Probabilistic modeling of tephra dispersal: Hazard assessment of a multiphase rhyolitic eruption at Tarawera, New Zealand

The Tarawera Volcanic Complex comprises 11 rhyolite domes formed during five major eruptions between 17,000 B. C. and A. D. 1886, the first four of which were predominantly rhyolitic. The only historical event erupted about 2 km(3) of basaltic tephra fall (A. D. 1886). The youngest rhyolitic event erupted a tephra fall volume more than 2 times larger and covered a wider area northwest and southeast of the volcano (similar to A.D. 1315 Kaharoa eruption). We have used the Kaharoa scenario to assess the tephra fall hazard from a future rhyolitic eruption at Tarawera of a similar scale. The Plinian phase of this eruption consisted of 11 discrete episodes of VEI 4. We have developed an advection-diffusion model (TEPHRA) that allows for grain size-dependent diffusion and particle density, a stratified atmosphere, particle diffusion time within the rising plume, and settling velocities that include Reynolds number variations along the particle fall. Simulations are run in parallel on multiple processors to allow a significant implementation of the physical model and a fully probabilistic analysis of inputs and outputs. TEPHRA is an example of a class of numerical models that take advantage of new computational tools to forecast hazards as conditional probabilities far in advance of future eruptions. Three different scenarios were investigated for a comprehensive tephra fall hazard assessment: upper limit scenario, eruption range scenario, and multiple eruption scenario. Hazard curves and probability maps show that the area east and northeast of Tarawera would be the most affected by a Kaharoa-type eruption.