Source area morphometry and high depletion rate of landslides may indicate their coseismic origin

Ancient coseismic landslides indicate prehistoric earthquakes and may also be a measure of their intensity. Their identification in the landscape remains, however, challenging. We used field geologic observations of source areas of recent rainfall-induced and earthquake-induced landslides to define a morphometric index that can distinguish between these two types of triggers. Morphometries of 129 landslides from tropical, subtropical and temperate zones developed in a variety of bedrock types were described by using the following parameters: i) depletion index (I_d), i.e. the ratio between the length of the depleted part and the length of the source area; ii) relative slope height (H_rel) and iii) source area angle (φ_S), i.e. the angle between the top of the line connecting the scarp top with the toe of the source area and the horizontal plane. They were obtained using globally available digital elevation models and satellite images. Their combination into an Index of Potential Trigger (I_PT) successfully distinguished between the two types of landslide triggers with an overall accuracy of 87%. The rainfall-induced landslides had a significantly lower average I_PT value of 0.01 as compared with 0.37 for the coseismic landslides. The index of potential trigger may be a useful and readily implemented morphometric tool for preliminary identification of prehistoric coseismic landslides recorded in the current topography.