Landslide-triggering Rainfall Thresholds after Major Earthquakes for Early Warning

Abstract

Early-warning systems for natural disasters are important tools for disaster risk reduction. In 2005, the Japanese government initiated a new nationwide early-warning system for landslide disasters. The system uses the RBFN method to set the criterion (Critical Line: CL) primarily based on rainfall data recorded as not triggering disasters. Moreover, after large earthquakes, the thresholds (CL) are scaled down by 20-50%, considering the seismic disturbances in the slopes. In this study, we examined the validity of these ratios by analyzing disasters which occurred during the seven months after the Great East Japan Earthquake. As a result, we found that the use of interim CL improved the landslide capture rate by 18.8%, indicating that the interim standard is effective. The required minimum ratio of scaling down the interim CL is 20-30% in areas affected by more than level 5+ earthquakes without changes of landslide capture rates. The required minimum ratios are expected to reduce false predictions.

Characteristics of landslides caused by the 2018 Hokkaido Eastern Iburi Earthquake

“…A field survey revealed that many fissures were formed and a large amount of sediments remained unstable on ridgelines and hill slopes (Osanai et al 2019). It is known that slope failures and debris flows, which can affect residents downstream, are more likely to occur during heavy rain after a major earthquake due to fissures and unstable landslide materials produced by the earthquake (Nomura et al 2014;Sidle et al 2018). A prolonged increase in sediment and driftwood discharge, which can cause flooding due to riverbed aggradation and bridge blockage, followed by water quality deterioration, can occur after devastation of an upper stream (Koi et al 2008;Uchida et al 2014;Marc et al 2015).…”

The 2016 Kumamoto Earthquakes Landslide Disasters and Response

“…After large earthquakes (more than level 5+ on the Japanese scale), it is well-known that the thresholds (CL) of landslide occurrence should be scaled down [Nomura et al ., 2014]. So that, after the main shock with a seismic intensity of 7 (at 1 : 25, 16 th April, M 7.3) in Mashiki-town, and Nishihara-village, 70% of the original threshold was applied to 23 towns and villages in which the earthquake with intensity of more than minor 6 was observed while 80% of the original was applied to 8 towns and villages in which the earthquake with intensity over 5 was observed (Fig.…”