What is an "earthquake"?
An earthquake is any shaking of the ground - usually intense shaking of the ground - caused by either natural sources or by humans. The most common types of earthquakes are those caused by ruptures of geologic faults or earthquakes associated with volcanic activity.

What are the different types of earthquakes?
There are three major types of earthquake. Firstly, geologic earthquakes - sometimes called tectonic earthquakes - are caused by the rupturing of faults in the earth's crust. There are volcanic earthquakes, caused by the movement of magma in volcanoes. Lastly, there are various types of earthquakes caused by man-made activities, for example, explosions or the collapse of large structures.

Can earthquakes be predicted?
Whether earthquakes can be predicted is a very difficult question for we seismologists to answer. Certainly, there are things that we can predict about earthquakes. For example, we know where earthquakes are going to occur: primarily on large geologic faults. We know how big those earthquakes might be. For example, on the San Andreas Fault here in California, we know earthquakes can be as large as magnitude 8. The difficulty is that we cannot predict when large earthquakes will occur with any kind of reliability or precision needed to warn communities of impending disasters.