Chapter 8 Earthquakes and Earths Interior. What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy. –Example:

Chapter 8Earthquakes and Earth’s Interior

What is an earthquake?

• An earthquake is the vibration of Earth produced by the rapid release of energy.– Example: 1906 San Francisco Earthquake.

• Earthquakes usually occur when rocks under stress suddenly shift along a fault.– Stress: A force that can change the size and shape of rocks.– Fault: Fractures in the Earth where movement occurred.

• The area along a fault where slippage first occurs is called the focus of an earthquake.

• The point on the earth’s surface directly above the focus is called the epicenter.

• When an earthquake occurs, seismic waves radiate outward in all directions from the focus.


A fault is

A. A place on Earth where earthquakes cannot occur.

B. A fracture in the Earth where movement has occurred.

C. The place on Earth’s surface where structures move during an earthquake.

D. Another name for an earthquake.

An earthquake’s epicenter is

A. The place on the surface directly above the focus.

B. A spot halfway between the focus and the surface.

C. The spot below the focus.

D. Any spot along the nearest fault.

When an earthquake occurs, energy radiates in all directions from its source, which is called

theA. Epicenter.

B. Focus.

C. Fault.

D. Seismic Center.

Earthquakes are usually associated with

A. Violent weather.

B. Faults.

C. Large cities.

D. The east coast of North America.


• Geologists explain many earthquakes by the elastic rebound hypothesis.

• This hypothesis states that when the stress in rocks becomes to great, they fracture, separate, and spring back to their original shape, or rebound.

• As they fracture and slip into new positions, rocks along a fault release energy in the form of vibrations called seismic waves.


Which of the following causes earthquakes?

A. Elastic Rebound.

B. Richter Scale.

C. Release of Heat.

D. Frictional Heating.

The hypothesis that explains the release of energy during an

earthquake is called theA. Richter Hypothesis.

B. Moment Magnitude Hypothesis.

C. Vibration Hypothesis.

D. Elastic Rebound Hypothesis.

Most earthquakes are produced by the rapid release of which kind of energy stored

in rock subjected to great forces?

A. Chemical

B. Thermal

C. Elastic

D. Mechanical

During an earthquake, the ground surface

A. Moves only in a horizontal direction.

B. Moves only in a vertical direction.

C. Can move in any direction.

D. Does not move.


• This release of energy often increases the stress in other rocks along the fault, causing them to fracture and spring back.

• This reaction is the reason that major earthquakes are usually followed by a series of smaller tremors called aftershocks.

• These aftershocks are usually much weaker than the main earthquake, but they can sometimes destroy structures weakened by the main quake.


• Small earthquakes called foreshocks often come before a major earthquake.– These can happen days or even years before the major quake.

• The San Andreas Fault is the most studied fault system in the world.– Studies have shown that displacement has occurred along segments

that are 100 to 200-kilometers long (63 to 125-miles).

• Some segments move slowly, which is known as fault creep.• Other segments regularly slip and produce small

earthquakes.• Some segments stay locked and store elastic energy for

hundreds of years before they break and cause great earthquakes.

The adjustments of materials that follow a major

earthquake often generate smaller earthquakes calledA. Foreshocks.

B. Surface waves.

C. Aftershocks.

D. Body waves.

Major earthquakes are sometimes preceded by smaller

earthquakes calledA. Aftershocks.

B. Focus shocks.

C. Surface waves.

D. Foreshocks.

The slow continuous movement that occurs along some fault

zones is referred to asA. Slip.

B. Creep.

C. Fracture.

D. A foreshock.

Small foreshocks that precede a major earthquake occur

A. From the day of the major earthquake to days after the earthquake.

B. Only on the day of the major earthquake.

C. Days or years before the major earthquake.

D. Only on the day before the major earthquake.

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.



• To determine how far an earthquake is from a given seismograph, scientists plot the difference between arrival times of the two 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.

Destruction from earthquakes

• The damage to buildings and other structures from earthquake waves depends on several factors.– Intensity and duration of vibrations.– The nature of the material on which the structure

is built.– The design of the structure.

• Engineers have learned that unreinforced stone or brick buildings are the most serious safety threats during earthquake.


