Chapter 8Chapter 8

Earthquakes

The New Madrid EarthquakesThe New Madrid Earthquakes

Eyewitnesses to the 1811-1812 earthquakes in New Madrid, Missouri, reported seeing bright flashes of light and a dull glow in the sky over a wide area. Reeking sulfurous odors also accompanied the quakes. Many survivors were convinced that the quakes were a heavenly sign meant to frighten the local citizens back to church. As a result, church attendance in the area skyrocketed between 1811 and 1812.

IS IT A FACT!IS IT A FACT!

The strongest earthquake recorded to The strongest earthquake recorded to date occurred in Chile in 1960. It date occurred in Chile in 1960. It measured 9.5 on the Richter scale. measured 9.5 on the Richter scale. This is equivalent to detonating more This is equivalent to detonating more than 1 billion tons of TNT!than 1 billion tons of TNT!

What Are Earthquakes?What Are Earthquakes?

There is a branch of Earth Science There is a branch of Earth Science devoted to earthquakes called devoted to earthquakes called seismology. seismology. Seismology is the study is the study of earthquakes. Earthquakes are of earthquakes. Earthquakes are complex, and they present many complex, and they present many questions for questions for seismologists,, the the scientists that study earthquakes.scientists that study earthquakes.

Where Do Earthquakes Where Do Earthquakes Occur?Occur?

Most earthquakes take place near the edges Most earthquakes take place near the edges of tectonic plates. of tectonic plates. Tectonic platesTectonic plates are giant are giant masses of solid rock that make up the masses of solid rock that make up the outermost part of the earth.outermost part of the earth.

A A fault fault is a break in the Earth’s crust along is a break in the Earth’s crust along which blocks of the crust slide relative to one which blocks of the crust slide relative to one another. Earthquakes occur along faults due another. Earthquakes occur along faults due to this sliding. This is why earthquakes are to this sliding. This is why earthquakes are so common near tectonic plate boundaries.so common near tectonic plate boundaries.

The largest and most active earthquake zone lies along the plate boundaries surrounding the Pacific Ocean.

What Causes Earthquakes?What Causes Earthquakes?

As tectonic plates push, pull, or As tectonic plates push, pull, or scrape against each other, stress scrape against each other, stress builds up along the faults near the builds up along the faults near the plates’ edges. In response to this plates’ edges. In response to this stress, rock in the plates deforms.stress, rock in the plates deforms.

DeformationDeformation

Deformation is the change in the shape of the rock in response to stress. Rock along a fault deforms in mainly two ways – in a plastic manner, like a piece of molded clay, or in an elastic manner, like a rubber band.

Deformation contd.Deformation contd. PlasticPlastic deformationdeformation does not lead to does not lead to

earthquakes. earthquakes. Elastic deformationElastic deformation, , however, however, does lead to earthquakes. While rock can does lead to earthquakes. While rock can stretch farther than steel without breaking, it will stretch farther than steel without breaking, it will break at some point. Think of elastically break at some point. Think of elastically deformed rock as a stretched rubber band. You deformed rock as a stretched rubber band. You can stretch a rubber band only so far before it can stretch a rubber band only so far before it breaks. When the rubber band breaks, it breaks. When the rubber band breaks, it releases energy, and the broken pieces return to releases energy, and the broken pieces return to their un-stretched shape.their un-stretched shape.

Elastic ReboundElastic Rebound

Like the return of the broken rubber band pieces to their un-stretched shape, elastic rebound is the sudden return of elastically deformed rock to its original shape. Elastic rebound occurs when more stress is applied to rock than the rock can withstand. During elastic rebound, rock releases energy that causes an earthquake.

MISCONCEPTION ALERT!!MISCONCEPTION ALERT!!

