The deeper they are, the less they are felt at the surface. It is not always the place that has the most damage. An aftershock is an earthquake that occurs after the main earthquake. There are generally hundreds or even thousands of aftershocks, and they may occur for months or even years afterwards. They can be almost as powerful as the main earthquake, and can cause further damage to buildings which were damaged by the main quake.
Foreshocks are earthquakes that occur before the main earthquake.
If there are a series of earthquakes, generally the most powerful one is the main earthquake and all of the others are either foreshocks or aftershocks. The portion of California west of the San Andreas Fault is firmly attached to the Pacific Plate and the portion east is firmly attached to the North American Plate. San Diego is moving north towards Los Angeles at the rate of about 2 inches per year and is in no danger of falling into the ocean. The Los Angeles City Hall is 10 feet closer to San Francisco than when it was built in 1924. In about 500,000 years San Diego could be an island off the coast of San Francisco.