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What is a Seismograph? - Definition, History & Facts

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  • 0:00 What's a Seismograph?
  • 1:00 How a Seismograph Works
  • 2:00 Measurement Scales
  • 3:15 The First Seismograph
  • 3:55 Fun Facts
  • 4:50 Lesson Summary
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Lesson Transcript
Instructor
Mary Ellen Ellis
Expert Contributor
Maria Airth

Maria has a Doctorate of Education and over 20 years of experience teaching psychology and math related courses at the university level.

Read about the seismograph, what it measures, how it works, and how it has helped scientists to better understand earthquakes both in the past and today.

What's a Seismograph?

Earthquakes are fascinating and frightening at the same time. Have you ever wondered how scientists study these earth-shaking events? They use what's called a seismograph, also called a seismometer, which is an instrument that measures and records seismic waves that move through the earth as the result of an earthquake.

A modern seismograph can help scientists detect earthquakes and measure several aspects of the event:

  • The time at which the earthquake occurred
  • The epicenter, which is the location on the surface of the earth below which the earthquake occurred
  • The depth below the earth's surface at which the earthquake occurred
  • The amount of energy released by the earthquake

Scientists measure and record this data to learn more about earthquakes, tectonic plates, and Earth's layers. Earthquakes are difficult to predict, but scientists studying them hope to use seismographic measurements to be able to make more accurate predictions.

How a Seismograph Works

The idea behind a seismograph is fairly simple. A basic seismograph includes a solid base and a heavy weight suspended from a spring over the base. A pen hangs from the weight and a rotating drum with paper sits below it on the base. The tip of the pen touches the drum. When the earth shakes from an earthquake, the drum rotates, and the weighted pen moves back and forth due to the motion of seismic waves. The pen records the movement on the drum. The paper recording of an earthquake is called a seismogram.

The most high-tech seismographs used by scientists studying earthquakes today are sophisticated and precise. They are based on the same concept as a basic, simple seismograph, but make use of electronics, magnets, and amplifiers in order to accurately and precisely measure the smallest ripples in the earth caused by earthquakes.

Measurement Scales

You have probably heard of the Richter scale, a popular unit for measuring the magnitude of an earthquake. It was invented by Charles F. Richter of the California Institute of Technology in 1935 and uses a logarithmic scale to measure seismic wave magnitude. Because the scale is logarithmic, a difference of one unit represents a tenfold difference in the magnitude of a seismic wave. For instance, an earthquake measured as 6.0 on the Richter scale is 10 times more intense than a 5.0 earthquake. A 7.0 earthquake is 100 times more intense than the 5.0 earthquake.

The Richter scale is still often cited in news reports of earthquakes, but scientists studying these events use another scale that allows for more accurate measurements. This is called the moment magnitude scale and it can be applied to a wider range of types and sizes of earthquakes. It is logarithmic, like the Richter scale, and similar to the older scale for earthquakes up to about a magnitude of 8.0. For larger quakes, the moment magnitude scale begins to differ from the Richter scale and provides more accurate measurements.

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Additional Activities

Build Your Own Seismograph Project

In this extended project, students will attempt to build a unit that can measure simulated earthquakes. All materials will be determined by the students.

Instructions

  • Review the video lesson with your students.
  • Conduct a brief class discussion reviewing all major terms highlighted in the lesson (these are in bold in the lesson transcript).
    • You may wish to give your students print-outs of the transcript of the video lesson.
  • Now, tell your students that they are going to device their own machine to measure a simulated earthquake (a shaking desk).
  • Give your students these parameters:
    • The machine should not be motorized.
    • The machine must be able to differentiate between strong and weak simulated earthquakes.
    • The machine must fit on a table top or desk
  • Encourage your students to refer back to the information in the video lesson to give them guidance for how they should build their machines.
  • If desired, divide your students into paired working groups.
  • Assign a deadline for the project to be completed.
  • Encourage your students to be creative with their designs.

Testing

  • When students submit their machines, set up a testing table.
  • To test each machine, allow the students to set up their machine, explain how it works and then demonstrate that it does work by shaking the table on which the machine is set up.
  • After all machines have been tested, instruct your students to write a brief summary of what designs worked best and why.

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