I believe, in my most unscientific way, that the earth releases stress in a systematic way. So, when earthquakes occur, they occur in patterns until the energy within has been dispersed. The waves generated by the earthquake must travel in and through the crust, mantle and core of the earth until they reach the opposite point on the sphere. Then reflection would happen. Other related quakes.

Then again, what the hell do I know? Geology 101.