Measuring Earthquake - Seismograms tell seismologists how strong an earthquake is and how far away it is. At least three seismograms must be used to calculate where the epicenter is located. 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 seismometer is a machine that records seismic waves. In the past, all seismometers were seismographs because they produced a graph-like representation of the seismic waves they received. The paper record is called a seismogram. Modern seismometers record ground motions using electronic motions detectors. The data are then kept digitally on a computer. Seismographs have a pen suspended from a stationary frame, while a drum of paper rotates beneath it. The pen is weighted so that it is suspended and not attached to the ground. The drum is attached to the ground. As the earth shakes in an earthquake, the pen remains stationary but the drum moves beneath it. This creates the squiggly lines that make up a seismogram.
Seismograms contain information on how strong an earthquake was, how long it lasted, and how far away it was. The wiggly lines that are produced in a seismogram clearly show the different arrival times of P- and S-waves. As with words on a page, the seismogram record goes from left to right. First, there is a flat line, where there was no ground shaking. The first waves to be recorded by the seismogram are P-waves since they are the fastest. S-waves come in next and are usually larger than P-waves. The surface waves arrive just after the S-waves. If the earthquake has a shallow focus, the surface waves will be the largest ones recorded.
If a seismogram has recorded P-waves and surface waves, but not S-waves, the seismograph was on the other side of the planet from the earthquake. Scientists know that the earth’s outer core is liquid because S-waves cannot travel through liquid. The liquid outer core creates an S-wave shadow zone on the opposite side of the planet from the earthquake’s focus where no S-waves reach. The amplitude (height) of the waves can be used to determine the magnitude of the earthquake. How magnitude is calculated will be discussed in a later section.
Finding the Epicenter
A single seismogram can tell a seismologist how far away the earthquake was but it does not provide the seismologist with enough information to locate the exact epicenter. For that, the seismologist needs at least three seismograms. Determining distance to an earthquake epicenter depends on the fact that different seismic waves travel at different speeds. P-waves always arrive at a seismometer first, but the amount of time it takes for the S-waves to arrive after the P-wave indicates distance to the epicenter. If the epicenter is near the seismometer, the P-waves, S-waves and surface waves will all arrive in rapid succession. If the epicenter is further away, the S-waves will lag further behind. In other words, the longer it is between the arrival of the P-wave and S-wave from an earthquake, the farther the epicenter is from the seismometer.
After many years of study, geologists know the speed at which the different types of waves travel through various earth materials. Based on the difference in the arrival times of the first P wave and the first S wave, seismologists determine the distance between the epicenter and a seismometer. Once the distance to the epicenter is known, scientists can identify each point that is that distance away. Let’s say that they know that an earthquake’s epicenter is 50 kilometers from Kansas City. When each point that is that distance away from Kansas City is marked, the marks create a circle. This circle can be drawn with a compass.
To locate the earthquake epicenter, seismologists must have data from at least three seismometers. A circle drawn at the correct distance to the epicenter from a second seismometer will intercept the first circle in two places. A third circle showing the distance to the epicenter from a third seismometer will intercept the other two circles at a single point. This point is the earthquake epicenter. While this method was extremely useful for locating epicenters for decades, the technique has been replaced by digital calculations.
Earthquake Intensity
People have always tried to quantify the size of and damage done by earthquakes. Early in the 20th century, earthquakes could only be described in terms of what nearby residents felt and the damage that was done to nearby structures. This was called the Mercalli Intensity Scale and was developed in 1902 by the Italian seismologist Giuseppe Mercalli. The Mercalli Scale is sometimes used today in conjunction with the more modern intensity scales described below.
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Number
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Descriptions
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I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
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Not felt except by a very few under especially favorable
conditions.
Felt only by a few persons at rest, especially on
upper floors of buildings.
Felt noticeably by people indoors, especially on
upper floors of buildings. Many people don’t know it’s an earthquake.
Standing automobiles may rock lightly. Vibrations similar to the passing of a
truck. Duration estimated.
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 cars rocked
noticeably.
Felt by nearly everyone; many awakened. ome dishes,
windows broken. Unstable objects overturned. Pendulum clocks may stop.
Felt by all, many frightened. Some heavy furniture
moved; a little fallen plaster. Damage slight.
Damage negligible in buildings of good design and
construction; slight to moderate in well-built ordinary structures;
considerable damage in poorly built or designed structures; some chimneys
broken.
Damage slight in specially designed structures;
considerable damage in ordinary substantial buildings with partial collapse. Damage
great in poorly built structures. Fallen chimneys, factory stacks, columns, monuments,
walls. Heavy furniture overturned.
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.
Some well-built wooden structures destroyed; most
masonry and frame structures destroyed with foundations. Rails bent.
Few if any structures still standing. Bridges destroyed.
Rails bent greatly.
Damage total. Lines of sight and level distorted.
Objects thrown into air.
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There were many problems with the Mercalli scale. What people feel and see in an earthquake is affected by how far they are from the earthquake’s focus, the type of rock that lies beneath them, the construction type of the nearby buildings, and many other factors. Different observers will also perceive the experience differently. For example, one might exaggerate while the other downplays the damage done. With the Mercalli scale, comparisons between earthquakes are difficult to make.
To address these problems, in 1935 Charles Richter developed his Richter magnitude scale. The Richter scale measures the magnitude of the largest jolt of energy released in an earthquake. Because Richter’s scale is logarithmic, the amplitude of the largest wave increases 10 times from one integer to the next. For example, 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. One integer increase in magnitude roughly correlates with a 30-fold increase in the amount of energy released. A difference of two integers on the Richter scale equals a 1,000-fold increase in released energy.
Seismologists recognize that the Richter scale has limitations, since it measures the height of the greatest earthquake wave. A single sharp jolt will measure higher on the Richter scale than a very long intense earthquake that releases more energy. In other words, earthquakes that release more energy are likely to do more damage than those that are short, but have a larger single jolt. Using the Richter scale, a high magnitude may not necessarily reflect the amount of damage caused.
The moment magnitude scale is the current method of measuring earthquake magnitudes. This method measures the total energy released by an earthquake and so more accurately reflects its magnitude. Moment magnitude is calculated from the area of the fault that is ruptured and the distance the earth moved along the fault. Like the Richter scale, the moment magnitude scale is logarithmic. An increase in one integer means that 30 times more energy was released, while two integers means that 1,000 times the energy was released released. The Richter and moment magnitude scales often give very similar measurements.
In a single year, more than 900,000 earthquakes are recorded. 150,000 of them are strong enough to be felt. About 18 per year are major, with a Richter magnitude of 7.0 to 7.9. Each year, on average, one earthquake with a magnitude of 8 to 8.9 strikes. Remember that many of these earthquakes occur deep in the crust and out in the oceans and do not cause much or any damage on land.
Earthquakes with a magnitude in the 9 range are rare. The United States Geological Survey lists six such earthquakes on the moment magnitude scale in historic times. All but one of them, the Great Indian Ocean Earthquake of 2004, occurred somewhere around the Pacific Ring of Fire.
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