The power–law relationship between landslide occurrence and rainfall level

Zhejiang Province is highly prone to the occurrence of landslides during the annual rainy season from May to September. We studied the relationship between landslide frequency and rainfall level based on 1414 shallow landslides that have detailed records on the location, the date of occurrence, and the corresponding daily rainfall data from 1257 rain gauges during the 1990–2003 period in the region. Cumulative rainfall levels were examined for the periods from the failure day to the prior 10 days. The results show that the cumulative frequency of the landslide occurrence correlates well with the landslide-triggering rainfall and that their relationship can be described using two power–law relations with different scaling exponents for two different ranges of the rainfall level. The rainfall level corresponding to the intersection point of two fitted correlation lines can be defined as the upper bound for shallow landslide-triggering cumulative rainfall threshold (THCR). A large number of shallow landslides may be induced when the amount of cumulative rainfall reaches the THCR value. The 814 landslides that occurred from 2004 to 2007 in the region are used to examine the method, which shows that about 80% of these landslides are induced when the cumulative 1-day rainfall is 200 mm. We suggest that the power–law relationship between rainfall level and landslide occurrence can be used to estimate the probability of various levels of landslide activity during a rainfall event as a guide for making decisions related to emergency preparedness.