Weight-Induced Stresses and the Recent Seismicity at Lake Oroville, California

Lake Oroville is a large artificial lake created by the construction of a 235-m-high earth dam on the Feather River, California, near the city of Oroville. Its storage capacity is about 4.4 × 10^9 m^3, and its maximum depth is about 200 m. There was no significant increase in seismic activity in the lake region following impoundment of the dam late in 1967 until the occurrence of many small seismic events which began in June 1975. This activity lead to a M = 5.7 main shock on August 1, 1975 with an epicenter about 11 km SSW of the Oroville dam. The main shock produced significant damage in the city of Oroville which lies about 7 km NNW of the epicenter. With several cases of reservoir-induced activity already documented, it is natural to inquire whether the Oroville seismicity was due to the presence of the reservoir. As part of such a study, the stresses induced in the neighboring lithosphere by the weight of Lake Oroville are determined. On the basis of present geological data, it is unlikely that these stresses were responsible for the main shock of August 1, 1975. The weight-induced shear stress across the fault plane in the hypocentral region has a component of about 0.04 bar parallel to the reported fault movement but in opposition to this movement. The greatest weight-induced shear stress is about 3.4 bars and this occurs under the deepest portion of the lake. The greatest vertical deflection at the surface due to the weight of Lake Oroville is calculated to be about 5.5 cm