Multiphysics Modeling of Volcanic Unrest at Mt. Ruapehu (New Zealand)

Pre-eruptive signals at the crater lake-bearing Mt. Ruapehu (New Zealand) are either absent or hard to identify. Here, we report on geophysical anomalies arising from hydrothermal unrest (HTU) and magmatic unrest (MU) using multiphysics numerical modeling. Distinct spatio-temporal anomalies are revealed when jointly solving for ground displacements and changes in gravitational and electrical potential fields for a set of subsurface disturbances including magma recharge and anomalous hydrothermal flow. Protracted hydrothermal injections induce measurable surface displacements (>0.5 cm) at Ruapehu's summit plateau, while magmatic pressurization (5-20 MPa) results in ground displacements below detection limits. Source density changes of 10 kg/m(3) (MU simulations) and CO2 fluxes between 2,150 and 3,600 t/d (HTU simulations) induce resolvable residual gravity changes between +8 and -8 mu Gal at the plateau. Absolute self-potential (SP) anomalies are predicted to vary between 0.3 and 2.5 mV for all unrest simulations and exceed the detection limit of conventional electric surveying. Parameter space exploration indicates that variations of up to 400% in the Biot-Willis coefficient produce negligible differences in surface displacement in MU simulations, but strongly impact surface displacement in HTU simulations. Our interpretation of the findings is that monitoring of changes in SP and gravity should permit insights into MU at Ruapehu, while HTU is best characterized using ground displacements, residual gravity changes and SP anomalies. Our findings are useful to inform multiparameter monitoring strategies at Ruapehu and other volcanoes hosting crater lakes. Plain Language Summary Eruptions at Mt. Ruapehu in New Zealand often occur without any warning amid an absence of what are called pre-eruptive geophysical signals. In order to study the detectability of relevant geophysical signals, we use physics-based models to simulate two distinct subsurface processes at Mt. Ruapehu: magma accumulation and flow of hydrothermal fluids. Both processes involve fluid flow, density variations and pressure changes in the ground and have the potential to trigger eruptive activity. Here we identify distinct measurable sets of geophysical signals from either subsurface process: changes in the electrical and gravitational potential fields, or surface deformation. Our study highlights the benefit of computer models to provide useful information on the link between subsurface processes and measurable geophysical signals prior to eruptive activity. Our findings may have implications for volcano monitoring efforts at Mt. Ruapehu and other crater lake volcanoes.