Scientists estimate that over 10,000 earthquakes occur in California each year. Most of these go unnoticed since they are minor. For example, only several hundred have a magnitude greater than 3.0 and of these,only 15 to 30 have a magnitude in excess of 4.0.
Research conducted by the U.S. Geological Survey in 2008 reported that California can expect an earthquake of 6.7 magnitude or greater once every 6.7 years. The southern portion of the San Andreas system of faults is of particular concern to geologists who feel this to be the site of the next major quake that they consider a certainty within the next 20 years.