Linear features are uncommon in nature, therefore when we examine an image of the earth, they tend to capture our attention. Viewed from above, certain fault systems appear linear. When examining this image, you should notice several things. The red circles represent earthquake events, and the yellow circles, magnitude. Notice that the red circles appear clustered.
Earthquakes are common along active major fault systems.
The main fault system in California is the San Andreas Fault. It enters California at the Mexican border and goes off-shore north of San Francisco. You might also notice other differences such as changes in vegetation or altitude. Here to more closely study changes in vegetation and altitude.