Magnitude is determined by various methods. Magnitude is an index of the amplitude of the seismic waves created by an earthquake. The intensity scale most commonly used in the United States. The first widely used magnitude scale. Return periods of the larger, infrequent, earthquakes can be estimated from that of the smaller ones, and sometimes from geologic clues in sediments that were disturbed by ancient shocks.
Return periods are shorter in the West than the East, and also shorter for smaller earthquakes.
A return period is an average, and does not imply that earthquakes happen on any schedule. It often breaks, like a huge surf-wave. Popularly called a tidal wave.