How are Earthquakes measured?

Seismograph

We always hear that particular earthquakes have a certain Magnitude.  But what is that, what does that mean? In this post we will see how Magnitude numbers portrayed in the news are misleading, and how we actually do measure Earthquakes.  Enjoy.

How are Earthquakes measured?

A device called a seismograph is used to record earthquakes.  The base of the seismograph is inserted into the Earth and shakes accordingly with an earthquake while a pen that is attached to a motionless weight does not.  The difference in movement is recorded on the paper directly below the pen, and the recording is thus known as a seismograph.

Seismograph (5)

The Richter Scale

The greater the size of the earthquake (magnitude) the greater the squiggle will be on the seismograph. Using the seismograph, in the 1930’s, Charles F. Richter developed a logarithmic scale to determine the size of earthquakes.  It was believed that all earthquakes released the same amount of energy at different periods of time.  However, it was discovered that very large earthquakes release more “long-period” energy (shake the ground longer), and that the Richter scale underestimated the size of very large earthquakes by only measuring surface.  Thus, two new magnitude scales were developed. One for the size of the earthquake Moment Magnitude (Mw), and one for the amount of energy released (Me).

My amateur interest in astronomy brought out the term ‘magnitude,’ which is used for the brightness of a star.” -Charles Richter

Moment Magnitude

To calculate the moment magnitude (the size of the earthquake), the seismic movement (Mo) must first be determined.

MO = µS‹d›

Where µ is the shear strength of the faulted rock, S is the area of the fault, and <d> is the average displacement on the fault” (6)  In other words:

seismic movement= strength of rock x area of fault x fault offset

The seismic movement is the measure used in science, because it has fewer limitation than Richter magnitudes.  Richter magnitudes “often reach a maximum value (we call that magnitude saturation). To compare seismic moment with magnitude We use the Moment Magnitude (Mw) formula constructed by Hiroo Kanamori of the California Institute of Seismology” (7).

MW = 2/3 log10(MO) – 10.7

The units of both seismic moment and moment magnitude are force x distance, or dyne-cm.

The Amount of Energy Released

The amount of potential damage an earthquake can create is measured by the amount energy released from an earthquake.  The formula for the magnitude which is based on based on energy radiated by an earthquake, Mis:

Me = 2/3 log10E – 2.9.

In addition, every increase in magnitude is correlated with an energy increase by about 32 times.  An earthquake with a magnitude of 2 releases 32 times more energy than a magnitude of 1, and a magnitude 8 earthquake releases 1,024 (32 x 32) times more energy than a magnitude 6.  This means that it would take about 1,024 earthquakes of magnitude 6 to equal the energy released by one magnitude 8 earthquake.

“Wait, what Larry?” In essence, MW and Me are both magnitudes, but represent different properties. MW measures the size of the area affected by the earthquake, and Me is a measure of seismic potential for damage.

“And what does this all mean?”  Well let’s take a look at the Modified Mercalli Intensity Scale.

*Note: The press usually reports magnitudes as Richter magnitudes.

Modified Mercalli Intensity Scale

The Modified Mercalli Intensity Scale defines what kind of effects and damage are caused by the earthquakes.  After earthquakes, that have been felt by a lot of people have occurred, the U.S. Geological Survey sends out a questionnaire to gather these values. These numbers can be compared to the typical Richter Scale magnitudes we hear about all the time.(8)

Magnitude Typical Maximum
Modified Mercalli Intensity
1.0 – 3.0 I
3.0 – 3.9 II – III
4.0 – 4.9 IV – V
5.0 – 5.9 VI – VII
6.0 – 6.9 VII – IX
7.0 and higher VIII or higher

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

Pseudo predictions

Sometimes there are false reports that float around declaring an earthquake is imminent. Current science can not predict when an earthquake will occur in the near future, but they can make strong forecasts of when earthquakes will occur in the distant future.

“It is unlikely they [scientists] will ever be able to predict them [earthquakes]. Scientists have tried many different ways of predicting earthquakes, but none have been successful. On any particular fault, scientists know there will be another earthquake sometime in the future [forecasting], but they have no way of telling when it will happen.” (3)

Example of a false report

And this is an example of what can happen when people misunderstand the capabilities of science and earthquake predictions.

We Can’t Predict Earthquakes—And We Shouldn’t Scapegoat Scientists

Summary 

To wrap this all up

  •  Seismograph measures the amount of movement caused by an earthquake
  • The Richter Scale uses a logarithmic function to analyze movement, but underestimates large earthquakes.
  • seismic movement(MO)=strength of rock x area of fault x movement of fault
  • The Moment Magnitude (MW) is a measurement of the size of the area affected by the earthquake)
  • Me stands for the amount of energy released which is a measurement for the amount of potential an earthquake can create.
  • Every increase in magnitude is correlated with an exponential energy increase by about 32 times which means that it would take about 1,024 earthquakes of magnitude 6 to equal the energy released by one magnitude 8 earthquake.
  • The Modified Mercalli Intensity Scale defines the intensity of an earthquake and the effect it is has on people and the environment (Considerably intense earthquakes are considered V and higher)

It is important to understand that current science can not predict when an earthquake will occur in the near future, but they can make strong forecasts of when earthquakes will occur in the distant future.  Thus, if you hear that an earthquake is imminent in the next couple of months, engage in a little skepticism.  The most practical way to understand the magnitudes you hear in the news is to compare them to the Modified Mercalli Intensity Scale.  This scale will let you know how intense the earthquake is.  In the next post we’ll take a look on what kinds of damage earthquakes create, and how they do it.

In an interview in 1971, Charles Richter was asked: How do you define earthquake prediction? 

I don’t define it. I think that harping on prediction is something between a will-o’-the-wisp and a red herring. Attention is thereby diverted away from positive measures to eliminate earthquake risk.

Citations

3. Wald, Lisa. "The Science of Earthquakes." The Science of Earthquakes. U.S. Geological Survey, n.d. Web. 06 Apr. 2014.

4. Lynch, David. "Plate Tectonics, Continental Drift, Spreading Centers, Subduction Zones." Plate Tectonics, Continental Drift, Spreading Centers, Subduction Zones. N.p., 2010. Web. 07 Apr. 2014

5. Michna, Paul. "Earthquakes." Earth Science Australia. N.p., n.d. Web. 07 Apr. 2014.

6. Spence, William, Stuart A. Sipkin, and George L. Choy. Earthquakes and Volcanoes. Rep. no. 1. Vol. 21. N.p.: n.p., n.d. Measuring the Size of an Earthquake. United States Geological Society. Web. 14 Apr. 2014.

7. Ammon, Charles J. "Earthquake Size." Earthquake Size. Penn State Department of Geosciences, n.d. Web. 14 Apr. 2014.

8. "Magnitude / Intensity Comparison." Magnitude / Intensity Comparison. U.S Geological Survey, 09 Jan. 2013. Web. 03 June 2014.
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