Hi Canie. The reservoir was first filled in the late 60s. It has been filled many times since then. The fault most likely to have caused this quake is the Ortigalita Fault. There is also an outside chance that it was caused by the Great Valley Fault.

The Tesla- Ortigalita Fault is a step over fault. The southern section of the fault is near the west end of the lake and the northern section of the fault is more towards the east. Not sure if anyone knew of the fault before they built the dam.

The following was taken from a report on tectonic wedging.

“Wentworth and others (1984) interpreted the juxtaposition of Franciscan assemblage and a coeval section consisting of Coast Range ophiolite and Great Valley sequence as having occurred during landward movement of the Franciscan assemblage as a tectonic wedge. They reinterpreted the "Coast Range thrust fault" of Bailey and others (1970), a subduction megathrust between the Coast Range ophiolite and the Franciscan assemblage, as the roof thrust of the wedge. More recently, the thrust nature of the "Coast Range thrust fault" has been reevaluated. Jayko and others (1987), testing an hypothesis by Platt (1986), produced abundant evidence that the contact between Franciscan assemblage and Coast Range ophiolite is a detachment surface along which the upper plate was extended during uplift of the Franciscan assemblage. Their evidence is the consistent attenuation, as opposed to repetition, of geologic section across this discontinuity and associated faults above it. They proposed the term "Coast Range fault" for this discontinuity which we adopt here. Evidence of attenuation is present even on transect C2, in that the two outcrops of the Coast Range ophiolite in the eastern Diablo Range (Figure. 8.4A) represent an abridged section of ophiolite: The western outcrop is partially serpentinized ultramafic rock of the basal part of an ophiolite, whereas the eastern outcrop is the sill complex and volcanic flows of the upper part of an ophiolite. These two parts of the ophiolite are now juxtaposed across the crooked, steeply dipping Tesla-Ortigalita fault. Although this fault now offsets the Coast Range fault, it may represent reactivation of a normal fault that originally soled into the Coast Range fault (compare Raymond, 1973).”

I know basalt can be found near the base of the dam and in an around the hills south of the dam.

• Teconic wedging