Exploring the pre-eruptive geodetic signature of a future M7 explosive eruption
Recent eruptions of intermediate magmas in volcanic arcs with well-documented pre-eruptive geodetic signatures were of small to modest magnitude. The pre-eruptive ground deformation of a future large magnitude intermediate eruption similar to that of Tambora in 1815 is thus poorly understood. Here, I explore a potential pre-eruptive geodetic signature of a M7 intermediate magma chamber using constraints from the (data-poor) Tambora case in contrast to the (data-rich) case of the current small-magnitude eruptive period of Soufriere Hills volcano (SHV) using a set of analytical and numerical mechanical models.
I establish a chamber failure criterion based on rock tensile strength and forward model pre-failure ground displacements starting with the simple assumption of elastic mechanical behaviour of surrounding rocks. Accounting for gravitational loading the results demonstrate that a static failure criterion is inadequate to explain cyclic eruptive behaviour at SHV-type systems, given observed pre-eruptive deformation amplitudes and petrologically deduced storage conditions.
The same applies for a Tambora-type system, where forward models of permissible (but unrealistically large) chamber pressures predict several meters of pre-eruptive uplift with a wavelength of tens of kilometers, when assuming elastic crustal mechanics. Results indicate that pressurisation of a small and shallow-seated chamber (SHV-type) is more likely to rupture and repeatedly feed intrusions or small magnitude eruptions. However, even in this case anelastic effects appear to be important to explain the dynamic behaviour during the current activity at SHV.
Although, there is a first order influence of edifice load, topography, and mechanical heterogeneity of encasing rocks on the stress distribution and the resultant deformation field which are explored using a Finite Element Analysis, time-dependent stress dissipation must play a first-order role in growing an intermediate magma chamber of substantial size and thereby inhibiting pre-mature failure.
The modeling shows that a static failure criterion based on the tensile strength of encasing rocks is problematic in the context of elastic mechanical behavior of rocks as its influence is found to be second order for magma stored at lithostatic pressures of between 50 and 220 MPa, i.e., within a pressure window constrained petrologically for many recent compositionally intermediate magmas. Combining the modelling results with published thermal and petrological findings, I deduce an upper limit for the permissible volumetric strain rate upon magma pressuriation of 10E-3/s to enable growth of a large body of eruptible magma of intermediate composition and thereby preventing its pre-mature failure. Resultant ground displacements induced by incremental magma input into a relatively short-lived (compared to silcic) intermediate magma chamber is likely well below 1 cm/year when accounting for mechanical heterogeneity and time-dependent stress relaxation. Models assuming elastic mechanical behaviour of encasing rocks to explain pre-eruptive ground deformation may provide unrealistic and misleading constraints on chamber characteristics.