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Recording and Measuring Earthquakes

Earthquakes generate vibrations called seismic waves that travel through the earth in all directions from the focus, the point beneath the earth's surface where the earthquake begins. The point on the earth's surface directly above the focus, where the strongest shaking occurs, is called the epicenter (Fig. 4). Shaking decreases with distance from the epicenter.

Figure 4--Spatial relationship between focus and epicenter of an earthquake.

The focus is the point withing the Earth where the quake starts; the epicanter is the point on the surface directly above the epicanter

Seismologists use sensitive instruments called seismographs to record these waves. Seismographs can electronically amplify seismic waves more than 10,000 times and are sensitive enough to detect strong earthquakes originating anywhere in the world. The time, location, and magnitude of an earthquake can be determined from a graphical plot of the data called a seismogram.

To measure the strength of an earthquake, seismologists use two different scales: the Modified Mercalli Intensity Scale and the Richter Magnitude Scale. The Modified Mercalli Intensity Scale gauges earthquakes by their effect on people and structures. It was originally developed in 1902 in Italy, and it relies on newspaper and eyewitness reports. This scale is used to gauge the size of earthquakes that occurred before sensitive instruments existed to measure them. It has 12 levels designated by roman numerals, ranging from imperceptible shaking (I) to catastrophic destruction (XII).

The other measure of earthquake size is the Richter Magnitude Scale. It was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. Using the record of the seismic waves plotted on the seismograph, seismologists determine the magnitude mathematically from the size of the recorded waves and from the calculated distance between the earthquake focus and the seismograph recording station. The Richter Scale expresses magnitude in whole numbers and decimal fractions. Each increase in magnitude by a whole number represents a tenfold increase in measured wave size. In terms of energy, each whole-number increase represents 31 times more energy released--for instance, a Richter 5.3 earthquake releases 31 times more energy than an earthquake of Richter 4.3.

The world averages about 20 earthquakes each year that have Richter magnitudes of 7 or larger. The largest earthquake ever recorded in the world was a magnitude 9.5 in Chile in 1960. Sensitive seismographs are capable of recording nearby earthquakes with Richter magnitudes of -1 or smaller. A person with a sledge hammer can generate the equivalent of a Richter magnitude -4 earthquake.

An earthquake's magnitude does not necessarily express the damage it caused. In a densely populated area, an earthquake may do far more damage than one of greater magnitude that occurs in a remote area. For example, the magnitude 6.8 earthquake that hit Kobe, Japan, on January 16, 1995, killed 6,308 people and injured thousands of others. Though it was the deadliest earthquake in 1995, its magnitude was lower than 25 other earthquakes recorded that year.

Although the Richter Magnitude Scale and the Modified Mercalli Intensity Scale are not strictly comparable, they can be roughly correlated for locations near the epicenter of an earthquake (Fig. 5). In Kansas, for example, the Palco earthquake of June 1989 had a Modified Mercalli (MM) intensity of IV and a Richter magnitude of 4.0, and the 1867 Manhattan earthquake had a MM intensity of VII and a Richter magnitude that was probably between 5.0 and 5.5.

Figure 5--Approximate comparison for the U. S. Midcontinent of Modified Mercalli and Richter scales at locations very near the epicenter.

Chart compares the two ways of measuring an earthquake

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Kansas Geological Survey, Public Outreach
1930 Constant Ave., Lawrence, KS 66047-3726
Phone: (785) 864-3965, Fax: (785) 864-5317
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Web version July 1996