Insights Into the 2020 Instability Crisis of Merapi Through Numerical Modeling

Abstract The northwestern flank of Merapi volcano (Java, Indonesia) moved from July 2020 to January 2021. The cliff collapses at the summit, the size of the affected area (about 500 m in elevation and 1 km wide) and a flank displacement of about 14 m, never observed before, led to fears of a flank collapse. Field observations suggested a correlation between this flank movement and the opening of a NE‐SW fracture crossing the entire summit, filled with magma. In the present study, we developed numerical models based on the discrete element method to investigate the role of the NE‐SW fracture pressurization on the deformation of Merapi. Even though simplified, considering the complexity of the processes at stake, our approach manages to reproduce the observed deformations both in terms of kinetics and kinematics. Specifically, the model reproduces the inelastic deformation of the northwestern flank to a depth of about 500 m below the summit. Under the pressure of the magma, the northwestern flank tilted slightly, causing its sliding with displacement magnitudes increasing with elevation. As observed on‐site, the sliding eventually stopped so that the flank reached a new stable state, demonstrating how topographic readjustments of a volcano, driven by magma pressure, can produce significant plastic deformation without leading to a major flank collapse.