Dynamic models of magma storage within a damaging, strain-softening crust and their application to Sierra-Negra's pre-eruptive inflation pattern

Prior to its 2005 and 2018 eruptions Sierra Negra's caldera displayed decades-long inflation characterized by successive transient uplifts culminating in accelerating uplifts in the 1-2 years prior to each eruption. These observations motivated the question: can these transient uplifts be explained by internal dynamics of the magma system rather than ad-hoc changes in magma influx from below? To understand these transients, we develop dynamical models governing the coupled time evolution of magma transport, magma storage and associated crustal strain. Our models account for changes in the effective mechanical properties of the volcanic edifice due to damage manifest through earthquakes, and the resulting strain-softening behavior: a decline in stress with increasing strain. Our models assume that the damage increases as a function of crustal strain resulting from magma storage within a shallow reservoir. The strain rate is primarily controlled by magma influx driven by the fluid pressure gradient between the shallow reservoir and its deeper source. As the reservoir’s volume increases due to the magma influx, the crust is deformed and damaged, lowering its effective mechanical properties and affecting its capacity to store magma. This mechanism allows us to simulate volcanic inflation with fluctuating rates matching key features of Sierra Negra's pre-2018 eruption uplift and seismicity patterns. Our study demonstrates that damage can exert top-down control over the magma system. It induces changes in the ascending magma flux resulting in deformation transients consistent with observations at Sierra Negra without the need to invoke ad-hoc changes in the magma supply rate.