Extracting region-specific runout behavior and rainfall thresholds for massive landslides using seismic records: a case study in southern Taiwan

Typhoon Morakot struck Taiwan on 7 August 2009, bringing severe rainfall and landslides. Of the many landslides caused by the storm, 12 large-scale landslides were discovered based on seismic signals recorded from broadband seismic stations. Landslides inventoried from both seismic records and satellite imagery are used as baseline data to evaluate region-specific forensic analyses of landslide features, e.g. disturbed area, width and runout distance (D_runout), associated with landslide-induced seismic signals and signal durations (Δt). The results of these analyses show that (i) the landslide-disturbed area correlated well with signal duration when Δt was over 60 s; (ii) the landslide-disturbed area correlated well with the ratio of the landslide width to runout distance when Δt was under 40 s; (iii) the runout distance of sliding mass exhibited a positive relation with Δt², satisfying the energy conserved during mass movement. This case study presents a region-specific examination of the Δt-D_runout relationship based on 12 large-scale landslides that occurred during Typhoon Morakot, providing insight into the sliding processes. In addition, the time of landslide initiation was extracted from seismic signals in order to better understand region-specific, large-scale, landslide rainfall thresholds. These relationships indicate that (a) long-duration rainstorms with high cumulative rainfall tend to dictate the occurrence of large-scale landslides; (b) the rainfall threshold for large-scale landslides is found to be higher than in previous studies that also include smaller landslides, defined in this region as I = 60D^−0.31, where I is the rainfall intensity (mm/h) and D is the duration (h) of the rainstorm.