• Where loosely consolidated sediments are saturated with water, earthquakes can cause a process known as liquefaction.– Liquefaction: A phenomenon, sometimes

associated with earthquakes, in which soils and other unconsolidated materials saturated with water are turned into a liquid that is not able to support buildings.

• Buildings/Bridges may settle and collapse.• Underground storage tanks and sewer lines

may float toward the surface.

Which of the following affects the amount of destruction

caused by earthquake vibrations?

A. The design of structures.

B. The intensity and duration of the vibrations.

C. The nature of the material on which structures are built.

D. All of the above.

In which of the following areas would the damage from an earthquake measuring 6.8

likely be the greatest?A. Lightly populated rural area.

B. Area with older brick structures.

C. Area with modern steel-framed structures.

D. Area with wood-framed structures.

Which of the following areas would most likely be the

safest during a major earthquake?

A. Area with granite bedrock.

B. Area with loosely consolidated soil.

C. Area with structures built on a landfill.

D. Area with steep slopes of unconsolidated sediments.

In areas where unconsolidated sediments are saturated with water, earthquakes can turn

stable soil into a fluid through a process calledA. Tidal effect.

B. Fault creep.

C. Liquefaction.

D. Underwater landslide.

A building that settles unevenly after an earthquake

is evidence of

A. A tsunami.

B. Liquefaction.

C. An underwater landslide.

D. Fault creep.


• A major earthquake with an epicenter on the ocean floor sometimes causes a giant ocean wave called a tsunami.– This name comes from the Japanese word for

“harbor wave.”

• Scientists think that most tsunamis are caused by two events related to undersea earthquakes: – Faulting– Underwater landslides.

http://news.bbc.co.uk/nol/shared/spl/hi/world/04/tsunami/swf/tsunami.swf?path=/nol/shared/spl/hi/world/04/tsunami/


• A tsunami travels across the ocean at speeds of 500 to 950 km/hr (310 to 590 mph).

• A tsunami can also go unnoticed in the open ocean because its height is usually less than 1 meter (3.3 feet), and the distance between wave crests can range from 100 to 700 km (62 to 435 miles).

• When the wave enters shallower coastal water, it is slowed and begins to pile up to heights that sometimes are greater than 30 meters (98 feet).



• Disastrous earthquakes and tsunamis have encouraged the expansion and improvement of the Pacific Tsunami Warning Center (PTWC).

• This network of seismograph stations around and in the Pacific Ocean alerts scientists to the location and magnitude of earthquakes.

• If a tsunami seems possible, scientists estimate its arrival times at different locations.

• They can then issue warnings immediately to these areas.• However, there will not be enough time to issue tsunami

warnings to areas very near the epicenter of the earthquake.• On average, only one or two destructive tsunamis are

generated worldwide every year.• Only about one tsunami in every 10 years causes major

damage and loss of life.

Tsunamis are

A. Often generated by movements of the ocean floor.

B. Waves that are produced by tidal forces.

C. Waves that cannot cause damage on land.

D. Also known as tidal waves.

A succession of ocean waves set in motion by an submarine

earthquake is called a (an)

A. Compression wave.

B. Underwater landslide.

C. Tsunami.

D. Liquefaction.

A tsunami can occur when there is vertical movement at

a fault under

A. A mountain range.

B. The San Andreas Fault.

C. The ocean floor.

D. A small inland lake.


• The vibrations from earthquakes cause other dangers:– Landslides.– Ground subsidence.– Fires.

1. Landslides:• With many earthquakes, the greatest damage to structures

is from landslides and ground subsidence, or the sinking of the ground triggered by the vibrations.

2. Fire:• Fires are started when gas and electrical lines are cut

during an earthquake.• They are hard to stop due to the water lines also being cut

by an earthquake.


• Humans have long dreamed of being able to accurately predict earthquakes.• There are two different predictions that scientists can make about earthquakes:

– Short-Range Predictions.– Long-Range Forecasts.

1. Short-Range Predictions:• The goal of these predictions is to provide an early warning of the location and

magnitude of a large earthquake.• Researchers monitor possible precursors – things that precede and may warn

of a future earthquake.• Examples:

– Uplift.– Subsidence.– Strain in the rocks near active faults.– Water levels and pressure in wells.– Radon gas emissions from fractures.– Changes in electromagnetic properties of rocks.

