Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: Precursor, motion and aftermath

Abstract

Landslide hazard motivates the need for a deeper understanding of the events that occur before, during, and after catastrophic slope failures. Due to the destructive nature of such events, in situ observation is often difficult or impossible. Here, we use data from a network of 58 seismic stations to characterise a large landslide at the Askja caldera, Iceland, on 21 July 2014. High data quality and extensive network coverage allow us to analyse both long- and short-period signals associated with the landslide, and thereby obtain information about its triggering, initiation, timing, and propagation. At long periods, a landslide force history inversion shows that the Askja landslide was a single, large event starting at the SE corner of the caldera lake at 23:24:05gUTC and propagating to the NW in the following 2gmin. The bulk sliding mass was 7-16g×g1010gkg, equivalent to a collapsed volume of 35-80g×g106gm3. The sliding mass was displaced downslope by 1260g±g250gm. At short periods, a seismic tremor was observed for 30gmin before the landslide. The tremor is approximately harmonic with a fundamental frequency of 2.3gHz and shows time-dependent changes of its frequency content. We attribute the seismic tremor to stick-slip motion along the landslide failure plane. Accelerating motion leading up to the catastrophic slope failure culminated in an aseismic quiescent period for 2gmin before the landslide. We propose that precursory seismic signals may be useful in landslide early-warning systems. The 8gh after the main landslide failure are characterised by smaller slope failures originating from the destabilised caldera wall decaying in frequency and magnitude. We introduce the term "afterslides" for this subsequent, declining slope activity after a large landslide.