Magnitude: Changing an Earthquake's Magnitude (downgrade/upgrade)

6min 50s Novice Spanish

Why do seismologists change the magnitude of an earthquake?

There are often misunderstandings about why seismologists change the magnitude of an earthquake. The magnitude isn't a simple assessment of a squiggle on a seismogram.

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. In this animation we look at two magnitude scales, and use two examples of magnitude changing.

CLOSED CAPTIONING: .srt file is included with the download. Use appropriate media player to utilize captioning.

Keypoints:

Factors in calculating the magnitude of an earthquake:

  • Amplitude of seismogram
  • Frequency of seismic wave
  • Time between P, S, and surface wave arrivals
  • Equation used to determine the magnitude
  • Orientation of the fault and how it moved
  • Distance the fault moved
  • Area of the moving fault
  • Rigidity of rock on both sides of the fault
  • Duration of the earthquake rupture
  • Kind of bedrock/soil on which the seimograph is built
  • How the waves traveled through the Earth
  • Characteristics & calibration of the seismograph instrument
  • Types of waves recorded
  • Size of the earthquake

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Related Fact-Sheets

Knowing precisely where an earthquake occurred is an important piece of scientific information. It can help seismologists identify and map seismic hazards. It is also a fundamental piece of information necessary for facilitating studies of Earth's internal structures. This fact sheet provides an overview of the S-P process to locate an earthquake.

NOTE: Out of stock; self-printing only.

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A seismograph is a device for measuring the movement of the earth, and consists of a ground-motion detection sensor, called a seismometer, coupled with a recording system. This fact sheet provides an overview of the basic components of a seismometer and physical science principles behind its operation.

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Learn about the earthquake magnitude scale and changes in the amount of energy released at each step by breaking different size bundles of uncooked spaghetti noodles! Students can both see and feel the differences in the energy released from a M4 - M8 quake. 

 

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Related Videos

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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