• So far, methods for short-range predictions of earthquakes have not been successful.


2. Long-Range Forecasts:• Give the probability of a certain magnitude

earthquake occurring with 30 to 100-plus years.• Based on the idea that earthquakes are repetitive or

cyclical.• Scientists study historical records of earthquakes to

see if there are any patterns of recurrence.• They also study seismic gaps – an area along a

fault where there has not been any earthquake activity for a long period of time.

• Scientists don’t yet understand enough about how and where earthquakes will occur to make accurate long-term predictions.

Violent shaking from an earthquake can cause the soil and rock on slopes to fail and

cause a A. Fault.

B. Landslide.

C. Tsunami.

D. Sinkhole.

Why do earthquakes often cause damaging fires?

A. Lightning strikes are common during earthquakes.

B. Earthquake vibrations can break gas lines, water lines, and electrical lines.

C. Tsunamis from earthquakes generate enough heat to start fires.

D. Magma from deep underground escapes through faults.

Which of the following is used in an attempt to make short-

range predictions of when earthquakes will occur?

A. Strain in rocks near faults.

B. Height of ocean waves after earthquakes.

C. Changes in the color of rocks near faults.

D. Study of historical records.

Long-range earthquake forecasts are based on the idea that earthquakes are

A. Random.

B. Destructive.

C. Fully understood.

D. Repetitive.

Earth’s layered structure

• Most knowledge of the interior of the Earth comes from the study of earthquake waves.

• If the Earth were made of the same materials throughout, seismic waves would spread through it in straight lines at constant speed.

• Seismic waves travel at different speeds due to the differences in the composition of the Earth.

• The general increase in speed with depth is due to increased pressure, which changes the elastic properties of deeply buried rock.

• As a result, the paths of seismic waves are refracted, or bent, as they travel.



• Earth’s interior consists of three major zones defined by its chemical composition:

– Crust– Mantle– Core

1. Crust:• The crust is the thin, rocky outer layer of Earth, which is divided into

oceanic and continental crust.• The oceanic crust is roughly 7-kilometers thick and composed of the

igneous rocks basalt and gabbro.• The continental crust is 8 – 75-kilometers thick (average 40-km), and

consists of many types of rock.• The average composition of the continental crust is granitic rock called

granodiorite.• Continental rocks have an average density of about 2.7-g.cm3 and

some are over 4 billion years old.• The rocks of the oceanic crust are younger (180 million years or less)

and have an average density of about 3.0-g/cm3.


2. Mantle:

• The mantle is a solid, rocky shell that extends to a depth of 2890-km.

• Over 82% of Earth’s volume is contained in the mantle.

• The boundary between the crust and mantle represents a change in chemical composition.

• The dominant rock in the uppermost mantle is perioditite, which has a density of 3.4-g/cm3.


3. Core:

• The core is a sphere composed of an iron-nickel alloy.

• At the extreme pressures found in the center of the core, the iron-rich material has an average density of almost 13-g/cm3 (13 times heavier than water).


Most of the information about Earth’s interior was obtained

by studyingA. Earthquake waves.

B. Rocks of the ocean crust.

C. Meteorites.

D. Rocks in deep wells.

Earth’s thin, rocky outer layer is its

A. Core.

B. Mantle.

C. Outer core.

D. Crust.

Earth’s core is made of an alloy of

A. Iron and nickel.

B. Copper and iron.

C. Zinc and magnesium.

D. Iron and zinc.

The continental crust has the average composition of

A. Gneiss.

B. Granite.

C. Basalt.

D. Limestone.


• Scientists have no direct way to measure temperatures deep within the earth.

• Analysis of seismic waves and heat flow near the earth’s surface and computer modeling allow scientists to estimate those temperatures.

• The following graph estimates the earth’s inner temperatures and pressures.

• The graph shows, the combined temperature and pressure in the lower part of the mantle keep the rocks located there below their melting point.



• Earth can be divided into 4 additional layers based on physical properties:

– The lithosphere.– The asthenosphere.– The outer core.– The inner core.

