Ever wondered why?
It's because of the population shift that happens every weekend. Most of California's population lives on the coast. On Friday, a significant percentage of the San Francisco Bay Area population gets on Highway 80 and heads for Tahoe/Reno. If it's summer, they go water skiing. If it's winter, then they go snow-skiing. On Sunday night, everyone heads back to the Bay Area. It's this constant teeter-totter effect that creates earthquakes.