Implications of Thermal Weakening and Magma Compressibility for Volcano Monitoring

Abstract Magmatic intrusion and volcanic deformation are traditionally approximated with an inflating cavity in an elastic halfspace. Various analytical models predict surface displacement as a function of pressure or volume change, depth, and elastic crustal properties. While this approach allows for rapid inversion of monitoring data, it neglects important components such as the visco‐elastic behavior of thermally weakened host rock, magma transport, and magma compressibility. Here, we use 3D thermomechanical models to investigate the temporal evolution of overpressure and surface deformation due to magma influx into a pre‐existing reservoir. Instead of applying an internal source of pressure or volume in the magma body, we introduce a cylindrical feeding system which allows us to track the self‐consistent development of overpressure and investigate the effects of magma compressibility. Our model reproduces analytical solutions for an elastic host rock and a visco‐elastic shell. For thermally weakened, visco‐elastic host rock, we observe two effects: (a) The same overpressure causes more deformation, and (b) the same injected volume causes less overpressure. These effects partially cancel out, with a net uplift reduction due to thermal weakening. Considering magma compressibility further reduces the overpressure. Stacking all three effects results in up to 80% less surface deformation in our models compared to the elastic assumption. Consequently, intrusion volumes derived from surface deformation may be significantly underestimated, especially in young/cold magmatic systems. In thermally mature systems, magma overpressure is significantly overestimated. We propose modifications to the elastic solutions which can capture thermal weakening effects to first order.