RESULTS OF A PROBABILISTIC SEISMIC HAZARD STUDY FOR THE SANTA BARBARA AREA, SOUTHERN CALIFORNIA

The Santa Barbara area is seismically active with damaging earthquakes occurring in 1800, 1812, 1925, 1941, and 1978. The U.S. Bureau of Reclamation has four relatively small dams in the area that were constructed in the early 1950's. As part of the agency's dam safety program, a probabilistic seismic hazard evaluation was recently conducted for these dams. The study identified over 20 potentially active faults within about 25 km of Santa Barbara. The fault sources include major thrust faults and strike-slip faults.

Major north-dipping thrust faults in the Santa Barbara area with relatively high slip rates (1-3 mm/yr) include the North Channel, Red Mountain, and Pitas Point faults. These faults are capable of producing very high accelerations at the subject dams. However, for most of the dams, the earthquake hazard is dominated by the nearby faults of the More Ranch, Mission Ridge, Arroyo Parida, and Santa Ana fault system. This left-oblique slip system is about 70 km long, is considered capable of M 6.6-7.2 earthquakes, and has a slip rate of approximately 1 mm/yr. Based on information gathered as part of site specific paleoseismic studies at Lauro Dam (a 42-m-high, zoned-earthfill embankment), either a splay or the direct western continuation of the Mission Ridge fault bisects the foundation of the dam. Thus, Lauro Dam may be subjected to fault displacement through the foundation and outlet works. Assuming a slip rate on the fault of about 1 mm/yr, the annual probability of 1 m or more of surface displacement could be higher than 0.002/yr.

For all the subject dams, the results of this study indicate that high accelerations occur relatively frequently. At a return period of 500 years, peak accelerations range from 0.6 to 0.8 g whereas for a return period of 2500 years, peak accelerations range from 1.0 to 1.3g.