W Ei Ght- I Nduced Stresses and the Recent Seismicity at Lake Oroville, California

Lake Oroville is a large artificial lake created by the construction of a 235-mhigh earth dam on the Feather River, California, near the city of Oroville. Its storage capacity is about 4.4 × 109 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, 19,75 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 lithosphel:e 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 weightinduced shear stress is about 3.4 l~ars 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. I N T R O D U C T I O N There are now several well-documented cases of a significant increase in seismic activity following the impounding of certain large reservoirs, for example, Roth6 (1969), Gupta et al. (1972), Shen et al. (1973) and Judd (1974). In four of these cases, Lake Kariba (central Africa), Lake Kremasta (Greece), Hsinfengkiang Reservoir (China) and Koyna Reservoir (India), the principal earthquakes had magnitudes of about M = 6 . Lake Oroville, California (Figure 1)is a deep, relatively large reservoir created by the construction of a 235-m-high earth dam on the Feather River at a site in the foothills of the Sierra Nevada mountain range. When full, the lake holds 4.4 x 1 0 9 m 3 of water with a surface area of 64 km 2. Its maximum depth is about 200 meters. There was no significant increase in local seismicity within a radius of 30 km of the dam following the impounding of the reservoir at the end of 1967 until a sudden increase occurred in June 1975. During the preceding 4 months the water level had risen at its gre~,test rate and with the greatest increment since impoundment, but it is, not clear whether this was a factor behind the increase in seismic activity. The greatest earthquake in this series was a shock of magnitude 5.7 at i : 20 p.m. on August 1, 1975. The epicenter of this shock was about 11 km SSW of the dam. No damage to the dam was reported but there was significant damage in the city of Oroville. These events close to the lake have given rise to speculation that the seismic activity may be reservoir-induced. As a basis for further discussion, estimates of the incremental stresses induced in the neighboring lithosphere by the weight of Lake Oroville are