In addition to what Brian said, you have to watch the quality of the hypocenter location. There can be significanr errors. You might want to limit yourself to "A" quality hypocenters. For some areas like Parkfield, even M1s should be precisely and accurately located, but you need to have the correct catalogue. For southern California, you would want to use the various catalogues of relocated earthquakes. But. Nicholson and others SCEC abstracts (2008?) show that the different relocations can be pretty different. Theer can be systematic shifts related to problems with the velocity model (for example, faster velocities on one side of the San Andreas.
etc.
Chris