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

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 pre-eruptive dynamics.