Stability assessment of the Crater Lake/Te Wai-ā-moe overflow channel at Mt. Ruapehu (New Zealand), and implications for volcanic lake break-out triggers

The presence of lakes on active volcanoes gives rise to several unique volcano-hydrologic hazards, including those directly related to volcanic activity (e.g. phreatic explosions) and non-volcanic phenomena (e.g. lake outbursts). Mt. Ruapehu volcano hosts the 107 m3 Crater Lake/Te Wai-ā-moe in the active summit crater, making it prone to lahars due to the abundance of water stored at high elevation. Apart from eruptive periods when water is commonly ejected from the lake, large lahar events can occur when portions of the crater rim fail during quiescent periods, leading to partial lake drainage; the latter occurred recently in 1953 and 2007. Here, we investigate the potential of future lahars due to rim failure at the current overflow channel considering several triggering scenarios during non-eruptive periods. The geomechanical properties of a matrix-dominated breccia unit composing part of the overflow channel were analyzed using physical and mechanical laboratory methods. These properties, combined with previous geomechanical studies of lava units in the overflow channel, were used in finite element and limit equilibrium method-based groundwater and stability models. Groundwater models indicate that a low-permeability barrier inside the crater rim is preventing water from seeping through the unconsolidated and potentially permeable breccia unit of the overflow channel. The removal of this barrier, or weathering of a resistive lava cap, could cause water to seep through the overflow channel, elevating the potential of collapse. Slope stability models indicate that under certain conditions, the overflow channel is below a conservative threshold of stability and can be destabilized with continued mechanical or chemical weathering that reduces strength properties. This is also true during high magnitude tectonic or volcanic earthquakes, where large ground accelerations could result in several centimeters of displacement at the overflow channel, corresponding to high hazard categories of stability. In the worst-case scenario modeled here, full collapse of the overflow channel could result in the release of c. 2.4 million m3 of lake water. This could drive a lahar substantially larger than well-documented historic events, including the more common lahars produced during eruptions. The identification of key factors affecting stability can be used to inform future monitoring and the risk of these high impact events.