Earthquake Facts

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.

What is the probability of a severe earthquake striking a major city?

The probability of a severe earthquake striking a city is typically very small, and of course that probability depends on which city you're talking about. Some cities, such as Los Angeles, are in very geologically active regions where earthquakes are common. In those locations, the probability of an earthquake is higher. Some cities, like New York City on the east coast of the United States, are not in a geologically active region, and therefore the probability of an earthquake occurring is lower.

Where do earthquakes most often occur?

Earthquakes most often occur on the boundaries between major tectonic plates. The earth's surface is broken into a dozen or so major plates, plus a number of smaller plates and those plates are moving with respect to each other. On the boundaries between two plates is where that motion occurs and that's where we find the big earthquakes. For example, in California there is a major plate boundary between the pacific plate to the west and the North American plate to the east. The fault that marks that boundary is the San Andreas fault and that's where we have some of the largest earthquakes in the continental United States.

What are the most common earthquake myths?

There are many earthquake myths. One of my favorite is the myth of “earthquake weather.” Some people believe that certain weather patterns are condusive to the occurrence of earthquakes, and there is no scientific evidence that this is the case. Another earthquake myth is that large regions like California might break off and fall into the ocean. Even the largest earthquakes produce motions along geologic faults of only about 1-2 meters--not nearly enough to submerge California. Another earthquake is that animals can sense earthquakes coming in advance. Although there are many stories of this sort, there is, again, no scientific evidence that animals can detect earthquakes in ways that humans cannot.

What is the "Foreshock", "Aftershock", and "Main Shock" of an earthquake?

A foreshock is a small earthquake that occurs before a larger earthquake. Aftershocks are small earthquakes which occur following a larger earthquake. Main shock is a term that is used to distinguish a large earthquake from its foreshocks or its aftershocks.

How long do most earthquakes last?

Earthquakes can last different periods of time depending on their size. Small earthquakes typically last only a few seconds. A large earthquake, such as the 24 earthquake which occurred in Sumatra, can last for many minutes.