When a series of earthquakes with shallow focal depths occur within a limited time and area and if there is an event, which is much larger than the rest, this event is called the main shock. Earthquake occurring before the main shock are called aftershocks. A series of earthquakes without a main shock is called an earthquake swarm.
The patterns of earthquake occurrence in tome domain can be generally divided into three groups:
1. Foreshock-main shock-aftershock,
2. Main shock-aftershock, and
3. Earthquake swarm.
Foreshocks are rarely observed while aftershocks are always observed after every large earthquake. Foreshocks are important for earthquake prediction and aftershocks are vital for the study of the mechanism of the rupture of the main shock. The area where aftershocks occur is called the aftershocks region. The distances from the focus and the epicenter to an observation point are called focal distance and epicenter distance respectively. The observation point is termed the station. Seismic destruction propagates from the focus through a limited region of the surrounding earth body, which is called the focal region. The larger the earthquake, the larger the focal region. When evaluating earthquake parameters, it should be kept in mind that the data may contain inaccuracies. The dates of old earthquakes are often not accurate in the catalogs and descriptions of the size of the earthquake and its effects are sometimes imprecise. For recent earthquakes, the data are usually accurate. Origin time is known with an accuracy of a few seconds for earthquakes in the second half of the 20th century. The accuracy decreases with elapsed time and depends on the number of recording stations and on the accuracy of the time service. Most instrumental epicenters are now determined with a standard error of 10 – 20 km; the earlier instrumental determinations are known with a low accuracy varying according to the number, position, and reliability of recording stations.