The 2005 la conchita landslide, california: part 1 - geology

Abstract

This is Part I of a two-part causation analysis of the January 10, 2005 La Conchita, California, USA landslide. This paper describes the geology and geomorphology of the event, which killed 10 persons and damaged or destroyed 36 residences. The landslide is located in a complex and active tectonic setting. It was triggered by two weeks of heavy rainfall, which initiated a failure in the backscarp of a large slump that had occurred ten years earlier. The 2005 landslide displaced older landslide deposits derived from Tertiary sedimentary rocks. Over 30,000 m3 of wet debris was mobilized in the event, which formed two distinct lobes of debris. The main lobe, comprising 90% of the deposit, rapidly transformed into a large scale debris flow, eroded and entrained over 4,000 m3 of material along its path, overran a temporary wall, then flowed into the residential community. This lobe exhibits characteristic debris flow textures, including raised lateral levees and a surface pattern of ridges and troughs. The minor lobe, comprising about 10% of the total landslide volume, was deposited in pulses that arrived from different directions at different times. This material impacted and locally breached the temporary wall, but did not flow into the community below.