Nearly two years ago, on August 24, 2014, just south of Napa, California, a fault in the Earth suddenly slipped, 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, 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.