Measuring an Earthquake
Earthquakes are recorded by a network of
seismic stations. Each station measures the movement of the ground. Many things
can cause the ground to move: an earthquake, the wind, or a passing truck, for
examples. Therefore seismometers are usually placed in quiet locations. The slip
of a block of rock past another in an earthquake releases energy that makes the
rock near the fault vibrate. That vibration pushes the adjoining rocks, and thus
energy travels out from the earthquake in a wave. As the wave passes by a
seismic station, the ground vibrates and a signal is recorded. Earthquakes
produce two types of waves -- the P-wave (primary wave), a compressional wave
that travels fast but is not as large, and the S-wave (secondary wave), a shear
wave that is slower but larger and does most of the damage. These waves are
usually clear on seismograms, but people can tell them apart too. If you have an
estimate of the number of seconds between the P-wave and the S-wave, multiply by
5 miles (8 km) per second to get the distance to the earthquake. Knowing how
fast the waves travel, seismologists calculate a time and location of the
earthquake that gives the pattern of shaking that was recorded.They measure
the time of the wave that arrives first. That is the wave traveling from the
hypocenter, the first part of the fault to slip. Arrival times and locations can
be determined by a computer within minutes. Determining the location of the rest
of the fault plane, beyond the hypocenter, requires more complicated procedures
and can take several hours to days.
Horizontal Motion Seismograph
Measuring the size of an earthquake
How big was the earthquake? That should be easy. Why do scientists
have problems coming up with a simple answer to a simple question? Many Nevadans
have felt this frustration after earthquakes, as seismologists often seem to
contradict one another. In fact, earthquakes are very complex. Measuring their
size is something like trying to determine the "size" of an abstract modern
sculpture with only the use of a tape measure. Which dimension do you measure?
Magnitude is the most common measure of an earthquake's size. In the
1930s, Beno Gutenberg and Charles Richter borrowed the idea of a magnitude scale
from astronomers and defined it in terms of how big the signal was on a
particular seismograph, at a particular distance from the earthquake. The size
of the signal is related to how much the ground moved. Each time the magnitude
scale increases by one unit, for example from 4 to 5 or from 5 to 6, it means
the ground moved 10 times more.
Gutenberg and Richter also showed that magnitude is related to the energy released in the earthquake. A magnitude 6 earthquake has about 32 times more energy than a magnitude 5 and almost 1000 times more energy than a magnitude 4 earthquake. This does not mean that there will be 1000 times stronger shaking at your home. A bigger earthquake will last longer and release its energy over a much larger area.
Seismologists measure different earthquake "dimensions" with different magnitude scales. Each scale measures how much the ground moves at a different distance and in a different frequency band of vibration. Each scale has its uses, but all are limited because they measure only a part of the ground motion.
In recent years, seismologists have developed a new scale, called moment magnitude. Moment is a physical quantity related to the area of the fault that moved during an earthquake and the average amount that it slipped. It can be estimated by geologists examining the geometry of a fault in the field or by seismologists analyzing a seismogram. The moment is a cumbersome number to work with. It has been converted to a magnitude scale (moment magnitude) for better communication to the public.
Moment magnitude has many advantages over other magnitude scales. First, we can measure all earthquakes, large and small, near and distant, with the same scale. Second, because it can be determined either from instruments or from geology, we can use it to measure old earthquakes and compare them to the instrumentally recorded events. Third, because it is more reliable, we can compare the size and energy of different earthquakes with more confidence, and better estimate what might happen in the future.
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"Where was the earthquake?"
This is a question that seems reasonable, but any answer can be misleading.
We define the epicenter of an earthquake with the latitude and longitude of a point, but the earthquake is bigger than that point. The fault's rupture surface can be hundreds of miles long and several miles wide, and even the epicenter can only be determined within a few tenths of a mile, at best. Giving location of an earthquake in terms of its epicenter is like giving the location Vegas by the address of City Hall.
We name earthquakes after map locations near epicenters to have a convenient way to refer to them. We could, for example, "the earthquake of 12:23, September 12, 1994," but it's easier to say the "Double Spring Flat Earthquake."
