We know that the San Andreas fault produces large earthquakes and that many other faults are also hazardous. However, it is often difficult to understand how to incorporate this information into our lives. Should we care only if we live near the San Andreas fault? Is every place just as dangerous? This section describes where and how often earthquakes happen in Southern California. It also explains how earthquakes will shake the ground and cause other hazards such as liquefaction and landslides.

Southern California earthquakes

Southern California has thousands of earthquakes every year. A few are damaging, but most are not even felt. However, almost none are on the San Andreas fault. The last significant earthquake on the Southern California stretch of this fault was in 1857. It is still storing energy for some future earthquake. Other faults produce most of our earthquakes.

Larger imageEarthquakes plotted using data from the Southern California Seismic Network. Significant earthquakes are numbered according to the table below.

The Southern California Seismic Network (operated jointly by the U.S. Geological Survey and the California Institute of Technology) recorded several hundred thousand earthquakes greater than magnitude 1 in Southern California between 1981 and 2003. The epicenters of these earthquakes are shown by the dark blue dots on the map of Southern California on this page. The colored circles are earthquakes larger than magnitude 4 since 1932, with the size of the circle increasing with magnitude. Small earthquakes are much more common — the map shows 360,000 earthquakes smaller than magnitude 4 in 22 years, but less than 1600 earthquakes above magnitude 4 in 70 years. The largest earthquakes (in red) are the 1992 magnitude 7.3 Landers and 1999 magnitude 7.1 Hector Mine earthquakes in the Mojave Desert and the 1952 magnitude 7.5 Kern County earthquake near Bakersfield.

By comparing this map with the fault map on the next page, we can see that there are very few small earthquakes along many of the major faults, including the San Andreas, Garlock, and Elsinore faults. Other major faults, such as the San Jacinto fault, have both small and large earthquakes. This shows how difficult it can be to predict future earthquakes from patterns of earthquakes.

“BIG EARTHQUAKES ALWAYS HAPPEN IN THE EARLY MORNING.”

This myth may be so common because we want it to be true. Several recent damaging earthquakes have been in the early morning, so many people believe that all big earthquakes happen then. In fact, earthquakes occur at all times of day. The 1933 Long Beach earthquake was at 5:54 pm and the 1940 Imperial Valley event was at 8:37 pm. More recently, the 1992 Joshua Tree earthquake was at 9:50 pm and the 2003 San Simeon event was at 11:15 am. It is easy to notice the earthquakes that fit the pattern and forget the ones that don't.