Simulation of the 2012 Te Maari debris avalanche: Insight into the failure mechanics and the role of the hydrothermal system

Abstract

Composite volcanoes consist of alternating layers with varying mechanical properties, which contribute to the instability of the flanks. This instability can lead to the onset of mass flows down volcanic slopes, posing significant risks to nearby populations and infrastructures. Tongariro, an active andesite volcano, experienced one of New Zealand's most recent debris avalanches at the Upper Te Maari crater on August 6, 2012. This debris avalanche, initiated simultaneously with a small-magnitude earthquake, released a volume of 7 × 105 m3 of material from the source, which by unloading the pressurised vapour-dominated hydrothermal system, led to a phreatic eruption. This paper aims to better constrain the preparatory and triggering factors, along with the failure mechanics, that led to the 2012 debris avalanche. To achieve this, we applied slope stability finite-element modelling to assess the volcanic slope's sensitivity to varying groundwater, seismic and mechanical conditions. Model results closely match the observed failure when considering the strength of hydrothermally altered rocks subjected to an increased pore pressure at shallow depth. We found that even a relatively minor rise in pore pressure, ≈ 250 kPa in the upper layers, could replicate the observed failure at Te Maari. Our simulations also reveal that this debris avalanche might be a multiple-stage failure involving the progressive sliding of two distinct blocks. These findings enhance our understanding of Tongariro's structure and improve hazard assessments for future potential collapses at Tongariro and other New Zealand volcanoes.