As a rule of thumb, for each magnitude unit increase, there are 10 times less earthquakes during a specified time interval. For example there are about 20 major earthquakes each year but only one or two great earthquakes. Earthquake Sequences Earthquakes are not isolated events, they occur in sequences. We call the large event the mainshock, and the events that follow are called aftershocks. Sometimes, earthquakes occur in interesting sequences which we call doublets, triplets, multiplets, or swarms depending on how many similar-size events are in the sequence.
Earthquakes are the result of slow-moving processes that operate within Earth.
Earth's cooling causes the portions of Earth to move, and that movement is what we call an earthquake. We will discuss the details later. Earthquake Catastrophes Earthquakes in populated regions have killed many people and have destroyed entire communities. In the US, we have been fortunate to avoid such disasters, the largest earthquakes in U.S. history occurred decades ago, before the dramatic increase in population in earthquake-prone regions. During the last hundred years there has been an average of about 17 large earthquakes each year.