Earthquakes are not a rare phenomenon. Earthquakes are not a rare phenomenon. In fact, more than 3 million earthquakes In fact, more than 3 million earthquakes happen each year, about one every 10 happen each year, about one every 10 seconds! Most earthquakes are too weak seconds! Most earthquakes are too weak to be felt by humans. The Ring of Fire, a to be felt by humans. The Ring of Fire, a volcanic zone which lies along the plate volcanic zone which lies along the plate boundaries surrounding the Pacific Ocean, boundaries surrounding the Pacific Ocean, is also the world’s largest and most active is also the world’s largest and most active earthquake zone.earthquake zone.

Are All Earthquakes the Are All Earthquakes the Same?Same?

Earthquakes differ in strength and in Earthquakes differ in strength and in the depth at which they begin. These the depth at which they begin. These differences depend on the type of differences depend on the type of tectonic plate motion that produces tectonic plate motion that produces the earthquake. Examine the chart the earthquake. Examine the chart and the diagram below to learn how and the diagram below to learn how earthquakes differ.earthquakes differ.

WEIRD SCEINCEWEIRD SCEINCE Many people assume that major earthquakes Many people assume that major earthquakes

in the United States occur only on the West in the United States occur only on the West Coast. However, major quakes have occurred Coast. However, major quakes have occurred in South Carolina and Missouri---far from any in South Carolina and Missouri---far from any active plate boundaries. The four major active plate boundaries. The four major tremors of the 1811-1812 earthquakes in New tremors of the 1811-1812 earthquakes in New Madrid, Missouri, were so intense that, Madrid, Missouri, were so intense that, according to reports, they altered the flow of according to reports, they altered the flow of the Mississippi River and rang church bells in the Mississippi River and rang church bells in Boston!Boston!

How Do Earthquakes How Do Earthquakes Travel?Travel?

Remember rock releases energy when it springs Remember rock releases energy when it springs back after being deformed. This energy travels back after being deformed. This energy travels in the form of seismic waves. in the form of seismic waves. SeismicSeismic waves waves are waves of energy that travel through the are waves of energy that travel through the Earth. Seismic waves that travel through the Earth. Seismic waves that travel through the Earth’s interior is called Earth’s interior is called body wavesbody waves. . There are There are two types of body waves: P waves and S waves. two types of body waves: P waves and S waves. Seismic waves that travel along the Earth’s Seismic waves that travel along the Earth’s surface is called surface is called surface wavessurface waves. . Different types Different types of seismic waves travel at different speeds and of seismic waves travel at different speeds and move the materials that they travel through move the materials that they travel through differently.differently.

Earthquake WavesEarthquake Waves Seismographs reveal that two main

groups of seismic waves are generated by the slippage of a rock mass.

Surface Waves- travel along the surface

Body Waves- travel through the Earth’s interior

““L” - WavesL” - Waves Surface waves consist of two types of

wave motion. One motion produces a complex up-and-down motion similar to ocean swells, while the other surface wave whips the ground from side-to-side without any vertical motion. Surface waves have a longer time interval between crests and are referred to as L-Waves. These waves cause the greatest destruction.

““S” - WavesS” - Waves ““S” Waves- (secondary) shake the

particles at right angles to the direction of wave travel. This can be illustrated by tying one end of a rope to a post and shaking the other end. S waves do not travel through liquids and gasses, only through solids.

““P” - WavesP” - Waves ““P” Waves- (primary) are

compressional waves and travel through solids, liquids and gasses. This wave action is like that generated by human vocal chords as they move air to create sound.

EarthquakesEarthquakesThe epicenter is the location on the surface of the Earth directly above the focus (source) and can be measured by a seismograph.Seismograph- A device used to measure the low frequency waves generated by an earthquake. They are placed at over 100 locations throughout the world and provided geologists with data about the Earth’s composition, by understanding the characteristics of the low frequency waves generated by an earthquake.

