Initiation and Runout of Post‐Seismic Debris Flows: Insights From the 2015 Gorkha Earthquake

Abstract

Post‐seismic debris flows are an important hazard following large earthquakes, propagating destruction downstream from hillslopes where coseismic landslides occur and extending damage for years after shaking stops. Data sets of post‐seismic debris flows are necessary to predict initiation and runout characteristics but are presently scarce. We used satellite imagery supplemented by field observations to compile an inventory of >1,000 debris flows associated with the 2015 Gorkha Earthquake in Nepal. We identified two distinct debris flow types: (1) Material from a coseismic landslide was remobilized in a steep channel during a later monsoon; and (2) a new post‐seismic hillslope failure occurred in saturated conditions and became fluidized and channelized. Runout distance was constrained by channel confluences and may be related to confluence geometry. Unstable landslide debris was largely flushed from steep channels during the first monsoon following the earthquake, and the rate of new hillslope failures tailed off over a few years.

Enhanced Rainfall‐Induced Shallow Landslide Activity Following Seismic Disturbance—From Triggering to Healing

“…These relationships, particularly SI.2, demonstrate that the time rate of post‐seismic failure is a function of rainfall intensity; however, as other temporal factors such as evapotranspiration and drainage are not considered in this study, rainfall volume that causes post‐seismic yield is described in this section for illustrative purposes. While simple, it serves as a rational, conceptual approach toward evaluating hillslope damage and delayed landslide triggering in general (Figure 5), and is consistent with rainfall threshold observations regarding post‐seismic landsliding (e.g., Dahlquist & West, 2019; X. Fan et al., 2018).…”

Rapidly Evolving Controls of Landslides After a Strong Earthquake and Implications for Hazard Assessments

“…This rapid decay is consistent among all recent major earthquakes (Hovius et al., 2011b; Marc et al., 2015; Zhang et al., 2016), and partly constrained the postseismic sediment transport (Wang et al., 2017). In both the Wenchuan and Gorkha regions, researchers argue that most of the loose material generated by earthquake‐induced landslides remained confined to hillslopes and tributary channels of catchments, whereas only a minor proportion of it has been evacuated from the catchments thus far (Dahlquist & West, 2019). In Wenchuan, previous studies (X.…”

“…Rainfall‐induced remobilizations of coseismic landslide deposits, propagating from hillslopes to downstream (Dahlquist & West, 2019), are a typical hazard in areas affected by earthquake‐induced landslides (X. Fan et al., 2019a).…”

“…Such inventories are essential for understanding the spatiotemporal patterns of future landslides. To the best of our knowledge, only a few studies have mapped these occurrences systematically in worldwide earthquake cases (Dahlquist & West, 2019; R. L. Fan et al., 2018a; Lin et al., 2004; Shafique, 2020; Shou et al., 2011; Tian et al., 2020), and in limited areas affected by the Wenchuan Earthquake (M. Chen et al., 2020; X. Fan et al., 2019b; Shen et al., 2020; Tang et al., 2016; Yang et al., 2017, 2018), yet no multitemporal postseismic susceptibility and controlling factor assessments have been attempted thus far.…”

“…Such inventories are essential for understanding the spatiotemporal patterns of future landslides. To the best of our knowledge, only a few studies have mapped these occurrences systematically in worldwide earthquake cases (Dahlquist & West, 2019;R. L. Fan et al, 2018a;Lin et al, 2004;Shafique, 2020;Shou et al, 2011;Tian et al, 2020), and in limited areas affected by the Wenchuan Earthquake (M. X.…”