Dynamic velocity and seismic characteristics of gravitational rockfalls at the Merapi lava dome

Gravitational instability of discrete lava lobes in an active dome-building volcano may generate rockfalls that travel up to hundreds of meters from the summit. Understanding the behaviour of rockfalls is important for volcanic hazard assessment as rockfalls can precede large dome collapses that generate pyroclastic flows. Here, we investigate the mechanisms, trajectory, velocity and seismic characteristic of rockfalls at the Merapi volcano through high-resolution video and seismic-spectrum frequency analysis. High-resolution videos of rockfall events were converted to images for every second and were scaled with a georeferenced 3D point cloud to convert pixels into scaled units on the order of a meter. The scaled images were analysed using pixel tracking to investigate the mechanism, trajectory, and velocity of rockfalls. Results illustrate that the rockfall mechanisms include gravitational free fall, rolling and bouncing, airborne rotation and rolling over high-friction basal surfaces. The dynamic mechanisms of gravitational rockfalls at Merapi are strongly controlled by the topography and the condition of the basal surface, which is responsible for the continuous fluctuations in rockfall velocity. Maximum rockfall velocity occurs when rocks become airborne while rotating over the steepest area with velocity up to ~90 m/s. Material rolling over gently sloping, high-friction basal surfaces can significantly reduce the rockfall velocity to 2 m/s, which then ceases the rock movement. Our high-resolution video documents the relatively short durations of gravitational rockfall events at Merapi in the range of 24–61 s and has different time duration with the short-period seismic signals as the movement of small rock fragments over sandy area is not detected by seismic signal. Spectral analysis of seismic frequencies indicates that gravitational rockfalls at Merapi are characterized by high frequencies, with a range of 4–17 Hz. This study provides new insights and improves our understanding of the dynamic mechanism of rockfall events at dome-building volcanoes. © 2020 Elsevier B.V.