Earthquake Magnitude and Intensity

A fault slips (or moves) when the force of friction along the fault is overcome. This slip results in an earthquake. The size (or amount) of slip and the area of the fault that slips determine an earthquake's magnitude. An earthquake has one size (or magnitude), but varying intensities. Intensity is the shaking you feel during an earthquake. The shaking you feel depends on several factors, described in this module. 
The success of an earthquake early warning system depends on the quick calculation of an earthquake's location and magnitude. Location and magnitude help to determine intensities. ShakeAlert delivery partners trigger automated actions and public alerts when an earthquake exceeds a specified magnitude and intensity. If you feel the bump of an earthquake, it's important to get to a safe area and Drop, Cover, and Hold On.

  • 8 Number of Resources

Is seismic intensity really different than an earthquake's magnitude? Find out here!

Take 2: Magnitude vs. Intensity
Time: 2m / Level: Novice

Magnitude and intensity are both related to the size of an earthquake, but they each measure different aspects. One is measured using seismometers; the others is felt. Let's use a lightbulb as an analogy.

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The magnitude of an earthquake is a measure of its size. The Richter (or local) magnitude depends on the amplitude of the seismic waves. The moment magnitude depends on the physical parameters of an earthquake. Let's find out more!

Pasta Quake: Exploring Earthquake Magnitude
Time: / Level: Novice

Magnitude (M) describes the size of an earthquake. The energy released in an earthquake relates to the difference in magnitude. Each step in magnitude represents a 32 times increase in the amount of energy released.

Breaking spaghetti illustrates this concept. If a Pasta Quake M5 earthquake is one spaghetti noodle, 32 noodles is a M6 earthquake! And 1024 noodles is a M7 earthquake - try to break that with your hands!

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Pasta Quake: Modeling magnitude scale using Spaghetti
Time: 2m 41s / Level: Novice

Understanding the magnitude change, thus the relative energy released from say, magnitude 7 to magnitude 8 can be challenging. Dr. Robert Butler (Univ. of Portland) uses spaghetti to illustrate the concept by breaking pasta to show how each step up in magnitude represents a huge jump in the size of the pasta bundles. Each step in magnitude is represented by 32 times more spaghetti noodles.



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Magnitude: Changing an Earthquake's Magnitude (downgrade/upgrade)
Time: 6m 50s / Level: Novice

Why do seismologists change the magnitude of an earthquake? Obtaining an accurate preliminary magnitude can be difficult due not only to the complex processes that occur deep within the Earth, but because there are over a dozen techniques of for calculating the magnitude of an earthquake. 

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Earthquake: Foreshock—Mainshock—Aftershock
Time: 6m 32s / Level: Novice

There is often confusion surrounding the terminology of earthquakes. Large earthquakes are usually followed by hundreds and even thousands of smaller earthquakes, called aftershocks. 

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Magnitude Explained: Moment Magnitude vs. Richter Scale
Time: / Level: Novice

The "moment magnitude" scale has replaced the Richter scale for large earthquakes. Scientists have developed far-more sensitive seismometers that, with faster computers, have enabled them to record & interpret a broader spectrum of seismic signals than was possible in the 1930's, when the Richter magnitude was developed. Find out what scientists learn from seismograms.

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The shaking you feel during an earthquake depends on four geologic factors. You may also feel different shaking depending on the building you are in.

Earthquake Intensity—Introduction to 4 modules
Time: 3m 10s / Level: Novice

Seismic intensity is the shaking you experience during an earthquake. An earthquake's magnitude is a measure of the energy released and is the same for all locations. Unlike magnitude, the intensity of shaking you feel depends on your location.

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Buildings in Earthquakes: Construction affects the intensity
Time: 6m 29s / Level: Novice

In earthquake-prone regions, buildings can be designed resist being deformed, The way the building is constructed and your position in the building can have an impact on the shaking you feel. Learn more about construction as well as mitigation practices.

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