬂow in and down-bending of the ice. We show that such a ﬂow is likely within the caldera as a result of the stress induced by the 45-km-long regional dyke emplaced (primarily in vertical magma ﬂow) during the episode. This conclusion is further sup-ported by the model results suggesting that the ring-fault (piston-like) displacements must have been much less than the total 60 m ice subsidence, or else faults with tens-of-metres displacements would have cut through the ice (these are not observed). We suggest that the ring-fault subsidence was triggered by small doming of the volcanic ﬁeld and system hosting the Bardarbunga Caldera and that this doming occurred as a result of magma inﬂow and pressure increase in a deep-seated reservoir. The doming is conﬁrmed by GPS measurements and supported by the seismicity results. The magmatic pressure increase in the reservoir was, in terms of the present model, responsible for the regional dyke emplacement, the Holuhraun eruption, and part of the stress concentration around, and displacement of, the Bardarbunga Caldera.
Agri Dagi volcano, Turkey
Although seismic tomography indicates a magma reservoir at great depths (>20–30 km) below the Ağrı Dağı volcano, geochemical constraints on some of the later-formed rocks suggest an interaction between a shallow chamber (at 8–10-km depth) and the deep reservoir approxi-mately 0.5 Ma. We provide numerical models whose results indicate that dykes injected from the lateral margins of the deep-seated reservoir are more likely to reach the surface directly rather than replenish the shallow magma chamber, suggesting also that the compartment for the second eruption was at the margin of the reservoir.
Triple junction tectonics
Few places on Earth are as tectonically active as the Karlıova region of eastern Turkey. In this region, complex interactions between the Arabian, Eurasian and Anatolian plates occur at the Karlıova Triple Junction (KTJ). The relationship between tectonics and magma propagation in triple-junction tectonic settings is poorly understood.
I utilize Finite Element methods to calculate stress and strain in materials both in the design stage of deformation apparatus and to understand volcanic and seismic processes.
FEM numerical modelling
Forecasting magma chamber rupture
How much magma needs to be added to a shallow magma chamber to cause rupture, dyke injection, and a potential eruption? Models that yield reliable answers to this question are needed in order to facilitate eruption forecasting. Development of a long-lived shallow magma chamber requires periodic influx of magmas from a parental body at depth. This redistribution process does not necessarily cause an eruption but produces a net volume change that can be measured geodetically by inversion techniques. Using continuum-mechanics and fracture-mechanics principles, we calculate the amount of magma contained at shallow depth beneath Santorini volcano, Greece.