Volcanic precursors at Mt. Ruapehu (New Zealand): a numerical modelling approach

For the purpose of forecasting eruptive activity at volcanoes that host active hydrothermal systems (HTS), it is important to discriminate anomalies caused by processes related to either magma or fluid flow. Here we develop 2D axisymmetric numerical models to study magmatic and non-magmatic unrest at Mt. Ruapehu where ascending fluids feed an active HTS including a summit crater lake. We forward model ground displacements, and changes in gravitational and electrical potential fields from two distinct scenarios (S1 and S2) of subsurface processes: S1) pressurisation of a transcrustal mush zone from magmatic rejuvenation coupled with poroelastic responses of the HTS and S2) thermo-poroelastic processes due to the injection of hot, multi-component and multiphase fluids (H2O and CO2) at the bottom of Ruapehus HTS. All models account for topography and subsurface mechanical and hydraulic heterogeneities. Preliminary results show that geophysical observables are distinct in their magnitude and wavelength in both unrest scenarios. For a set of plausible reference parameters, ground displacements predicted by both approaches are below accepted detection levels (1 cm in the vertical and 0.5 cm in the horizontal). Concurrent residual gravity changes are detectable but of opposite polarity, with maximum amplitudes of 8 Gal (S1, for a source density change of 10 kg/m3) and -45 Gal (S2). The concurrent maximum magnitude of the self-potential (SP) anomaly in S2 is ~3 times larger than in S1 with detectable SP signals rapidly falling off with distance from the HTS. Parameter testing for S1 shows that surface displacements become detectable for reservoir strengths (V/P) of >0.7 km3/MPa with SP signal strength (variations over several orders of magnitude) and polarity strongly dependent on the hydro-electric coupling coefficient. Concurrent residual gravity changes are predominantly controlled by reservoir density changes. The largest geophysical anomalies for S2 are predicted to occur directly above the HTS. Signal magnitudes correlate with simulated fluid fluxes. Preliminary interpretations of the findings are that detectable gravity and deformation anomalies accompany magmatic processes at Mt. Ruapehu, whereas gravity and self-potential anomalies are readily detectable indicators for changes in the HTS.