Earthquakes in northern California are mostly concentrated along the San Andreas fault (blue line) at relatively shallow levels (<25 km). At greater depths, the rocks along the boundary between the Pacific and North American plates are sufficiently hot that they flow and deform plastically rather than breaking brittly to generate earthquakes. Another prominent clustering of somewhat deeper earthquakes occurs just offshore of Cape Mendocino, where the San Andreas fault turns west and meets a trench and a transform fault in a plate-tectonic "triple junction". Also, note the seismic activity on the Hayward fault east of San Francisco Bay. This fault runs directly through the University of California football stadium, where 2-5 mm of right-lateral slip takes place each year.
