The mainshock began on the San Andreas fault off the coast near San Francisco at 5:12 in the morning of 18 April 1906. Many years later, researchers assigned it a magnitude of 7.8 or so. Violent shaking went on for some 60 seconds. The rupture extended far to the north and south, 430 kilometers in all. Parts of the fault were offset by as much as 6 meters. By day's end and dozens of aftershocks later, much of San Francisco was a shambles and large fires had broken out, threatening the rest.