The effects of thermomechanical heterogeneities in island arc crust on time‐dependent preeruptive stresses and the failure of an andesitic reservoir

Abstract

Using ground deformation data from Soufrière Hills volcano (SHV), we present results from numerical modeling of the temperature‐ and time‐dependent stress evolution in a mechanically heterogeneous crust prior to reservoir failure and renewed eruptive activity. The best fit models do not allow us to discriminate between a magmatic plumbing system consisting of either a single vertically elongated reservoir or a series of stacked reservoirs. A prolate reservoir geometry with volumes between 50 and 100 km3, reservoir pressure changes between 4 and 7 MPa, and reservoir volume changes between 0.03 and 0.04 km3 with magma compressibility between 4 × 10−11 and 1 × 10−9 Pa−1 provide plausible thermomechanical model parameters to explain the deformation time series; around an order of magnitude less overpressure than is generally inferred from homogeneous, elastic crustal models. Reservoir failure is predicted to occur at the crest of the reservoir except for reservoirs with highly compressible magma (≳4×10−9 Pa) for which subhorizontal sill formation is predicted upon reservoir failure. Introducing a deep‐crustal hot zone modulates the partitioning of strains into the hotter underlying crust and results in a further reduction in overpressure estimates to values of around 1–2 MPa upon reservoir failure. Deduced volume fluxes are consistent with constraints from thermal modeling of active subvolcanic systems and imply dynamic failure of a compressible magma mush column feeding eruptions at SHV. Our interpretation of the results is that the combined thermomechanical effects of a deep‐crustal hot zone and hot encasing rocks around a midcrustal andesitic reservoir fundamentally alter the time‐dependent subsurface stress and strain partitioning upon reservoir priming. These effects substantially influence surface strains recorded by volcano geodetic monitoring.

Sustained Uplift at a Continental Rift Caldera

“…External triggers represent a source of unpredictability of potential eruptions and are not considered here. At Corbetti, there is no evidence of an eruption in recent decades, or major form of reservoir failure, meaning that the reservoir must be large and/or compressible enough to accommodate the strain associated with the intruding magma (Degruyter et al, ; Gottsmann & Odbert, ). External evidence for a large (∼10 2 km 3 ) magma reservoir comes from the MT observations and the high degree of fractionation required to produce the peralkaline, aphyric erupted products.…”

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

“…Depth-and temperature-dependent changes of Young's modulus of the crust are ignored for simplicity. We chose a Young's modulus of 30 GPa, representing an average value for crustal rocks in arcs for depths up to 8 km as derived from seismic velocity data (Gottsmann and Odbert, 2014). We test for two typical aquifer types found in volcanic regions: unconsolidated pyroclastic deposits, commonly composed of coarse ash to fine lapilli sized clasts, and Table 3 Water compressibility χ f 4 × 10 −10 Pa −1 changed acc.…”

Deposit loading and its effect on co-eruptive volcano deformation

“…Loading stresses caused by ice (Pinel et al, 2007) and lava flows (Grapenthin et al, 2010) via long-term (decades), sub-crustal viscous flow have been modelled to explain deformation measured in Iceland, where the elastic crust is comparably thin. Models of ground deformation due to surface loading by a relatively small (few million cubic metres) volume of erupted lava at Merapi volcano suggested that displacements observed outside the crater were not significantly affected (Beauducel et al, 2000).…”