Application of New Developments in Seismic Hazards Assessment: Revised Safety Element of the General Plan of the City of Santa Monica

Douglas Bausch, Eldon M. Gath, Tania Gonzalez, James F. Dolan, & Kerry Sieh

Published 1992, SCEC Contribution #79

When recently documented seismic hazard data impact a heavily urbanized area, planners, consultants and local government officials find that they must address a wide range of issues relating to land-use management, emergency preparedness and response. For example, new data on the state-of-activity and location of the Santa Monica fault in the Los Angeles Basin, must be presented in a manner that can be used by city agencies, private firms and residents. To apply the new data, overlays that show the relationship of seismic hazards to the existing urban development were developed for the city of Santa Monica. These overlays can be implemented through the city’s GIS database, and used, for example, by city emergency services personnel in emergency preparedness and response programs that may include delineating emergency access and evacuation routes to avoid hazardous areas. The overlays could also be used in landuse decisions for redevelopment or “infilling” projects, to identify and plan for potential hazards, and to locate critical facilities in areas less susceptible to damage. In addition, these overlay zones may be used to define areas that require more study.

The present data regarding the Santa Monica fault is limited. Additional research is required to further refine the recurrence interval, potential magnitude and location of surface displacement per event. The Santa Monica fault becomes a more diffuse system within the city of Santa Monica, where it is manifested by a series of enechelon topographic scarps that range in width from about 100 to 900 feet. Previous investigators have mapped the fault as two distinct branches, the North and South, based on ground water barrier information, and by projecting oil well data from the Sawtelle Oil Field. Since 1975, “Hazard Management Zones”, requiring geotechnical investigations that address the potential for surface rupture, have been established around the North and South branches of the Santa Monica fault. A new “Hazard Management Zone” that encompasses the fault locations of previous investigators, and the topographic fault scarps recently identified, includes about one-third of the city of Santa Monica. The large width of this zone is unsuitable to implement or enforce effective legislation requiring investigation of potential fault rupture hazards for proposed redevelopment or infilling projects.

Site specific investigations, similar to Alquist-Priolo required studies, have been ineffective in evaluating the risk associated with ground rupture potential in the city of Santa Monica. Occasional redevelopment projects within the Hazard Management Zones do not provide the regional picture concerning the tectonic significance of the Santa Monica fault. The authors are presently working on a regional study of the Santa Monica fault system designed to define the recurrence interval, magnitude and location of past displacements (JD & KS), and to identify and develop a cost-efficient exploratory method for faults that is particularly suited to an urbanized environment (DB, EMG, & TG). In urbanized environments, where structures are already located within hazardous areas, education may be the only tool in preventing loss of life and property. “Hazard Abatement Districts”, formed within potentially hazardous areas, may provide the most effective and equitable funding methodology for evaluation and mitigation of geologic hazards within urbanized areas.

Bausch, D., Gath, E. M., Gonzalez, T., Dolan, J. F., & Sieh, K. (1992). Application of New Developments in Seismic Hazards Assessment: Revised Safety Element of the General Plan of the City of Santa Monica. Presentation at 35th Assoc. of Engineering Geologists National Convention.