Insights into the mobility characteristics of seismic earthflows related to the Palu and Eastern Iburi earthquakes

Abstract

As an important indicator for landslide hazard and risk assessments, landslide mobility has attracted considerable attention in the literature, yet few studies have investigated seismic earthflows. Therefore, the earthflows induced by the 2018 Mw 7.5 Palu earthquake (Indonesia) and the 2018 Mw 6.5 Eastern Iburi earthquake (Japan) and their mobility characteristics were analyzed. Long-term irrigation for cultivation of wet rice and heavy rainfall produced by Typhoon Jebi introduced substantial moisture into alluvial fans (Palu earthquake) and pyroclastic fall deposits (Eastern Iburi earthquake), respectively, and seismic shaking liquefied these moist deposits, which consequently traveled long distances. Four earthflows that were triggered on very gentle slopes of 1–3° by the Palu event exhibited very high mobility levels (H/L) of 0.0129, 0.01125, 0.0209 and 0.0271 (average H/L = 0.018), whereas 7058 earthflows triggered by the Eastern Iburi event exhibited an average mobility of 0.33. Different liquefied layers were located below or above the low permeability layer, and movement restrictions and complex final flow disturbances could be responsible for obvious mobility differences triggered by the two seismic events. However, the earthflow mobility induced by the Eastern Iburi earthquake was obviously higher than that of coseismic rock landslides worldwide and is described by the following relationship: H/L = 1.46–0.24 × log10 (earthflow_area). Furthermore, the earthflow area, peak ground acceleration (PGA) and slope angle all have increase influences on mobility.