Poroelastic Mechanical Behavior of Crystal Much Reservoirs: Insights Into the Spatio-Temporal Evolution of Volcano Surface Deformation

Characterizing the physical properties and mechanical behavior of melt reservoirs is essential for enhancing geophysical models that aim to understand the evolution of subvolcanic systems and support hazard forecasting. Increasing evidence suggests that shallow magmatic reservoirs consist of variably packed crystal frameworks with small volumes of interstitial melt, commonly referred to as "mushes." Current volcano deformation models often implement static magma sources with a cavity and thus provide little insight into dynamic internal reservoir processes; they also ignore the presence of crystals, melt and other fluids, and therefore the likely poroelastic mechanical response to melt addition or withdrawal. Here we investigate the influence of poroelastic mechanical behavior on reservoir pressure evolution and resultant spatio-temporal surface deformation. We consider the melt reservoir to be largely crystalline (10%-50% melt fraction) with melt distributed between crystals; we show that the presence of crystals affects the spatial and temporal mechanics of magma reservoir behavior. In contrast to classical models for volcanic surface deformation, our results suggest that a poroelastic surface deformation response continues to develop after withdrawal/upward emplacement of melt has terminated, and importantly that the withdrawal/injection point can affect the evolution of the relative magnitudes of vertical and radial deformation over time. These protracted displacements are caused by melt diffusion, which depends principally on mush hydraulic properties and melt characteristics. Following an intrusion/withdrawal event, a steady state is eventually reached when the fluid pressure is uniform in the mush reservoir. Plain Language Summary Predicting where and when volcanic eruptions may occur is the biggest challenge in volcanology; an important tool in this is the assessment of volcanic surface deformation. Ground deformation often accompanies magma accumulation/withdrawal into/out of a shallow reservoir. It is often modeled with the assumption that the shallow magmatic reservoir is a large melt-dominated body embedded in perfectly elastic rock. However, recent geophysical imaging studies suggest that magma reservoirs instead consist of small fractions of interstitial melt distributed within frameworks of crystals, called crystal mush. We present a new numerical model that accounts for the mechanical behavior of mush by considering it to behave as a poroelastic material. In our model, melt injection/withdrawal events produce deformation within the surrounding crust. The melt flow within the pores is coupled with the deformation of the solid crystal matrix via poroelasticity theory. Melt diffusion in the mush reservoir produces a time-dependent ground deformation that depends on the mush diffusivity coefficient, a function of the mush and the melt characteristics. Critically, our study confirms that continuous melt diffusion can cause reservoir inflation and surface deformation without ongoing intrusion or withdrawal of melt.