Earthquake Facts

1. What are earthquakes?

An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.

2. What causes earthquakes?

According to the USGS, an earthquake occurs when plates grind and scrape against each other. In California there are 2 plates: the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. These primary boundary between these 2 plates is the San Andreas fault. The San Andreas fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas fault zone. The Pacific plate grinds northwestward past the North American Plate at a rate of about 2 inches per year. Parts of the San Andreas fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing great earthquakes when it finally releases.

A fault is a thin zone of crushed rock between two blocks of rock, and can be any length, from centimeters to thousands of kilometers. It is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Most faults are the result of repeated displacements over a long period of time.

There are 3 different kinds of faults: (1) Normal, dip-slip fault. The fault plane of a normal fault dips away from the uplifted crustal block. Faulting occurs in response to extension.
(2) Reverse, dip-slip fault. The fault plane of a reverse fault dips beneath the uplifted crustal block. Faulting occurs in response to compression.
(3) Strike-slip fault. Crustal blocks slide past each other. The slip may be left lateral or right lateral. This example shows a left-lateral, strike-slip fault.

Earthquakes occur on faults. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. The slip direction can also be at any angle. Because of this, there are two different types of earthquake that can occur. The strike-slip earthquake occurs on an approximately vertical fault plane as the rock on one side of the fault slide horizontally past the other. The dip-slip earthquake happens when the fault is at an angle to the surface of the earth and the movement of the rock is up or down.

Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. Not all earthquakes result in surface rupture. Fault rupture almost always follows preexisting faults which are zones of weakness. Rupture may occur suddenly during an earthquake or slowly in the form of fault creep, which is the slow movement of faults in the earth's crust. Sudden displacements are more damaging to structures because they are accompanied by shaking.

There are several ways of finding a fault: (1) the earthquake left surface evidence, such as surface ruptures or fault scarps (cliffs made by EQs); (2) a large earthquake has broken the fault since we began instrumental recordings in 1932; and (3) the fault produces small earthquakes that we can record with the denser seimographic network established in the 1970s.

3. Can earthquakes be prevented?

No, but they can be mitigated. With proper planning now, you can avoid a lot of damage to your home, business and yourself when an earthquake strikes. To learn more about protecting your home or business from an earthquake, please see the home mitigation and business mitigation pages on our site.

Learn about how scientists are attempting to predict earthquakes.

4. How many earthquakes occur in the US every year?

The National Earthquake Information Center reports from 12,000 to 14,000 earthquakes yearly. Approximately, that averages out to 35 earthquakes every day.

5. How are earthquakes measured?

Earthquakes are measured with a seismographic network. Each seismic station in the network measures the movement of the ground at the site. The slip of one block of rock over another in an earthquake releases energy that makes the ground vibrate. That vibration pushes the adjoining piece of ground and causes it to vibrate as well, and thus the energy travels out from the earthquake in a wave. Most earthquakes are measured by magnitude. Magnitude measures the energy produced by the EQ, rather than what people feel during the event.

The instruments used to record the motion of the ground during earthquakes are called seismographs. First developed during the 1890's, these instruments are installed in the ground throughout the world and operate as seismographic network. At the heart of the seismograph is a seismometer, which is a pendulum or a mass mounted on a spring, and which moves during earthquakes. Seismographs produce seismograms, which is the paper copy with squiggly lines that almost everyone has seen. Here is a seismogram from the January 17,1994 earthquake in Northridge, CA. The movement of the seismometer is converted into a seismogram through three ways: (1) a pen drawing an ink line on paper revolving on a drum; (2) a light beam making a trace on a moving phographic film or (3) electromagnetic system generating a current that is recorded electronically on tape.

Seismograms show how the ground moves with the passage of time. The HORIZONTAL axis or the seismogram shows time (measured in seconds). The VERTICAL axis shows ground displacement (usually measured in millimeters). When there is no earthquake, the seismogram usually shows a straight line except for small wiggles caused by local disturbance or "noise."

Follow these links to learn about both the Richter Magnitude Scale and Modified Mercalli Intensity Scale for measuring earthquake intensity.

For a much more detailed description of how earthquakes are studied, visit the USGS site on studying earthquakes.

6. How are the exact locations of earthquakes pinpointed?

7. What do all these seismologic terms mean?

Visit the Glossary of Some Common Terms in Seismology to find out.

The above information is from the USGS's Frequently Asked Questions about earthquakes, a very informative site. To find out more, visit it yourself!