The Mercalli scale is not widely used for modern earthquakes as it is inaccurate in areas of low population density and in cities which lack stringent building codes, and has a range of values with distance from the epicenter. Great, major and strong earthquakes are differentiated by Richter magnitude. The Richter scale measures earthquake magnitude, the amplitude of S-waves recorded on a seismograph following an earthquake. Earthquakes are concentrated in narrow seismic belts along plate boundaries. The largest earthquakes are typically associated with convergent boundaries.
Shallow earthquakes occur along all plate boundaries.
Most U.S. earthquakes occur in Alaska and California but several smaller quakes occur along old fault zones in the continental interior. Earthquake magnitude is related to fault length. Longer faults yield larger earthquakes that shake the ground for longer periods. Future large earthquakes are anticipated where strain has accumulated along faults that have not experienced recent seismic activity. Scientists predict the long-term probability of earthquakes for specific locations based upon information about strain accumulation and recurrence interval.