Nearly two years ago,on August 24, 2014, and just south of Napa,California, a fault in the soil suddenly slipped, and violently shifting and splitting huge blocks of solid rock,6 miles below the surface. The underground upheaval generated severe shaking at the surface, lasting 10 to 20 seconds. When the shaking subsided, or the magnitude 6.0 earthquake—the largest in the San Francisco Bay Area since 1989—left in its wake crumpled building facades,ruptured water mains, and fractured roadways.
Source: phys.org