Landslides

Extreme ground shaking along the steep slopes of the Santa Cruz Mountains during the 1989 earthquake led to the reactivation of many ancient landslides and formed a large number of new ones. These landslide masses, which range from tens of meters to several kilometers in width, moved downslope several meters and caused damage to many homes and roads. Fortunately, the 1989 earthquake occurred in the midst of a several year drought and exceptionally dry subsurface conditions probably aided in preventing any large-scale disastrous slope failures. In contrast, the great 1906 earthquake occurred in the wake of an unusually wet winter and saturated subsurface conditions in combination with extreme ground shaking led to several fatal catastrophic slope failures in the Santa Cruz Mountains. A return to normal high rainfall conditions could precipitate dangerous landsliding in the Santa Cruz Mountains, causing extensive damage to homes, roads, and other structures. In Santa Cruz county, the eventual damage due to these landslides may exceed the damage suffered during the earthquake itself. It is estimated that over 300 houses may be threatened.

House destroyed by landslide, Santa Cruz Mountains (photo by Jeff Marshall): These landslides have become a major public policy issue in the county. The debate centers around whether to allow homeowners to rebuild on threatened property or not. Recently, the county board of supervisors approved rebuilding, if proper geologic investigation shows that there is no immediate threat of injury or death. The homeowners must then sign a waiver releasing the county from any liability, and any geologic hazard must be clearly indicated in the deed for the property. The average house in the Santa Cruz Mountains has a value of at least $200,000, which naturally introduces a number of interesting legal, economic, and emotional complications to this situation.

Roadcut failure on Old Soquel-San Jose Highway, Santa Cruz Mountains (photo by Jeff Marshall): This rockfall covers the southbound lane of Old Soquel-San Jose Highway. Santa Cruz is connected to the south San Francisco Bay region by three main arteries (all narrow mountain roads): Highway 17, Highway 9, and the Old Soquel-San Jose Highway. During the earthquake there were landslides on all three, severely limiting access to Santa Cruz. The only unaffected route to the South Bay was Mt. Charlie Rd., the 19th century stagecoach road.

Laurel curve landslide, State Highway 17, Santa Cruz Mountains (photo by Jeff Marshall): This landslide, initiated by the earthquake, covered both of the northbound lanes of Hwy 17, the major commuter route between Santa Cruz and San Jose. This landslide was the most extensive of numerous slope failures that caused a month long closure of Highway 17. The concrete wall in this photo is the highway center divider.

The numerous landslides and extensional cracks caused by the quake presented geologists with a complex picture of ground deformation in the Santa Cruz Mountains. In the weeks following the earthquake, these features were mapped and assessed for hazards through a cooperative effort by geologists from the County of Santa Cruz, University of California at Santa Cruz, the U.S. Geological Survey, the California Division of Mines and Geology, the Army Corps of Engineers, and several private consulting firms. An ongoing monitoring program has been established to assess any potential landslide hazards produced by rainfall or additional earthquakes.