Insights Into the Edifice Stability of Anak-Krakatau (Sunda Strait, Indonesia) Before the Lateral Collapse in December 2018 from Direct Shear Experiments and Finite-Element Models

The lateral collapse of oceanic volcanoes poses a high risk for the population living in coastal areas since the sudden displacement of large amount of material can trigger tsunami waves impacting the surrounding coastlines. One recent example is the lateral collapse of the SW-flank of Anak-Krakatau (Sunda Strait, Indonesia) in December 2018 that generated a tsunami wave impacting the Sunda Strait coastlines and causing several hundred fatalities. Even though, the lateral collapse of oceanic volcanoes are hazardous events, the precursors of such events are poorly understood. It is suggested that external triggers such as the movement of a décollement, the rise of magma during enhanced activity, or earthquakes can cause a lateral collapse. Yet, the internal state of stability of a volcano needs to be known to evaluate the impact of external triggers. We carry out direct shear tests on samples from Anak-Krakatau and implement the results into finite-element models to evaluate the influence of the volcano’s geometry and the rock mechanical properties on the stability of Anak-Krakatau’s flank before the collapse in 2018. The preliminary results suggest that the volcanic edifice of Anak-Krakatau was unstable before the collapse in 2018 solely due to the geometry of the volcano and the rock mechanical properties. Whether the instability of the volcanic edifice is enough to cause the lateral collapse of Anak-Krakatau in 2018 or whether an external trigger is needed, needs further testing.