Measuring an Earthquake's Strength
Measuring an Earthquake's Strength
Scientists use seismographs to determine where an earthquake's epicenter is. They also use seismographs
to measure an earthquake's strength, or magnitude. During a small earthquake, Earth's surface may shake
only a little. The seismogram shows these relatively low-energy seismic waves as little wiggles. During
a big earthquake, Earth's surface shakes a lot harder. The seismogram shows these high-energy waves as
big zigzags.
The Richter scale is another way scientists measure an earthquake's magnitude. The Richter scale
assigns a number to an earthquake based on the largest seismic wave recorded for that earthquake. The
higher the Richter scale number, the stronger the earthquake. For example, a magnitude 5.0 earthquake
on the Richter scale causes 10 times as much ground shaking as a magnitude 4.0 earthquake. A
magnitude 6.0 earthquake causes 10 times more shaking than a 5.0, and so on.
Damage caused by earthquakes
The Modified Mercalli Intensity Scale also uses numbers to measure earthquake strength. The numbers
are based on survivors' descriptions and the amount of earthquake damage. The higher the number, the
stronger the earthquake. The Mercalli scale is less scientific than the Richter scale, as few people
describe events in the same way.
Pressure along faults can build up for years, even centuries. When blocks of rock along a fault finally
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Measuring an Earthquake's Strength
move, the resulting earthquake happens very quickly. Most earthquakes last just a few seconds. Still, the
trouble may not be over after the ground stops shaking. Large earthquakes are often followed by
aftershocks. Aftershocks are like mini-earthquakes. They are usually smaller and weaker than the main
earthquake event. Aftershocks happen as blocks of rock along the newly slipped fault settle into place.
The Mercalli scale is less scientific than the Richter scale.
ReadWorks.org · © 2016 ReadWorks®, Inc. All rights reserved. This article is based on an original work of the Core Knowledge® Foundation made available through licensing under a Creative Commons
Attribution-NonCommercial-ShareAlike 3.0 Unported License. This does not in any way imply that the Core Knowledge Foundation endorses this work. This article is licensed under a Creative Commons
Attribution-NonCommercial-ShareAlike 3.0 Unported License. https://creativecommons.org/licenses/by-nc-sa/3.0/