Most earthquakes in California cause little, if any, damage, but exceptions exist. As of February 2015, the most damaging earthquake in California is the Great San Francisco Earthquake of 1906. At a 7.8 magnitude, it leveled the San Francisco.
Most earthquakes aren't even felt. In the southern California area alone, seismographs pick up between 2,000 and 10,000 earthquakes per year that are larger than 1 magnitude on the Richter scale. The location of the epicenter, or starting point, of a quake, the size and duration of the quake, and the type of soil involved determine if an earthquake can be felt or is likely to cause damage.
The 1906 quake was felt as far away as Santa Rosa and San Jose. Throughout the San Francisco Bay area, more than 3,000 people died, 225,000 were made homeless and 28,000 buildings were destroyed. Estimated monetary damage, in 1906 dollars, was more than $400 million.
The 1989 Loma Prieta quake had a magnitude of 6.9, killed 63 people and caused property loss of between $6 billion and $10 billion. Most of the deaths occurred in the East Bay, where the Nimitz freeway collapsed. The Bay Bridge was also damaged. The 1994 Northridge quake, at 6.7 magnitude, killed 57 people and caused $20 billion worth of damage.