# measuring and predicting earthquakes

## earthquake prediction

Scientists are not able to predict earthquakes. Since nearly all earthquakes take place at plate boundaries, scientists can predict where an earthquake will occur (Figure 7.37). This information helps communities to prepare for an earthquake. For example, they can require that structures are built to be earthquake safe. Predicting when an earthquake will occur is much more difficult. Scientists can look at how often earthquakes have struck in the past. This does not allow an accurate prediction for the future. Small tremors, called foreshocks, often happen a short time before a major quake. The ground may also tilt as stress builds up in the rocks. Water levels in wells also change as groundwater moves through rock fractures. These do not usually allow accurate predictions. Folklore tells of animals behaving strangely just before an earthquake. Most people tell stories of these behaviors after the earthquake. Chinese scientists actively study the behavior of animals before earthquakes to see if there is a connection. So far nothing concrete has come of these studies. Once an earthquake has started, many actions must take place. Seismometers can detect P-waves a few seconds before more damaging S-waves and surface waves arrive. Although a few seconds is not much, computers can shut down gas mains and electrical transmission lines. They can initiate protective measures in chemical plants, nuclear power plants, mass transit systems, airports, and roadways.

## the richter magnitude scale

Charles Richter developed the Richter magnitude scale in 1935. The Richter scale measures the magnitude of an earthquakes largest jolt of energy. This is determined by using the height of the waves recorded on a seismograph. Richter scale magnitudes jump from one level to the next. The height of the largest wave increases 10 times with each level. So the height of the largest seismic wave of a magnitude 5 quake is 10 times that of a magnitude 4 quake. A magnitude 5 is 100 times that of a magnitude 3 quake. With each level, thirty times more energy is released. A difference of two levels on the Richter scale equals 900 times more released energy. The Richter scale has limitations. A single sharp jolt measures higher on the Richter scale than a very long intense earthquake. Yet this is misleading because the longer quake releases more energy. Earthquakes that release more energy are likely to do more damage. As a result, another scale was needed.

## earthquake intensity

The ways seismologists measure an earthquake have changed over the decades. Initially, they could only measure what people felt and saw, the intensity. Now they can measure the energy released during the quake, the magnitude. Early in the 20th century, earthquakes were described in terms of what people felt and the damage that was done to buildings. The Mercalli Intensity Scale describes earthquake intensity. There are many problems with the Mercalli scale. The damage from an earthquake is affected by many things. Different people experience an earthquake differently. Using this scale, comparisons between earthquakes were difficult to make. A new scale was needed.

## finding the epicenter

One seismogram indicates the distance to the epicenter. This is determined by the P-and S-wave arrival times. If a quake is near the seismograph, the S-waves arrive shortly after the P-waves. If a quake is far from the seismograph, the P-waves arrive long before the S-waves. The longer the time is between the P-and S-wave arrivals, the further away the earthquake was from the seismograph. First, seismologists calculate the arrival time difference. Then they know the distance to the epicenter from that seismograph. Next, the seismologists try to determine the location of the earthquake epicenter. To do this they need the distances to the epicenter from at least three seismographs. Lets say that they know that an earthquakes epicenter is 50 kilometers from Kansas City. They draw a circle with a 50 km radius around that seismic station. They do this twice more around two different seismic stations. The three circles intersect at a single point. This is the earthquakes epicenter (Figure 7.35).

## what we learn from seismograms

Seismograms contain a lot of information about an earthquake: its strength, length and distance. Wave height used to determine the magnitude of the earthquake. The seismogram shows the different arrival times of the seismic waves (Figure 7.34). The first waves 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 are the largest ones recorded. A seismogram may record P-waves and surface waves, but not S-waves. This means that it was located more than halfway around the Earth from the earthquake. The reason is that Earths outer core is liquid. S-waves cannot travel

## seismographs

A seismograph is a machine that records seismic waves. In the past, seismographs produced a seismogram. A seismogram is a paper record of the seismic waves the seismograph received. Seismographs have a weighted pen suspended from a stationary frame. A drum of paper is attached to the ground. As the ground shakes in an earthquake, the pen remains stationary but the drum moves beneath it. This creates the squiggly lines that make up a seismogram (Figure 7.33). Modern seismographs record ground motions using electronic motion detectors. The data are recorded digitally on a computer.

## measuring seismic waves

Seismic waves are measured on a seismograph. Seismographs contain a lot of information, and not just about earthquakes.

## earthquake magnitudes

Each year, more than 900,000 earthquakes are recorded. 150,000 of them are strong enough to be felt by people. About 18 each year are major, with a Richter magnitude of 7.0 to 7.9. Usually there is one earthquake with a magnitude of 8 to 8.9 each year. Earthquakes with a magnitude in the 9 range are rare. The United States Geological Survey lists five such earthquakes on the moment magnitude scale since 1900 (see Figure 7.36). All but one, the Great Indian Ocean Earthquake of 2004, occurred somewhere around the Pacific Ring of Fire.

