Understanding volcano hydrothermal unrest from geodetic observations: Insights from numerical modeling and application to White Island volcano, New Zealand

In this work we assess how volcano geodetic observations can be used to gain insights into hydrothermal system dynamics. We designed a range of numerical models of hydrothermal unrest and associated ground deformation caused by the thermo-poro-elastic response of the substratum. Throughout an episode of unrest, ground deformation is consistently first controlled by the poroelastic response of the substratum to pore pressure increase near the injection area. Later, thermal expansion may become the dominant process if the injection is sustained. We inverted these synthetic geodetic data using simple conventional pressure source models and compared the retrieved source characteristics with that of the synthetic hydrothermal systems. Simple pressure source models can reproduce well ground deformation caused by pore-pressure increase at depth. Most importantly, the pressure source's depth retrieved from the inversions corresponds to those of the area of injection of the hot magmatic fluids into the hydrothermal system. When the thermoelastic contribution to ground deformation becomes significant through time, simple point or spherical finite sources cannot reproduce the ground deformation signal. This allows one to determine whether observed ground deformation events due to hydrothermal unrest are distinct episodes of unrest and injection at depth, or whether one may correspond to the late, thermally-controlled phase of a previous event. Finally we applied this strategy to White Island volcano, New Zealand, to gain insights into the processes driving the last two episodes of ground uplift.