the San Andreas faultSan An·dre·as fault, the /ˌsæn ændreɪəs ˈfɔːlt/ a crack in the Earth’s surface which runs from the north to the south of California in the western US. The large areas of rock on either side of this crack are slowly moving in different directions, and this sometimes causes earthquakes (=when the Earth’s surface suddenly shakes, causing a lot of damage). → see also San Francisco, plate tectonics