chapter 8 earthquakes and earth’s interior. section 8.2 measuring earthquakes

Chapter 8Earthquakes and Earth’s Interior

Section 8.2

Measuring Earthquakes

Measuring earthquakes

• The study of earthquake waves, or seismology dates back almost 2000 years.– The first attempts to discover the direction of earthquakes

were made by the Chinese.

• Seismic waves can be detected and recorded by using an instrument called a seismograph.– Seismos = shake; graph = write.

• A seismograph consists of three separate sensing devices.– One device records the vertical motion of the ground.– The other two record horizontal motion in the east-west

and north-south directions.


• Modern seismographs amplify and electronically record ground motion, producing a trace, called a seismogram.– Seismos = shake; gram

= what is written.

The trace that records an earthquake from seismic instruments is called a

A. Seismograph.

B. Seismogram.

C. Richtergram.

D. Magnitude.

What instrument records earthquake waves?

A. Seismogram

B. Seismograph

C. Richter scale

D. Barometer


• Scientists have determined that earthquakes generally produce three major types of seismic waves.– Primary Waves (P-Waves).– Secondary Waves (S-Waves).– Surface Waves.

• Each type of waves travels at a different speed and causes different movements in the earth’s crust.


• Primary Waves (P-Waves):– Move the fastest and are therefore the first to be recorded

by a seismograph.• Travel 1.7 times faster than S-waves.

– P-waves moving through the earth can travel through solids and liquids.

– The more rigid the material, the faster the P-waves travel through it.

– P-waves are compression waves (push-pull waves), meaning that they cause rock particles to move together and apart along the direction of the waves.


• Secondary Waves (S-Waves):– Are the second waves to be detected on a

seismograph.– Unlike P-waves, S-waves can only travel through

solid material.– S-waves cannot be detected on the side of the

earth that is opposite the earthquake’s epicenter.– S-waves are shear waves, meaning that they

cause rock particles to move at right angles to the direction in which the waves are traveling.


• When P and S reach the earth’s surface, their energy can be converted into a third type of seismic wave.

• Surface Waves:– Are the slowest-moving waves and therefore are

the last to be recorded on the seismograph.• Travel at about 90% of the speed of the S-waves.

– Surface waves, which cause the surface to rise and fall, are particularly destructive when traveling through loose earth.

• Most destructive wave.


• Travel Time Graph -To determine how far an earthquake is from a given seismograph, scientists plot the difference between arrival times of the two waves. (P & S Waves)

• Then they consult a standard graph that translates the difference in arrival times into distance from the epicenter.

• For scientists to locate the epicenter, they need information from at least three seismograph stations at different locations.

Which seismic waves travel most rapidly?

A. P waves

B. S waves

C. Surface waves

D. Tsunamis

Which one of the following statements is true about P

waves?

A. They travel only through solids.

B. They travel faster than S waves.

C. They are the most destructive type of seismic wave.

D. They cannot be recorded on a seismograph.

Which seismic waves compress and expand in the direction the waves travel?

A. P waves

B. S waves

C. Surface waves

D. Transverse waves

A seismogram shows that P waves travel

A. At the same speed as surface waves.

B. More slowly than S waves.

C. At the same speed as S waves.

D. Faster than S waves.

Which of the following is NOT a characteristic of S waves?

A. They travel more slowly than P waves.

B. They temporarily change the volume of material by compression and expansion.

C. They shake particles at right angles to the direction the waves travel.

D. They cannot be transmitted through water or air.

Overall, which seismic waves are the most destructive?

A. P waves

B. S waves

C. Compression waves

D. Surface waves

What is the minimum number of seismic stations that is needed to determine the

location of an earthquake’s epicenter?A. 2

B. 1

C. 4

D. 3

A travel-time graph can be used to find the

A. Focus of an earthquake.

B. Strength of an earthquake.

C. Damage caused by an earthquake.

D. Distance to the epicenter of an earthquake.

The distance between a seismic station and the earthquake epicenter is

determined from theA. Calculation of the earthquake magnitude.

B. Intensity of the earthquake.

C. Arrival times of P and S waves.

D. Measurement of the amplitude of the surface wave.


• About 95% of the major earthquakes occur in a few narrow zones.– Circum-Pacific Belt, Mediterranean-Asian Belt,

and the Mid-Ocean Ridges.

• Most of the earthquakes occur around the outer edge of the Pacific Ocean.


• Historically, scientists have used two different types of measurements to describe the size of an earthquake – intensity and magnitude.– Intensity is a measure of the amount of earthquake

shaking at a given location based on the amount of damage.

– Magnitudes are a measure of the size of seismic waves or the amount of energy released at the source of the earthquake.

• Intensity is not a quantitative measurement because it is based on uncertain personal damage estimates.

• Magnitude is a quantitative measurement.

An earthquake’s magnitude is a measure of the

A. Size of seismic waves it produces.

B. Amount of shaking it produces.

C. Number of surface waves it produces.

D. Damage it causes.

The amount of shaking produced by an earthquake at a given location is called the

A. Intensity.

B. Magnitude.

C. Epicenter.

D. Richter magnitude.


• Seismologists express magnitude using a magnitude scale, such as the Richter Scale or the Moment Magnitude Scale.

• The Richter Scale is based on the amplitude of the largest seismic wave recorded on the seismogram.

• The Richter scale is a power of 10 scale. – Example: The amount of ground shaking for a 5.0 earthquake is 10

times greater than the shaking produced by an earthquake of 4.0 on the Richter scale.

• The Richter Scale is only useful for small, shallow earthquakes within about 500-kilometers of the epicenter.

• Most of the earthquake measurements you hear on news reports use the Richter Scale.


• The Moment Magnitude is derived from the amount of displacement that occurs along a fault zone.

• It is calculated using several factors:– Average amount of movement along the fault.– The area of the surface break.– The strength of the broken rock.– Equation form:

(surface area of fault) × (average displacement along fault) × (rigidity of rock)

• Moment magnitude is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes.

The scale most widely used by scientists for measuring

earthquakes is the

A. Seismic scale.

B. Richter scale.

C. Moment magnitude scale.

D. Epicenter magnitude scale.

How much of an increase in wave amplitude is seen from an

earthquake measuring 5.4 on the Richter scale compared to one

measuring 4.4?A. Two times

B. Ten times

C. 20 times

D. 100 times

The Richter magnitude of an earthquake is determined

from the

A. Duration of an earthquake.

B. Intensity of an earthquake.

C. Arrival times of P waves and S waves.

D. Measurement of the amplitude of the largest seismic waves.

chapter 8 earthquakes and earth’s interior. section 8.2 measuring earthquakes

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