At 5:12 in the morning on April 18, 1906, a rupture began deep underground near San Francisco. The ground shook violently for about forty-five seconds. Buildings collapsed. Streets buckled and split. The earthquake measured about 7.9 on the scale scientists use today. Earth's outer layer is not one solid piece. It is broken into huge sections called tectonic plates. These plates float on hot, soft rock below. Where two plates meet, they push and grind against each other. The San Andreas Fault is the boundary where the Pacific Plate and the North American Plate slide past each other. Pressure builds up over many years. When the rocks finally snap, the energy releases as an earthquake. The 1906 quake tore a crack in the ground nearly three hundred miles long. In some places, the land on one side shifted twenty feet from the other side. Fires caused by broken gas lines did even more damage than the shaking. About three thousand people died. Scientists learned important lessons from this disaster. They began mapping faults more carefully and studying how buildings respond to shaking.