The quake struck juts after 7:45 p.m. local time on Monday and was centered around the town of Dublin, according to the USGS.

The earthquake occurred at a depth of 4.6 miles and was on the San Andreas fault. CalTech researchers said the second quake was an aftershock. "Aftershocks may be expected in the next few days ...

This sudden jerk creates an earthquake. The San Andreas Fault is undoubtedly the most famous transform boundary in the world. To the west of the fault is the Pacific plate, which is moving northwest.

That two inches -- the average distance the San Andreas fault moves each year -- has the potential to unleash an earthquake of epic proportions directly under the San Francisco and San Jose area ...