Preliminary study of the effect of earthquakes on the rainfall threshold of debris flows

Abstract

Numerous landslides were triggered in earthquake-affected areas and the largest one was located close to the rupture fault. Abundant loose solid erodible materials from co-seismic rock falls and landslides were deposited in gullies, which formed the source material for debris flows. Numerous debris flows were triggered in the years after the earthquake during the rainy season. The Wenchuan and Chi-Chi earthquakes are examples where after these events the triggering thresholds for debris flows significantly decreased. Some studies emphasized that the increased debris flow activity in the earthquake-affected areas will last 20 to 40years. However it is still an open question because no detailed research was conducted on the temporal occurrence of debris flows over a long time span after a strong earthquake. In this paper the ratio of rainfall thresholds after and before earthquakes is studied in relation to the peak ground acceleration (PGA) of earthquakes and the number of rainy seasons after the earthquake. The debris flows in the Chi-Chi and Wenchuan earthquake areas were selected for this study. The preliminary results are: (1) the earthquakes caused an instantaneous decrease of the rainfall threshold for debris flows. The ratio (r) of threshold after the earthquake over the threshold before the earthquake decreases with a power function with the increasing dimensionless peak ground acceleration (PGA) of the earthquake; (2) in the first 7 rainy seasons after the earthquake the ratio (r) linearly increases with the increasing number of elapsed rainy seasons; and (3) the earthquake not only has an increasing effect on the debris flow activity but also has an influence on the duration of this activity. The duration of the enhanced activity increases with a larger PGA.