CALIFORNIA'S SEISMIC HAZARDS MAPPING ACT: REALIZING THE BENEFITS OF LANDSLIDE INVENTORIES FOR MITIGATING SLOPE HAZARDS

Landslide inventory maps can be a valuable resource for planning and development, and typically provide information on the location, size, shape, type, direction of movement and activity status about existing slope failures. At regional scales (1:24,000 or smaller) they serve to indicate where more detailed mapping may be necessary prior to development. Although a key component of landslide hazard mitigation, many communities have neither the landslide inventories, the means to produce them, nor the policy to effectively use them. California’s Seismic Hazards Mapping Act of 1990 (Public Resources Code Sec 2690 et seq.) provides a framework for mitigating slope hazards that provides for the production of landslide inventories, and the procedures and policy to incorporate their use into local land-use planning and construction decisions. Cities and counties are required to utilize products when preparing their general plan, and when adopting or revising land-use planning and permitting ordinances. Although the act is chiefly aimed at reducing earthquake-related losses, provisions for slope stability can be broadly applied. Characteristics of landslide-prone terrain and measures taken to stabilize slopes are similar regardless of the triggering agent of slope failures. State mandates to withhold construction permits subject to approval of a site-specific landslide hazard investigation and report recommending mitigation provides a mechanism to identify when provisions beyond building codes are necessary to ensure safe development and reduce future losses. Successful implementation of the act is unifying regulatory policy and process for hillside development, and is leading to a more regionally consistent application of a higher standard of geotechnical practice. Importantly, lead agencies are embracing the act while promulgating effective implementation by establishing standards for review and approval of site investigations and proposed mitigation via expert ad hoc committees having multi-jurisdictional and multi-sector participation.