Boca Dam and Reservoir are within an intermountain basin flanked on the west by the 9,000-foot-high Sierra Nevada and by the 10,000-foot-high Carson Range on the east. The basin is underlain by Tertiary lavas and pyroclastics north of latitude 39.5 N. and by Pliocene and Pleistocene basalts and latite flows to the south.
The Cenozoic volcanic rocks are in turn underlain by Jurassic granite bedrock at some undetermined depth. As determined from logs of exploration, Boca Dam is founded on predominantly volcanic tuff with subordinate interlayers of tuff breccia, andesite, basalt, and diabase.
Alluvial deposits of interbedded silt, sand, gravel, and cobbles were encountered to 20 feet thick in Little Truckee River channel, to 90 feet thick in the left abutment area, and to 66 feet thick in the right abutment area. Exploration extended approximately 100 feet below stream grade and approximately 200 feet deep in the abutment areas.
No information is available discussing faulting at the damsite, in the dam foundation area, or in the diversion tunnel. However, ground surface rupture occurred during the September 12, 1966 earthquake along a possible fault trace bearing approximately N. 20 E. only 5 miles northwest of Boca Dam.
After the earthquake, widely spaced cracks were observed over a Â±9-mile-long distance. A concealed, pre-Quaternary fault is mapped about 2 miles east of Boca Dam trending about N. 20 W. A Â±1-mile-long branch of the fault appears to traverse the left abutment area a few hundred feet east of the dam.