"San Ramon sits over a complex geologic environment, which is one theory, Williamson said, for why the area gets swarms every few years. The Calaveras Fault runs underneath the city in a transition zone. It is part of the San Andreas Fault system and is capable of a magnitude 6.7 earthquake. To the east, the Mount Diablo Thrust Fault begins. "All these earthquakes happen in that transition area," Williamson said. "Any small changes kind of cause that area to preferentially get more earthquakes.""
"The last big quake the Calaveras Fault produced was in 1984 in Morgan Hill with a magnitude of 6.2. But if the Hayward Fault and the Calaveras Fault, which UC Berkeley scientists found are connected, rock simultaneously, that could result in a magnitude 7.3 earthquake. Faults contain liquid that lubricates the earth, making it easier to move and causing earthquakes. Similar to a person wetting their hands and sliding them together, the fluid reduces the friction, and the rock "can slide more freely," Williamson said."
Two nighttime earthquakes on Dec. 8 woke local residents and nearly 1,600 people reported feeling the temblor as far away as San José. San Ramon lies above a complex geological transition zone where the Calaveras Fault runs beneath the city and connects with the San Andreas system; the Mount Diablo Thrust Fault begins to the east. The Calaveras Fault is capable of magnitude 6.7 shaking; simultaneous rupture with the Hayward Fault could produce about magnitude 7.3. The Calaveras Fault produced a magnitude 6.2 quake in Morgan Hill in 1984. Underground fluids lubricate faults, reducing friction and enabling more frequent swarms.
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