The Santiago Landslide and Associated Ridge-top Graben (Sackungen): Implications for Paleoseismic Landslide Studies

Abstract

Some recent paleoseismic studies have focused on dating ridge-top graben deposits to evaluate the timing of paleoseismic events. By contrast, our study of the Santiago landslide demonstrates that ridge-top grabens also can be associated with aseismic, deep-seated landsliding. The Santiago landslide in Anaheim Hills, California, failed during the winter of 1992–1993 in response to elevated groundwater conditions associated with intense rainfall. The head of the active landslide included a zone of extensional deformation along the bounding ridgeline. Interpretation of historical, aerial photographs indicates that the active landslide is a re-activated, ancient, deep-seated, translational landslide and an associated ridge-top graben. Large-diameter borings within the ridge-top graben encountered thick colluvium, steeply dipping colluvium-filled fractures, and shears with normal offsets. In contrast to the rupture surface within the central part of the landslide, the basal rupture surfaces in the graben area had significantly less gouge. We interpret this contrast in gouge development as an indication that the ridge-top graben developed later than the original landslide by upslope progression of the deformation. Our limit-equilibrium, slope-stability analyses indicate that either high groundwater or seismic ground motion could have previously activated the ancient landslide and ridge-top graben. Because colluvial deposits preserved within the ridge-top graben and produced by these two different types of triggering events could be misinterpreted as representing the late Quaternary paleoseismic record, these features are not useful for paleoseismic studies unless aseismic activation can be clearly precluded.