Deformation and Gravitational Instability at Anak Krakatau (Sunda Strait, Indonesia): Insights From Direct Shear Experiments and Finite-Element Models

Volcanic edifices are known to be unstable and their collapses have been observed several times in history. A collapse of island or coastal volcanoes is especially hazardous for the population living on regional shorelines due to the generation of destructive tsunami waves. A recent example of this is the tsunami-generating collapse of Anak Krakatau's (Sunda Strait, Indonesia) southwestern flank in December 2018. At Anak Krakatau, deformation through sliding of the SW-flank, preceding the collapse, is evident from published InSAR data. However, little is known about the lithology and internal structure of the pre-collapse edifice. Here, we combine direct shear experiments and finite-element models to understand edifice deformation and gravitational edifice instability at Anak Krakatau. Our model results suggest that Anak Krakatau deformed through a proto shear plane within the edifice prior to the collapse. To cause a gravitationally unstable edifice, our models indicate that the proto shear plane must dip >12°, or the rock mass or proto shear plane must be weakened to μ < 0.3. These threshold values however seem unlikely in nature and thus, an external factor is needed to trigger catastrophic collapse. Our direct shear experiments indicate that samples of powdered gouge from ash and scoria are strong, exhibiting μ > 0.6. Furthermore, the results of the direct shear experiments support catastrophic collapse along a proto shear plane due to velocity-weakening frictional behavior. Hence, assessment of flank collapse potential requires knowledge of material properties and the presence, strength, and geometry of weak planes within the edifice.