Residents of the Bay Area are well aware of the constant earthquake danger. The region lies on the boundary zone between two of the major tectonic plates (the Pacific and North American Plates) that make up the Earth's outer shell. The continuous motion between these plates against one another strains the crustal rocks of the Bay region creating stored energy. Eventually the stored energy is released through earthquakes normally centered along the seven major fault lines in the San Francisco Bay Area. According to the United States Geological Survey 22 major earthquakes have occurred in the area since 1970, as well as hundreds of less powerful earthquakes.
http://www.loc.gov/rr/scitech/tracer-bullets/earthquakestb.html) and the United States Geological Survey’s Earthquake Hazards Program-Northern California (http://quake.wr.usgs.gov )
Library of Congress; U.S. Geological Survey, 07/2005; 05/2005