1. Lithosphere and Asthenosphere:• Earth’s outermost layer consists of the crust and uppermost mantle and

forms a relatively cool, rigid shell called the lithosphere.• This layer averages 100-km in thickness.• Beneath the lithosphere lies a soft, comparatively weak layer known as

the asthenosphere.• The asthenosphere has temperature/pressure conditions that may

result in a small amount of melting.• The lower lithosphere and asthenosphere are both part of the upper

mantle.


• From a depth of about 660-km down to near the base of the mantle lies a more rigid layer known as the lower mantle.

• The rocks in the lower mantle are still very hot and capable of gradual flow.

• The bottom few hundred kilometers of the mantle, laying on top of the hot outer core, contains softer, more flowing rock like that of the asthenosphere.


2. Inner and Outer Core:• The core is divided into two regions with

different physical properties.• The outer core is a liquid layer 2260-km thick.• The flow of metallic iron within this zone

generates Earth’s magnetic field.• The inner core is a sphere having a radius of

1220-km.• The material in the inner core is compressed

into a solid state by the immense pressure.


What layers of Earth make up the lithosphere?

A. The crust and lower mantle.

B. The crust and upper mantle.

C. The continental crust and oceanic crust.

D. The upper and lower mantle.

Earth’s inner core is solid because of

A. The composition of its rock.

B. Its great diameter.

C. Extreme temperatures.

D. Immense pressure.


• In 1909 Andrija Mohorovičić, a Croatian scientist, discovered that the speed of seismic waves increases abruptly 32 km to 70 km beneath the earth’s surface.

• This change in speed of the waves marks the boundary between the crust and the mantle.

• The boundary is called the Mohorovičić discontinuity, or the Moho.

• The increase in speed at the Moho indicates that the earth’s mantle is denser that its crust.


• Below the Moho, at a depth of about 100 km, a decrease in seismic waves speed marks the boundary between the lithosphere and the less rigid asthenosphere.

• Seismic waves then increase in speed until, at a depth of about 2,900 km, P waves slow down again, while the S waves disappear entirely.

• These changes in seismic waves mark the boundary between the mantle and the outer core.

• Because S waves cannot travel through liquids and P waves slow down in less-rigid materials, scientists think the outer core may be a dense liquid.

• At a depth of 5,150 km, P waves speed up again, marking the boundary between the outer core and the inner core.

• This increase in speed suggests that the inner core is a dense, rigid solid.



• Recordings of seismic waves around the world reveal shadow zones on the earth’s surface.

• Shadow zones are locations on the earth’s surface where neither S waves nor P waves are detected or where only P waves are detected.

• Shadow zones occur because the materials that make up the earth’s interior are not uniform in rigidity.

• When seismic waves travel through materials of differing rigidities, their speed changes, causing the waves to bend and change direction.



• Through seismic data and drilling technology, the composition of the continental and oceanic crust was classified.– Continental = granitic– Oceanic = basaltic

• The composition of the rocks of the mantle and core is known from more indirect data.– Example: Magma from volcanic activity and lab experiments with the rock

peridotite showed similar properties.• Earth’s core is thought to be mainly dense iron and nickel, similar

to metallic meteorites.– Scientists assume that meteorites are composed of the same material from

which Earth was formed (Big Bang Theory). Since iron is a dense element, it is believed that it sank toward Earth’s center, with less dense elements floating to the surface of Earth.

• The surrounding mantle is believed to be composed of rocks similar to stony meteorites.

The Moho is

A. The boundary between the outer and inner core.

B. The boundary between the crust and the mantle.

C. The material of which the mantle is composed.

D. An area of the mantle that will not transmit seismic waves.

Through which Earth layer are S-waves not transmitted?

A. Continental crust.

B. Ocean crust.

C. Inner core.

D. Outer core.

Evidence that Earth’s core has a high iron content comes

from

A. Deep wells.

B. Deep-sea drilling.

C. The study of earthquake waves.

D. Meteorites.

The greatest concentration of metals occurs in Earth’s

A. Oceanic crust.

B. Continental crust.

C. Core.

D. Mantle.

Chapter 8 Earthquakes and Earths Interior. What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy. –Example:

Documents

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small earthquakes