PREHISTORIC LANDSLIDE COMPLEXES IN THE LANDSCAPE AND ASSOCIATED HAZARDS: LA CONCHITA, CALIFORNIA

Prehistoric landslides in the modern landscape, if reactivated, present hazards to our personal safety, infrastructure and homes. Examples in Southern California include the Bluebird Canyon, the Portuguese Bend, the Big Rock landslides. These slides can move relatively slowly months after heavy rains and others can move rapidly during heavy rains with potentially deadly debris flows. The 2004-2005 La Conchita landslide moved as a slower moving, upper slump with a faster moving, lower debris flows. The 2005 La Conchita landslide killed 10 people and destroyed about 30 homes in the community and is a reactivation of about one-half of the landslide mass that occurred in 1995. The 1995 and 2005 landslides form only a small percent of the much larger landslide complex. The slope that failed in 1995 and 2005 is a Holocene paleosea cliff and is the seaward edge of an ancient landslide that has produced prehistoric and historic slides, slumps, debris and mud flows. Examination of a 30m DEM suggests the 1995 and 2005 landslides are at the leading edge of a much larger slower moving landslide we call the La Conchita landslide complex. The slide complex consists of many smaller active landslides and is delineated by an active, arcuate southwest facing 50 –75 m high head scarp which is bounded laterally by deeply incised drainages. The top ~50m of the ~200 m high steep slope above the community of La Conchita is composed of prehistoric slide debris, that displaces and overlies the 45 ky marine terrace. Therefore, the landslide was initiated at least a few thousand years ago but is younger than 45 ky. The combination of, active faulting and folding, rapid tectonic uplift, very weak rocks, steep topography, and presence of springs render the La Conchita area particularly vulnerable to landslide. The question is not if, but when the next landslide will impact the community of La Conchita, California.