Landslide Length-Width Ratios as an Aid in Landslide Identification and Verification

Abstract

Regression analysis of the length-width ratios of 153 rotational, rotational-complex landslides occurring in facies of the semi-consolidated Plio-Pleistocene Santa Clara Formation of northwestern Santa Clara County, California indicate that specific length-width ratios are unique to individual facies. The landslide data upon which these analyses were performed included all previously mapped (regional study and site specific) landslides that had at least one dimension (length or width) in excess of 30 m. An individual length-width ratio was established for all landslide data from a particular facies. Individual landslide length-width ratios which departed greatly from the relationship established by regression analysis were identified as landslides which needed to be field checked. In all cases that were field checked, those tentatively identified as landslides with highly divergent length-width ratios proved to have been misidentified as landslides. As economic realities (time, personnel, and equipment) generally preclude the field checking of all landslides identified by air photo analysis, particularly in regional zoning studies, the use of regionally derived length-width ratios is suggested as one means of selecting those areas that warrant specific field evaluations.