Structures involved in the vertical deformation at Lake Taupo (New Zealand) between 1979 and 2007: New insights from numerical modelling

Since 1979, repeat levelling measurements have been conducted on the lake filling the caldera of the dormant Taupo rhyolitic volcano in the North Island of New Zealand. Interpretation of these data through numerical modelling provides information on the structures involved in the relative vertical crustal movement throughout the southern end of the Taupo Volcanic Zone (TVZ), an area of active back-arc extension. The best-defined feature is a long term global subsidence of the northern part of the lake (7 mm yr− 1) due to the cumulative effect of the crust stretching and a deep deflation source. This long term subsidence is occasionally disturbed by strong short-term uplifts linked with overpressure sources located below the northern part of the lake, near active geothermal fields. Episodes of uplift can be attributed to various combinations of the following two processes taking place beneath the geothermal field (1) Movement or formation of rhyolitic magma (deepest sources) (2) Pressurization of the shallow hydrothermal fluid reservoir that traps volatiles exsolved from a crystallizing rhyolitic magma (shallowest sources). The pressurization of the shallow hydrothermal system gives rise to tensional stresses in the upper crust, resulting in seismic and aseismic fault ruptures. Slow slip motion of the Kaiapo fault decouples on a short-term time scale the ground deformation pattern on both sides of the fault. Our results, discriminating what parts of the deformation are due to the regional setting, the hydrothermal circulations and the seismic activity, reveal that each seismic swarm is preceded by 1 to 3 years of inflation of the eastern part of the lake. This systematic behaviour may allow us in the future to better predict the occurrence of the seismic swarms below the lake.