Lesson Objectives

  • Describe how to find an earthquake epicenter.
  • Describe the different earthquake magnitude scales and what the numbers for moment magnitude mean.
  • Describe how earthquakes are predicted and why the field of earthquake prediction has had little success.

Introduction

Seismograms record seismic waves. Over the past century, scientists have developed several ways of measuring earthquake intensity. The currently accepted method is the moment magnitude scale, which measures the total amount of energy released by the earthquake. At this time, seismologists have not found a reliable method for predicting earthquakes.

Measuring Magnitude

A seismograph produces a graph-like representation of the seismic waves it receives and records them onto a seismogram (Figure below). Modern seismometers record ground motions using electronic motion detectors. The data are then kept digitally on a computer. This animation shows how a seismogram works: http://www.wwnorton.com/college/geo/egeo/flash/8_3.swf.
  • A seismograph records an earthquake 50 miles away:
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/17/Seismogram_RegionalEarthquake.mov.
  • This animation shows three different stations picking up seismic waves:
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/10/4StationSeismoNetwork480.mov.
Seismograms contain information that can be used to determine how strong an earthquake was, how long it lasted, and how far away it was.
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These seismograms show the arrival of P-waves and S-waves. The surface waves arrive just after the S-waves and are difficult to distinguish. Time is indicated on the horizontal portion (or x-axis) of the graph.
If a seismogram records P-waves and surface waves, but not S-waves, the seismograph was on the other side of the Earth from the earthquake. The amplitude of the waves can be used to determine the magnitude of the earthquake, which will be discussed in a later section.

Finding the Epicenter

To locate an earthquake epicenter:
1. Scientists first determine the epicenter distance from three different seismographs. The longer the time between the arrival of the P-wave and S-wave, the farther away is the epicenter. So the difference in the P and S wave arrival times, determines the distance between the epicenter and a seismometer. This animation shows how distance is determined using P, S, and surface waves: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/12/IRIStravelTime_Bounce_480.mov
2. The scientist then draws a circle with a radius equal to the distance from the epicenter for that seismograph. The epicenter is somewhere along that circle. This is done for three locations. Using data from two seismographs, the two circles will intercept at two points. A third circle will intercept the other two circles at a single point. This point is the earthquake epicenter (Figure below). Although useful for decades, this technique has been replaced by digital calculations.
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Circles are drawn with radii representing the distance from each seismic station to the earthquake's epicenter. The intersection of these three circles is the earthquake's epicenter.
Seismic stations record ten earthquakes in this animation: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/12/TravelTime_Sphere_10Stn_480.mov

Earthquake Intensity

Measuring Earthquakes

People have always tried to quantify the size of and damage done by earthquakes. Since early in the 20th century, there have been three methods. What are the strengths and weaknesses of each?
  • Mercalli Intensity Scale. Earthquakes are described in terms of what nearby residents felt and the damage that was done to nearby structures.
  • Richter magnitude scale. Developed in 1935 by Charles Richter, this scale uses a seismometer to measure the magnitude of the largest jolt of energy released by an earthquake.
  • Moment magnitude scale. Measures the total energy released by an earthquake. Moment magnitude is calculated from the area of the fault that is ruptured and the distance the ground moved along the fault.
The Richter scale and the moment magnitude scale are logarithmic.
  • The amplitude of the largest wave increases ten times from one integer to the next.
  • An increase in one integer means that thirty times more energy was released.
  • These two scales often give very similar measurements.
How does the amplitude of the largest seismic wave of a magnitude 5 earthquake compare with the largest wave of a magnitude 4 earthquake? How does it compare with a magnitude 3 quake? The amplitude of the largest seismic wave of a magnitude 5 quake is 10 times that of a magnitude 4 quake and 100 times that of a magnitude 3 quake.
How does an increase in two integers on the moment magnitude scale compare in terms of the amount of energy released? Two integers equals a 900-fold increase in released energy.
Which scale do you think is best? With the Richter scale, a single sharp jolt measures higher than a very long intense earthquake that releases more energy. The moment magnitude scale more accurately reflects the energy released and the damage caused. Most seismologists now use the moment magnitude scale.
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The way scientists measure earthquake intensity and the two most common scales, Richter and moment magnitude are described along with a discussion of the 1906 San Francisco earthquake in Measuring Earthquakes video (3d): http://www.youtube.com/watch?v=wtlu_aDteCA (2:54).

