A case study of shallow landslide occurrence and magnitude-frequency due to rainfall events after earthquakes in northwestern California

This project is a case study to examine trends in shallow, rapid landslide occurrence and magnitude-frequency in Humboldt County, California. It attempts to identify a trend of increasing landslide occurrence and magnitude-frequency correlated to large seismic events (magnitude [M] six and greater) followed by significant rain events yielding high antecedent soil moisture conditions with Antecedent Precipitation Index values greater than 10 centimeters. Shallow landslides were observed through aerial photographs for three earthquakes that occurred in 1954, 1980, and 1992. For each earthquake, non earthquake "control", earthquake "control", and earthquake/rainfall event related "treatment" study years were selected for observation. Earthquake event selection was based on aerial photograph coverage, precipitation events producing an Antecedent Precipitation Index value greater than 10 centimeters, and earthquakes magnitude six or greater. Recently harvested areas were selected in order to provide uniformity of land management practices and also to allow for the maximum observance of landslides, which are often not seen under a full canopy. The occurrence and magnitude-frequency rates of shallow, rapid landslides were found to increase after an earthquake (magnitude six or greater) followed by a precipitation event with an Antecedent Precipitation Index value greater than 10 centimeters than when compared to landslide occurrence and magnitude-frequency rates related to high precipitation events only.