The Sibalaya flowslide initiated by the 28 September 2018 M_W 7.5 Palu-Donggala, Indonesia earthquake

The 28 September 2018 M_W 7.5 Palu-Donggala earthquake caused massive flowslides within and nearby the city of Palu. We performed post-earthquake reconnaissance at the flowslide sites and examine a large flowslide that occurred approximately 30 km south of Palu in Sibalaya. We performed field mapping, unmanned aerial vehicle flights, and eyewitness interviews. After the post-earthquake reconnaissance, we performed cone penetration tests. We developed hypothesized flowslide initiation and progression mechanisms from our collective reconnaissance efforts. Based on observed morphological features and eyewitness accounts, the flowslide initiated near a change from steep to shallow terrain. Significant liquefaction of the underlying soils initiated the flowslide after strong ground motions ceased. We speculate progressive liquefaction and/or void redistribution contributed to the timing and delay of initiation.