Estimating severity of seismically induced landslides and lateral spreads using threshold water levels

The potential for an earthquake-induced landslide increases when the shear strength of a slope decreases and the hydrostatic pressure increases from the dynamic stresses induced by seismic shaking and/or heavy rainfalls. This paper presents an assessment of seismically induced slope failure in the St. Louis, MO, USA, area; it emphasizes water elevations as the controlling factor, realizing that such levels vary over space and time. We estimated the threshold water table depths to initiate seismically induced landslides in the uplands and liquefaction-induced lateral spreads in the alluvial floodplains under an M7.5 earthquake with a peak ground acceleration of 0.20 to 0.40 g. These threshold water table depths were computed as a function of ground steepness using the Newmark model for rigid block landslides and an empirical regression for lateral spreads. The seismic microzonation was prepared by comparing the map of threshold water table depths and maps of average water levels. The resultant hazard maps suggest that the river bluffs are prone to seismically induced landslides only when the water reaches its highest recorded levels, while much of the floodplains are prone to lateral spreads. Lateral spreads occur more extensively when the water exceeds its normal level.