## the moment magnitude scale

The moment magnitude scale is the favored method of measuring earthquake magnitudes. It measures the total energy released by an earthquake. Moment magnitude is calculated by two things. One is the length of the fault break. The other is the distance the ground moves along the fault.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

earthquake scale based on the height of seismic waves on a seismogram

``````a. S wave

b. moment magnitude scale

c. Mercalli intensity scale

d. seismogram

e. P wave

-->  f. Richter magnitude scale

g. seismograph
``````

If a seismograph records P-waves but not S-waves from an earthquake, it means

``````a. the quake wasnt very strong

b. the quake was very far away

-->  c. the quake was on the opposite side of the planet

d. the seismograph was in the wrong spot
``````

paper record of seismic waves produced by a seismograph

``````a. S wave

b. moment magnitude scale

c. Mercalli intensity scale

-->  d. seismogram

e. P wave

f. Richter magnitude scale

g. seismograph
``````

Where is the focus with respect to the epicenter?

``````-->  a. directly below the epicenter

b. directly above the epicenter

c. in the P wave shadow zone

d. in the S wave shadow zone
``````

type of seismic wave that cannot travel through Earths liquid outer core

``````-->  a. S wave

b. moment magnitude scale

c. Mercalli intensity scale

d. seismogram

e. P wave

f. Richter magnitude scale

g. seismograph
``````

Which of the following measures the observed effects on people and structures of an earthquake?

``````a. Richter scale

-->  b. Modified Mercalli scale

d. the moment magnitude scale
``````

Which of the following statements best describes the state of earthquake prediction?

``````a. scientists can accurately predict the time and location of almost all earthquakes

b. scientists can accurately predict the time and location of about 50% of all earthquakes

c. scientists can accurately predict when an earthquake will occur, but not where

-->  d. scientists can characterize the seismic risk of an area, but cannot yet accurately predict most earthquakes
``````

earthquake scale based on the total energy released in an earthquake

``````a. S wave

-->  b. moment magnitude scale

c. Mercalli intensity scale

d. seismogram

e. P wave

f. Richter magnitude scale

g. seismograph
``````

device that records seismic waves

``````a. S wave

b. moment magnitude scale

c. Mercalli intensity scale

d. seismogram

e. P wave

f. Richter magnitude scale

-->  g. seismograph
``````

If the arrival time of the first P-wave and the first S-wave is long, the epicenter is

``````-->  a. far away

b. very close

c. very deep

d. near the surface
``````

earthquake scale based on information such as the damage done to buildings

``````a. S wave

b. moment magnitude scale

-->  c. Mercalli intensity scale

d. seismogram

e. P wave

f. Richter magnitude scale

g. seismograph
``````

type of seismic wave that always arrives first at a seismometer

``````a. S wave

b. moment magnitude scale

c. Mercalli intensity scale

d. seismogram

-->  e. P wave

f. Richter magnitude scale

g. seismograph
``````

Seismographs can help to determine the intensity of an earthquake.

``````a. true

-->  b. false
``````

The intensity of an earthquake is directly related to its distance from the epicenter.

``````-->  a. true

b. false
``````

An earthquake with magnitude between 0 and 9 happens about once per year.

``````-->  a. true

b. false
``````

Scientists can better predict when an earthquake will occur than where it will occur.

``````a. true

-->  b. false
``````

The time difference between the P & S wave shows the intensity of an earthquake.

``````a. true

-->  b. false
``````

The strength of an earthquake can be measured in several different ways.

``````-->  a. true

b. false
``````

The pen of a seismograph moves back and forth over a paper roll during an earthquake.

``````a. true

-->  b. false
``````

Modern seismographs record seismic waves using a stationary drum.

``````a. true

-->  b. false
``````

The S waves on a seismogram are usually smaller than the P waves.

``````a. true

-->  b. false
``````

For some earthquakes, only P waves and surface waves show up on a seismogram.

``````-->  a. true

b. false
``````

The Richter scale measures the magnitude of an earthquakes largest jolt of energy.

``````-->  a. true

b. false
``````

A Richter magnitude 8 earthquake occurs about once a week.

``````a. true

-->  b. false
``````

Most Richter magnitude 9 earthquakes have occurred around the Pacific Ring of Fire.

``````-->  a. true

b. false
``````

The preferred scale for measuring an earthquakes magnitude is the moment magnitude scale.

``````-->  a. true

b. false
``````

Today, scientists can accurately predict most earthquakes.

``````a. true

-->  b. false
``````

Finding the amplitude of a seismic wave is one way of determining its

``````a. intensity.

-->  b. magnitude.

c. wavelength.

d. wave spee
``````

What can you determine about an earthquake from a single sonogram?

``````a. exact location of the epicenter

b. distance from the epicenter to the seismograph

c. strength of the earthquake

-->  d. two of the above
``````

When an earthquakes focus is close to the surface, the largest waves recorded on a seismogram are

``````a. primary waves.

-->  b. surface waves.

c. body waves.

d. S waves.
``````

If a seismogram records only P waves and surface waves for an earthquake, the earthquake must be

``````-->  a. on the opposite side of Earth from the seismograph.

b. extremely close to the seismograph.

c. very far below Earths surface.

d. very close to Earths surface.
``````

An S-wave shadow occurs because S waves travel

``````a. more quickly than surface waves.

b. more slowly than P waves.

c. only on the surface.

-->  d. only through solids.
``````

The moment magnitude of an earthquake is calculated from the

``````a. length of the fault.

b. distance the ground moves.

c. amplitude of the seismic waves.

-->  d. two of the above
``````

Assume that an earthquake has a magnitude of 4 on the Richter scale. An earthquake that is 100 times stronger has a magnitude of

``````a. 5.

-->  b. 6.

c. 40.

d. 400.
``````

## diagram questions

No diagram questions associated with this lesson