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How is its size measured?
Seismologists use three different ways to describe earthquake shaking and earthquake sizes.

  • Intensity: the strength of shaking motions and the damage they can do. This varies varies from place to place for the same earthquake, and is measured on an intensity scale
  • Magnitude: the size of the earthquake as compared to others. This is measured on a magnitude scale
  • Seismic moment: a way of describing an important combination of physical conditions at the earthquake source

More than a hundred years ago, when people began studying earthquakes scientifically, they needed a practical way of describing the strength of an earthquake's shaking motions. They began by describing the pattern of damage to buildings and making maps to show different levels of damage at different places.

Seismologists call this approach the study of earthquake intensities. Today they usually work with twelve different damage zones, using what is called the modified Mercalli Intensity Scale. Each zone is assigned a Roman numeral, and each of the descriptions corresponds to a particular intensity of shaking.

After an earthquake, seismologists make intensity maps based on people's experiences and the types of damage that have occurred. By referring to old newspaper accounts and personal diaries, it's even possible to make intensity maps for past earthquakes.

Intensity and damage from an earthquake can be abnormally high in certain places because of the type of soil or surface. Areas with soft sedimentary layers of material are more susceptible to severe damage from shaking than surrounding areas of harder rock. Extensive damage is likely to occur in landfill areas where sandy material has been dumped into a lake or a bay to create a surface upon which buildings are constructed. Buildings in these areas should be specially strengthened.

The intensity scale describes the strength of seismic motions in different places; it does not tell whether the earthquake that caused the motions was large or small. Shaking at intensity III, for example, could occur near the epicenter of a small earthquake or at a great distance from a large earthquake.

In the 1930s, the American seismologist Charles Richter studied thousands of seismograms of earthquakes that had occurred in southern California. He realized that it would be useful to have a numerical scale for comparing the size of earthquakes that went beyond describing them as just large or small. Richter knew that he would have to take two things into account in devising such a scale: the distance from the epicenter and the great difference in size of ground motion between small and large earthquakes.

The result of Richter's work was a method of assigning magnitude that we know today as the Richter scale. It's based upon a measurement of the size of the largest wave recorded on a certain type of seismometer that was commonly used in Richter's day. This is what he took into account in order to set up the scale:

  • The distance from the epicenter, which he determined by finding how to make a correction to the actual measurement so that he knew how big the seismic waves would be at a distance of exactly 100 kilometers (about 60 miles) from the epicenter.

  • The great difference in size of ground motion for different earthquakes by using a scale on which an increase by one unit (for example, from magnitude 7 to magnitude 8) meant an increase in earthquake shaking by a factor of ten. Thus a magnitude of 8 is 100 times greater shaking than a magnitude of 6, and a million times greater than a magnitude of 2.

  • He took 0 (zero) on his scale as the smallest earthquake he cared to work with at that time 100 million times smaller than an earthquake of magnitude 8.

Very small earthquakes are rated up to about 2.5 on the Richter scale. Moderate earthquakes rated up to magnitude 5 can cause minor damage. Earthquakes of magnitude 6 and higher are major earthquakes that can cause widespread damage and loss of life.

Today there are many different magnitude scales in addition to the Richter scale, all based on ways of measuring the sizes of different seismic waves on different seismometers. The largest earthquakes on these scales range up to about magnitude 8 or 9. Using sensitive instruments, seismologists have detected earthquakes as small as magnitude -3 or -4. An earthquake with magnitude of -3 is 1,000 times smaller than magnitude 0, the smallest number on the original Richter scale.

Seismic Moment
Seismologists often prefer to describe the size of an earthquake in terms of the physical conditions at the earthquake source itself rather than in terms of the shaking it produces. To do this, they use seismic moment rather than magnitude. The seismic moment of an earthquake is determined from three factors:
  • The fault slip — the distance that rock slides along a fault surface after it breaks.
  • The area of the fault surface that is actually broken by the earthquake.
  • The measurement of how rigid the rocks are near the broken fault.

We determine the seismic moment of an earthquake by multiplying together the fault slip, the fault area, and the rigidity. The seismic moment describes the essential combination of physical quantities that really matters at the earthquake source and that determines how strong the seismic motions will be.

In the greatest earthquakes, the fault slip can be many feet and the fault area can be thousands of square miles. In the smallest measurable earthquake, on the other hand, the fault slip might be as small as a fraction of an inch and the fault area may be only a few square feet. If the rigidities are about the same, the largest seismic moment is a trillion times larger than the smallest.

There are approximately 10,000 earthquakes of magnitude 4 or greater each year. There are only about 10 earthquakes of magnitude 7 or greater each year.

Can you predict earthquakes?
Although seismologists know that certain regions are more earthquake-prone than others, they do not know for certain just when an earthquake will occur. Today much of the effort of earthquake prediction goes into studying the geology of the earth and examining historical records of particular regions to determine exactly where and how often earthquakes occurred in the past. This information can then be used to make rough estimates of what to expect in the future in that same region. Such studies lead seismologists to think that there is between a 10-50% chance that a major earthquake will strike California within a person's lifetime.

Most earthquakes do little harm, but a few can cause great destruction and loss of life. Engineers study each major earthquake to learn how to build safer buildings, dams, and bridges so that destruction and loss of life can be reduced. Seismologists and other scientists continue their studies to learn more about what happens at the earthquake source and to discover more about the interior of the earth.

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