Abstract. One purpose of landslide research is to establish early
warning thresholds for rainfall-induced landslides. Insufficient observations
of past events have inhibited the analysis of critical rainfall conditions
triggering landslides. This difficulty may be resolved by extracting the
timing of landslide occurrences through analysis of seismic signals. In this
study, seismic records of the Broadband Array in Taiwan for Seismology were
examined to identify ground motion triggered by large landslides that
occurred in the years 2005 to 2014. A total of 62 landslide-induced seismic
signals were identified. The seismic signals were analyzed to determine the
timing of landslide occurrences, and the rainfall conditions at those times
– including rainfall intensity (I), duration (D), and effective rainfall
(Rt) – were assessed. Three common rainfall threshold models (I–D,
I–Rt, and Rt–D) were compared, and the crucial factors of a
forecast warning model were found to be duration and effective rainfall. In
addition, rainfall information related to the 62 landslides was analyzed to
establish a critical height of water model, (I-1.5)⋅D=430.2. The
critical height of water model was applied to data from Typhoon Soudelor of
2015, and the model issued a large landslide warning for southern Taiwan.
Rainfall thresholds for slope failures are essential information for establishing early-warning systems and for disaster risk reduction. Studies on the thresholds for rainfall-induced landslides of different scales have been undertaken in recent decades. This study attempts to establish a warning threshold for large-scale landslides (LSLs), which are defined as landslides with a disturbed area more massive than 0.1 km2. The numerous landslides and extensive rainfall records make Taiwan an appropriate area to investigate the rainfall conditions that can result in LSLs. We used landslide information from multiple sources and rainfall data captured by 594 rain gauges to create a database of 83 rainfall events associated with LSLs in Taiwan between 2001 and 2016. The corresponding rainfall duration, cumulative event rainfall, and rainfall intensity for triggering LSLs were determined. This study adopted the tank model to estimate conceptual water depths (S1, S2, S3) in three-layer tanks and calculated the soil water index (SWI) by summing up the water depths in the three tanks. The empirical SWI and duration (SWI–D) threshold for triggering LSLs occurring during 2001–2013 in Taiwan is determined as SWI = 155.20 − 1.56D and D ≥ 24 h. The SWI–D threshold for LSLs is higher than that for small-scale landslides (SSLs), those with a disturbed area smaller than 0.1 km2. The LSLs that occurred during 2015–2016 support this finding. It is notable that when the SWI and S3 reached high values, the potential of LSLs increased significantly. The rainfall conditions for triggering LSLs gradually descend with increases in antecedent SWI. Unlike the rainfall conditions for triggering SSLs, those for triggering LSLs are related to the long duration–high intensity type of rainfall event.
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