ANTHROPOGENIC TRIGGERING OF LANDSLIDES IN VENTURA COUNTY, CA: AN ANALOG FOR PLEISTOCENE CLIMATE

Renewed movement of ancient landslides may be triggered by undercutting or oversteepening of slopes, earthquake ground motion, or natural or anthropogenic increases of the groundwater table. In tectonically active areas, the confluence of all the possible mechanisms may make the trigger difficult to discern. The north-flank of Oak Ridge, along the southern flank of the rapidly deforming Ventura Basin, is composed of numerous large rotational slumps. In the last 20 years, four of these slumps have been reactivated. The thickness of these slumps is greater than 10 m implying triggering by rising groundwater. Many others along the slope have remained stable in spite of record annual precipitation (2005) and earthquakes (Mw – 6.7 Northridge). A 3-year running average for precipitation shows that the slumps were reactivated during or immediately following periods of decreased precipitation, thereby casting doubt on precipitation as a trigger. However, slump reactivation follows planting of slopes as avocado orchards, which require about 40 inches/year of irrigation or nearly three times annual precipitation of the last 50 years. Morphology and stratigraphic relations infer that many of the slumps were last active during the Pleistocene suggesting that large-scale slumping is a likely artifact of the greater Pleistocene precipitation.