Volcanic Facies From Probabilistic Multi-Physics Characterization of an Andesite Island Volcano, Whakaari/White Island, New Zealand

Collapse of hydrothermally weakened rock on the flanks of volcanic islands is a recognized cause of tsunamis generated by volcanoes. Here we use a multiphysics clustering method to derive a volcanic facies model for Whakaari/White Island, an andesite arc volcanic island in New Zealand. Through probabilistic inversion of magnetic and gravity data, combined with airborne electromagnetic data inversion we derive density, susceptibility, resistivity and saturation models of the island. Petrophysical relationships between density, P-wave velocity and mean effective stress extends the range of physical properties mapped. A clustering algorithm identifies four clusters, that is facies, related to rock volumes characterized by varying degrees of hydrothermal alteration and saturation that occupy specific spatial locations in the edifice. Two volumes of rock (0.05–0.1 km³) in the west and north of the island, with contrasting facies properties are identified as the most hydrothermally altered or fractured parts of the island. Saturation models derived from resistivity models show the upper flanks are at low saturation, reducing their likelihood of failure. The submerged flanks become progressively more saturated with depth, in line with existing models of the hydrothermal system that show significant seawater input. The gravity and magnetic models delineate subcrater boundaries and highlight regions with different styles of alteration, including pore filling that increases rock density, and rock dissolution that decreases density. The model identifies new areas of potential slope instability, context for interpreting volcano monitoring data and quantified rock volumes for generation of scenarios which simulate tsunamis caused by volcanic landslides.