TESTING A RAPID, GLOBAL ASSESSMENT OF EARTHQUAKE-INDUCED LANDSLIDES

Landslides are responsible for a significant part of the societal effects of large earthquakes in mountainous areas. Recent examples from Kashmir in 2005 and Eastern Sichuan China in 2008 highlight their direct impact in terms of human loss, damage to the built environment, and impediments to response efforts. We assess the spatial density and distribution of earthquake-induced landslides by combining global topographic data and geologic mapping with near-real-time estimates of ground shaking from large earthquakes. Results compared with an estimate of the affected population will provide emergency managers with timely information on the possible societal effects of earthquake-induced landslides anywhere in the world. Seven quantiles (1^st, 10^th, 30^th, 50^th, 70^th, 90^th, and 99^th) of topographic slope from 3-arcsecond SRTM (Shuttle Radar Topography Mission) data were calculated at 30-arcsecond spacing. Gaps in the SRTM data were filled using the 1-arcsecond National Elevation Dataset (NED) in the United States and the globally complete GTOPO30 (30-arcsecond) dataset elsewhere. An assessment of landslide susceptibility, based on existing 1:5M scale geologic mapping, is used to assign material strength parameters in a simple limit-equilibrium slope-stability analysis to calculate threshold acceleration and displacement necessary to initiate landsliding. This assessment of potential instability is then combined with a spatially distributed estimate of strong ground motion provided operationally from the U.S. Geological Survey’s global ShakeMap and Prompt Assessment of Global Earthquakes for Response (PAGER) systems. Results are depicted as the percentage of the area of a 30-arcsecond-grid cell potentially affected by landslides. We apply this approach to two historical earthquakes, the 1994 M_w 6.7 Northridge, California and the 1999 M_w 7.6 Chi Chi, Taiwan earthquakes for which detailed landslide inventories and shaking maps are available. Results capture the broad spatial pattern of earthquake-induced landslides for the two historical cases, but the quality of the estimates is a function, in part, of the resolution of the input topographic data and uncertainty in the ground shaking, material properties, landslide thickness, and groundwater conditions at the time of the earthquake.