Annual Earthquakes

In a single year, on average, more than 900,000 earthquakes are recorded and 150,000 of them are strong enough to be felt. Each year about 18 earthquakes are major, with a Richter magnitude of 7.0 to 7.9. and on average, one earthquake has a magnitude of 8 to 8.9.
Magnitude 9 earthquakes are rare. The United States Geological Survey lists six in historic times (see Figure below) and (Table below). All but the Great Indian Ocean Earthquake of 2004 occurred somewhere around the Pacific Ocean basin.
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The 1964 Good Friday Earthquake centered in Prince William Sound, Alaska, released the second most amount of energy of any earthquake in recorded history.
Location
Year
Magnitude
Chile
1960
9.5
Prince William Sound, Alaska
1964
9.2
Great Indian Ocean Earthquake
2004
9.1
Kamchatka, Alaska
1952
9.0
Africa, Peru (now Chile)
1868
9.0
Cascadia Subduction Zone
1700
9.0
(Data from: United States Geological Survey)

Earthquake Prediction

Scientists are a long way from being able to predict earthquakes. A good prediction must be accurate as to where an earthquake will occur, when it will occur, and at what magnitude it will be so that people can evacuate. An unnecessary evacuation is expensive and causes people not to believe authorities the next time.
Where an earthquake will be is the easiest feature to predict. Scientists know that earthquakes take place at plate boundaries and tend to happen where they’ve occurred before. Earthquake-prone communities should always be prepared for an earthquake. These communities can implement building codes to make structures earthquake safe (Figure below).
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The probabilities of earthquakes striking along various faults in the San Francisco area between 2003 (when the work was done) and 2032.
When an earthquake will occur is much more difficult. Since stress on a fault builds up at the same rate over time, earthquakes should occur at regular intervals (Figure below). But so far scientists cannot predict when quakes will occur even to within a few years.
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Around Parkfield, California, an earthquake of magnitude 6.0 or higher occurs about every 22 years. So seismologists predicted that one would strike in 1993, but that quake came in 2004 -- 11 years late.
Signs sometimes come before a large earthquake.
Small quakes, called foreshocks, sometimes occur a few seconds to a few weeks before a major quake. However, many earthquakes do not have foreshocks and small earthquakes are not necessarily followed by a large earthquake. Often, the rocks around a fault will dilate as microfractures form. Ground tilting, caused by the buildup of stress in the rocks, may precede a large earthquake. But not always. Water levels in wells fluctuate as water moves into or out of fractures before an earthquake. This is also an uncertain predictor of large earthquakes. The relative arrival times of P-waves and S-waves also decreases just before an earthquake occurs.
Folklore tells of animals behaving erratically just before an earthquake. Mostly these anecdotes are told after the earthquake. If indeed animals sense danger from earthquakes or tsunami, scientists do not know what it is they could be sensing, but they would like to find out.
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The geology of California underlies the state’s wealth of natural resources as well as its natural hazards. This video explores the enormous diversity of California’s geology (9a): http://www.youtube.com/watch?v=QzdBx9zL0ZY (57:50).

Lesson Summary

  • Seismograms indicate an earthquake’s strength, how far away it is, and how long it lasts.
  • Epicenters can be calculated using the difference in the arrival times of P- and S-waves from three seismograms.
  • Three different methods can be used to determine an earthquake's strength. The Mercalli Scale identifies the damage done and what people felt after an earthquake has occurred, the Richter scale measures the greatest single shock, and the moment magnitude scale measures the total energy released.
  • Seismologists have not come too far in their ability to predict earthquakes.

Review Questions

  1. How can a seismograph measure ground shaking if all parts of it must be attached to the ground?
  2. On a seismogram, which waves arrive first, second, third, and which arrive last?
  3. What information is needed to calculate the distance from a seismic station to an earthquake’s epicenter?
  4. If a seismogram records P-waves and surface waves but not S-waves, where was the earthquake epicenter located relative to the seismograph and why?
  5. On the Richter or magnitude moment scale: What is the difference in energy released by an earthquake that is a 7.2 versus an 8.2 in magnitude? A 7.2 versus a 9.2?
  6. Why do you need at least three seismographs to locate an earthquake epicenter?
  7. Why did Richter and moment magnitude scales need to be developed?
  8. Why is the moment magnitude scale thought to be more accurate than the Richter scale for measuring earthquake magnitudes?
  9. What is the difference in energy released between a 6 and a 7 on the Richter scale? How about a 6 and a 7 on the moment magnitude scale?
  10. How do seismologists use earthquake foreshocks to predict earthquakes? Why are foreshocks not always an effective prediction tool?
  11. For earthquake prediction to be really useful, what would need to be predicted?

Further Reading / Supplemental Links

Vocabulary

seismometer A seismometer is a machine that measures seismic waves and other ground motions. seismograph An older type of seismometer in which a suspended, weighted pen wrote on a drum that moved with the ground. seismogram A seismogram is the printed record of seismic activity produced by a seismometer.

Points to Consider

  • If you live in an earthquake prone area, how do you feel about your home now? What can you do to minimize the risk to you and your family? If you do not live in an earthquake prone area, what would it take to get you to move to one? Also, what risks from natural disasters do you face where you live?
  • What do you think are the most promising clues that scientists might someday be able to use to predict earthquakes?
  • What good does information about possible earthquake locations do for communities in those earthquake-prone regions?