Measuring Earthquake Waves
Earthquake waves vary greatly in size, from very tiny to several feet, and it has been difficult to design a single instrument that could measure this wide range. Such instruments are available now, but they weren't in the past. A common way to measure the weaker ground motions that are generated by small, everyday earthquakes is with an instrument that measures the displacement or velocity of the ground. For larger earthquakes with strong ground motion, it is easier to measure the ground acceleration. These different types of recordings can be related to each other, as shown in the box in the lower right. Engineers subject computer models of buildings to these different representations of earthquake waves to see how the building would perform. If there is a problem, they can adjust their design and feel confident the building can withstand certain levels of earthquake motion.
Further Reading:
Bolt (1999) Earthquakes
Brumbaugh (1999) Earthquakes, Science and Society
Shearer (1999) Introduction to Seismology (technical)
Types of Measurement of
Ground Movement
To the left are three representations of ground motion during the 1999 Taiwan earthquake (M7.6) from a seismograph near the fault. The acceleration is measured record. Analysis of this record gives the velocity and displacement of the ground. Permanent displacement of the ground as shown on the record below happens only close to the fault.
Larger earthquakes produce more intesnse and
longer lasting shaking.
Some of the main differences in shaking between
earthquakes of different magnitudes can be seen in the six seismograms to the
right from earthquakes ranging from magnitude 3.1 to 8.1. These earthquakes were
recorded in Mexico on solid rock about 15 miles (25 km) away from each event.
The strong shaking from bigger earthquakes commonly overwhelms seismometers at
such close distances, but these seismograms are from "strong-ground motion
instruments." They measure motion in acceleration, which can be more easily kept
on scale. The most obvious differences are that with increasing magnitude, the
shaking is stronger (you're moved further from side to side) and lasts much
longer. The magnitude 3.1 earthquake has a short, sharp jolt lasting only about
a second, whereas the seismogram from the magnitude 8.1 earthquake indicates
strong side-to-side shaking for over 45 seconds.
Earthquake moment increases with longer fault lengths, thus larger magnitude earthquakes tend to occur along longer faults, as shown in the table below. The longer a fault is, the longer it takes an earthquake to rupture it, so the longer the duration of shaking is (also indicated on the table). Also, as shown above, longer faults can produce larger amplitude and longer period seismic waves.
| A longer fault can produce a larger earthquake that lasts longer | |||||
| Length | Duration | ||||
| Magnitude | Date | Location | miles | (km) | seconds |
| 7.8 | January 9, 1857 | Fort Tejon, CA | 224 | (360) | 130 |
| 7.7 | April 18, 1906 | San Francisco, CA | 249 | (400) | 110 |
| 7.5 | July 21, 1952 | Kern County, CA | 47 | (75) | 27 |
| 7.3 | June 28, 1992 | Landers, CA | 43 | (70) | 24 |
| 7.3 | October 15, 1915 | Pleasant Valley, NV | 38 | (61) | 19 |
| 7.2 | December 16, 1954 | Fairview Peak, NV | 40 | (64) | 12 |
| 7.1 | December 21, 1932 | Cedar Mtn., NV | 40 | (65) | 21 |
| 7.1 | December 16, 1954 | Dixie Valley, NV | 29 | (46) | 11 |
| 7.0 | October 17, 1989 | Loma Prieta, CA | 25 | (40) | 7 |
| 6.7 | February 9, 1971 | San Fernando, CA | 10 | (16) | 8 |
| 6.7 | January 17, 1994 | Northridge, CA | 9 | (14) | 7 |
| 6.6 | July 6, 1954 | Rainbow Mtn., NV | 11 | (18) | 11 |
| 6.4 | October 15, 1979 | Imperial Valley, CA | 19 | (30) | 13 |
| 6.0 | August 16, 1966 | Caliente, NV | 9 | (11) | 6 |
| 5.9 | September 12, 1994 | Double Spring Flat, NV | 10 | (16) | 5 |
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