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Earthquakes: Measuring Magnitude

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Earthquakes: Measuring Magnitude

Earth & Space Science

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This Activity Object explains how the magnitude of an earthquake is calculated.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • After completing this Activity Object, learners will be able to:
  • Explain that the magnitude of an earthquake is the amount of energy that is being released at the earthquake’s focus and that the magnitude of an earthquake does not change with distance from the epicenter, the local geology, or the effect of the earthquake on man-made objects.
  • Explain that the intensity of an earthquake is measured by the amount of damage it caused and that it decreases with distance from the epicenter. It may change with the local geology and the types of man-made structures in the region.
  • Describe the steps taken when calculating the magnitude of an earthquake.

Everything You'll Have Covered

Geology is the study of the Earth, its materials, and the processes acting upon those materials. It also includes the study of organisms that have inhabited our planet since its origin. Geologists are scientists who work to understand geology. They work to learn more about the history of our planet so they can foresee how events and processes of the past might influence the future.

The Earth is made up of three distinct layers: the crust, mantle, and core. The crust is the thin, solid, outermost layer of the Earth. The layer below the crust is the mantle. The mantle has more iron and magnesium than the crust, making it denser. The uppermost part of the mantle is solid and, along with the crust, forms the lithosphere. The lithosphere is rocky and brittle, making it easy to fracture. This is the place where earthquakes occur. It is the lithosphere that breaks into the thick, moving slabs of rock that geologists call tectonic plates. Under the plates, deep in the center of the Earth, is the super-dense core. The core of the Earth is made of two distinct layers: a liquid outer layer and a solid inner core. The core is made of metallic iron-nickel alloy.

The Earth is also made of a dozen or so major plates and several minor plates. These tectonic plates are in constant motion. Some of the plates are moving as much as 15 centimeters (6 inches) per year. If you live where two plates meet, you may experience geologic phenomena, such as earthquakes or volcanic eruptions. Wherever plates grind against each other, earthquakes can be expected. When an earthquake occurs, there is a sudden movement in the Earth's lithosphere, which causes ripples to move out in waves across the plate.

Earthquakes produce two types of seismic waves, primary (P) and secondary (S) waves. P waves travel faster and shake the ground first. Then, the S waves follow and shake the ground too. If you are close to the earthquake, the P and S waves will arrive one right after the other, but if you are far away, there will be more time between the two. These seismic waves are measured using an instrument called a seismograph. The seismogram recordings allow scientists to determine how large an earthquake is. Scientists also measure the amount of time between the P and S waves on a seismogram, which tells them how far away the earthquake was from that location.

When scientists study an earthquake, they look at several things. They find out where the focus is located. The focus is the exact point where the earthquake started, usually many miles/kilometers below the surface of the Earth. Scientists also search for the epicenter. The epicenter is the point on the surface of the Earth directly above the focus.

The strength of the earthquake is called its magnitude. There is one magnitude for each earthquake. When measuring an earthquake's magnitude, scientists rely on a scale developed in 1935 by Charles Richter. The magnitude of an earthquake is determined by measuring the maximum amplitude of a seismic wave. Next, scientists determine the difference in arrival times of the primary and secondary waves. Using a time interval-distance graph, they find the distance from the seismograph station from the epicenter. Finally, they mark the distance and the amplitude values on the Richter scale. The intersection point of the line on the Richter scale is the magnitude of the earthquake.

On the Richter scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3.

Scientists also talk about the intensity of shaking from an earthquake; this varies depending on where you are during the earthquake. The Modified Mercalli Intensity scale is used when measuring the severity of an earthquake's effect. Intensity ratings are written as Roman numerals with an I at the low end and a XII at the high end. Each rating has a corresponding description. For example, if a person reports that objects hanging on the walls swung back and forth and the windows rattled, then the intensity is set to be VI. Ratings on the intensity scale do not require any instruments. This allows scientists to use personal reports from newspapers and other historical records to make intensity ratings of past earthquakes. The Modified Mercalli scale differs from the Richter scale in that the effects of any one earthquake vary greatly from place to place, so there may be many intensity values measured from one earthquake. On the other hand, each earthquake should have just one magnitude.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts earthquake, earthquake’s focus, epicenter, primary (P) waves, seismic waves, seismogram, seismograph
Course Earth & Space Science
Type of Tutorial Concept Development
Key Vocabulary amplitude, earth, earthquake