Detection of landslide timing, reactivation and precursory motion during the 2018, Lombok, Indonesia earthquake sequence with Sentinel-1

Earthquake-triggered landslides can be mapped using optical satellite images, but assessing how they evolve during earthquake sequences is difficult due to cloud cover in these data. This information is crucial for understanding their triggering conditions. Here we use Sentinel-1 amplitude and a new, coherence-based method to characterise the evolution of rapid landslides during an earthquake sequence that occurred over a 23 d period in 2018 in Lombok, Indonesia. While most new landslides were triggered during the largest earthquake in the sequence on 5 August, we also identified landslide activity associated with other, lower magnitude earthquakes on 28 July, 9 and 19 August, with around half of the landslides studied active in more than one earthquake. In particular, many landslides triggered by the 5 August earthquake were then reactivated later in the sequence. These reactivations were triggered by accelerations as weak as 0.1 g, while new failures generally did not occur below 0.15 g, suggesting a post-seismic weakening effect driven by the landslides themselves rather than general landscape weakening. We also identified an example where possible precursory motion detected during the first earthquake in the sequence was later followed by larger scale failure. Overall, we demonstrate that, although they are not sensitive to all landslides and are more likely to detect larger events, Sentinel-1 amplitude and coherence are valuable tools to study how landslide hazard and mass wasting evolve during sequences of triggers.