# Earthquakes, Measuring

Earthquakes happen every day. Thousands occur across the world every week. Most are not felt, but throughout history there have been many earthquakes that have been very strong and caused extensive destruction. Mathematics skills are invaluable to earthquake scientists, and with them they can sometimes predict when and where earthquakes will happen, and make measurements of earthquakes when they do occur.

Earthquakes happen when the tectonic plates of the Earth shift. As this movement happens, pressure builds on the plates and faults. Eventually this pressure is released through an earthquake. During an earthquake seismic waves radiate out from a central point in all directions. There are four basic types of seismic waves, two that travel through the Earth, and two that are felt at the surface. These waves are recorded on a seismograph, which is an instrument made up of sensitive detectors that produce a permanent recording.

## Size of Earthquakes

To determine the magnitude or size of an earthquake, scientists use the Richter scale. Developed by Charles Richter in 1934, this scale is based on a logarithmic increase of magnitude. With a scale such as this, for every whole number increase on the scale, the amplitude of the earthquake goes up ten times. For example, an earthquake with a reading of 5.0 on the Richter scale has a magnitude 105 (or 100,000) times as great as an earthquake with a magnitude of zero.

There are more than 1,000 earthquakes a day with recorded magnitudes of two or greater. An earthquake measuring between 6.1 and 6.9 can cause destruction in an area up to 100 kilometers in diameter. The San Francisco earthquake of 1906 measured 7.8 on the Richter scale.

Another way to measure an earthquake is to use the Mercalli Intensity Scale. This scale measures the intensity or energy of an earthquake. Though each earthquake has a fixed magnitude, the effects of it are different depending on location. This measurement is based on criteria such as structural damage and the observations of people who felt the earthquake. From these types of observations, the intensity of an earthquake can be estimated.

The Mercalli scale is not considered as scientific as the Richter scale, since it is based on factors that can change depending on where the information is derived. After an earthquake, for example, witnesses may exaggerate or not agree on what they saw. In addition damage does not always accurately measure the strength of an earthquake.

## Depth and Location of Earthquakes

Earthquakes occur between the Earth's surface and about 700 kilometers below the surface. The way to determine the depth of an earthquake is to look at wave characteristics on a seismogram, which is a graph of seismic waves. For example, an indication of a large earthquake with a deep focus would be surface waves with small amplitudes, and uncomplicated deep waves.

The point on Earth's surface directly above the origin of an earthquake is called its epicenter . To find out where an earthquake is located, scientists must examine its waves. The simplest method is to look at the different arrival times of wave types at multiple seismograph stations. Scientists then use standard travel-time tables and travel-time curves to find the distance to the earthquake from each station. Arcs are then drawn, with the distance from each station to the earthquake used as a radius. The point where all the arcs intersect is the epicenter of the earthquake.

## Calculating Earthquake Odds

Earthquakes are naturally recurring events, and scientists continue to develop better methods to predict when and where earthquakes might happen.* Earthquake probabilities are based on balancing the continual motions of the Earth's plates with the slip on faults, which occurs primarily during earthquakes. Scientists must also look at the history of a given fault to know when it last ruptured, potential quake sizes, and the rate at which the plate is moving. By combining geology, physics, and statistics, scientists continue to become more accurate in their earthquake predictions.

*When predicting earthquakes, some scientists study the behavior of animals, which can become very erratic before a quake strikes.

Brook E. Hall

## Bibliography

Bolt, Bruce A. Earthquakes. New York: W. H. Freeman, 1999.

Lay, Thorne, and Terry C. Wallace. Modern Global Seismology. San Diego: Academic Press, 1995.

### Internet Resources

Calculating the Earthquake Odds. United States Geological Survey. <http://geopubs.wr.usgs.gov/fact-sheet/fs152-99/calcodds.html>.

How Are Earthquake Magnitudes Measured? UPSeis. <http://www.geo.mtu.edu/UPSeis/intensity.html>.

The Science of Seismology. United States Geological Survey. <http://earthquake.usgs.gov/4kids/science.html>.

## MAJOR QUAKE LIKELY TO STRIKE BETWEEN 2000 AND 2030

Scientists believe that there is an 80-percent probability of at least one earthquake with a Richter magnitude between 6.0 and 6.6 striking the San Francisco Bay region before 2030. By keeping aware of earthquake threats, people can make informed decisions on how to prepare for future quakes.

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