As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the main shock. The frequency of these aftershocks decreases with time. Each step of one in magnitude is an increase of ten times the amount of ground motion amplitude, corresponding to thirty-two times the amount of 'elastic' energy in the form of seismic waves. So a magnitude 6 quake has over 1,000 times as much energy as a magnitude 4 quake, and a 100 fold increase in ground motion amplitude. Above magnitude 6.0, the ground motion amplitude can no longer increase, and the excess energy is expressed as a longer duration of shaking.
Tsunamis are sea waves generated by an abrupt displacement of large volumes of water.
Large subduction zone earthquakes of magnitude 7.5 or greater are the most frequent cause of tsunamis, as the vertical displacement of the sea floor along the subduction zone fault results in displacement of the water above. A tsunami can also be generated by other types of submarine faults, as well as by large coastal or submarine landslides. Not all submarine earthquakes will cause tsunamis. A submarine earthquake with pure strike-slip motion may not produce a tsunami, because water is less likely to be displaced unless the ocean bottom is rough. Click here for a more in depth discussion on tsunamis.