California experiences approximately 10,000 earthquakes every year, which works out to approximately 27 earthquakes a day. However, most of these are so small that people cannot feel them.
Earthquakes that are large enough to cause moderate damage happen approximately two to three times a year. These earthquakes measure a magnitude of 5.5 or higher, which is used to determine the strength of earthquakes. Earthquakes that have a magnitude of 6.7 or higher, which are commonly referred to as big earthquakes, occur even less frequently. Many earthquakes occur in uninhabited parts of California, so even larger quakes may not be felt by many people.
Earthquakes occur unpredictably, so there is no way to say for sure when the next one is going to strike. Although small earthquakes temporarily relieve pressure on the fault lines, larger earthquakes may still occur at any time. Seismologists use the earthquake history of an area as well as studies of pressure building up in the faults to help predict the frequency of large earthquakes. However, these estimates are often imprecise because earthquakes are still poorly understood due to the difficulty of studying them. Scientists have been predicting a big earthquake in southern California for decades, but as of January 2015, it has yet to occur. The last big earthquake in that area of California was in 1857.