Seismic and geologic controls on spatial clustering of landslides in three large earthquakes

The large, shallow earthquakes at Northridge, California (1994), Chi-Chi, Taiwan (1999) and Wenchuan, China (2008) each triggered thousands of landslides. We have determined the position of these landslides along hillslopes, normalizing for statistical bias. The landslide patterns have a co-seismic signature, with clustering at ridge crests and slope toes. A cross check against rainfall-induced landslide inventories confirms that crest-clustering is specific to seismic-triggering. In our three study areas, seismic ground motion parameters, and lithologic and topographic features have limited bearing on the observed patterns of landslide clustering. However, we show that at the scale of the epicentral area, crest- and toe-clustering occur in areas with specific geological features. Toe-clustering of seismically-induced landslides tends to occur along major faults. Crest-clustering is concentrated at sites where the lithology along hillslopes is approximately uniform, or made of alternating soft and hard strata, and without strong overprint of geological structures. Although earthquake-induced landslides locate higher on hillslopes in a statistically significant way, geological features strongly modulate the landslide position along the hillslopes. As a result the observation of landslide clustering on topographic ridges cannot be used directly as an indicator of seismic parameters such as ground shaking.