EarthquakesEarthquakes

Seismograph of an Earthquake

Locating EarthquakesLocating Earthquakes

SeismographsSeismographs are instruments located are instruments located at or near the surface of the Earth that at or near the surface of the Earth that record seismic waves. When the waves record seismic waves. When the waves reach a seismograph, the seismograph reach a seismograph, the seismograph creates a seismogram. A creates a seismogram. A seismogramseismogram is a tracing of earthquake motion is a tracing of earthquake motion created by a seismograph. created by a seismograph.

When Did It Happen?When Did It Happen?

Seismologists use seismograms to Seismologists use seismograms to calculate when an earthquake started. An calculate when an earthquake started. An earthquake starts when rock slips earthquake starts when rock slips suddenly enough along a fault to create suddenly enough along a fault to create seismic waves. Seismologists find an seismic waves. Seismologists find an earthquake’s start time by comparing earthquake’s start time by comparing seismograms and noting the difference in seismograms and noting the difference in arrival times of P waves and S waves.arrival times of P waves and S waves.

Where Did It Happen?Where Did It Happen?

Seismologists also use seismograms Seismologists also use seismograms to find an earthquake’s epicenter. An to find an earthquake’s epicenter. An epicenterepicenter is the point on the Earth’s is the point on the Earth’s surface directly above the surface directly above the earthquake’s starting point. A earthquake’s starting point. A focusfocus is the point inside the Earth where an is the point inside the Earth where an earthquake begins.earthquake begins.

QUIZQUIZ1.1. How is an earthquake’s epicenter related How is an earthquake’s epicenter related

to its focus?to its focus?

2.2. As seismic waves travel farther, what As seismic waves travel farther, what happens to the difference in arrival times happens to the difference in arrival times of P and S waves?of P and S waves?

1.The epicenter is the point on the Earth’s surface directly above the focus, which is where the earthquake originates.

2. It increases.

The Richter ScaleThe Richter Scale

The The Richter scaleRichter scale is commonly is commonly used to measure earthquake used to measure earthquake strength. It is named after strength. It is named after Charles Richter, an American Charles Richter, an American seismologist who developed seismologist who developed the scale in the 1930’s.the scale in the 1930’s.

Richter ScaleRichter Scale The Richter Scale is used worldwide to

describe earthquake magnitude. Using Richter’s scale, the magnitude is determined by measuring the amplitude of the largest wave recorded on the seismograph. Large-magnitude earthquakes will cause the seismograph pen to be displaced farther than small-magnitude earthquakes.

Earthquake EnergyEarthquake Energy There is a pattern in the Richter scale There is a pattern in the Richter scale

relating an earthquakes magnitude and relating an earthquakes magnitude and the amount of energy release by the the amount of energy release by the earthquake. Each time the magnitude earthquake. Each time the magnitude increases by 1 unit, the amount of energy increases by 1 unit, the amount of energy released becomes 31.7 times larger. For released becomes 31.7 times larger. For example, an earthquake with a magnitude example, an earthquake with a magnitude of 5.0 on the Richter scale will release of 5.0 on the Richter scale will release 31.7 times as much energy as an 31.7 times as much energy as an earthquake’s with a magnitude of 4.0 on earthquake’s with a magnitude of 4.0 on the Richter scale.the Richter scale.

Earthquake HazardEarthquake Hazard Earthquake hazardEarthquake hazard measures how measures how

prone an area is to experiencing prone an area is to experiencing earthquakes in the future. An area’s earthquakes in the future. An area’s earthquake-hazard level is earthquake-hazard level is determined by past and present determined by past and present seismic activity. The West Coast, for seismic activity. The West Coast, for example, has a very high earthquake-example, has a very high earthquake-hazard level because it has a lot if hazard level because it has a lot if seismic activity.seismic activity.

Earthquake ForecastingEarthquake Forecasting

Predicting when and where earthquakes Predicting when and where earthquakes will occur and how strong will be a difficult will occur and how strong will be a difficult task. However, by closely monitoring task. However, by closely monitoring active faults and other areas of seismic active faults and other areas of seismic activity, seismologists have discovered activity, seismologists have discovered some patterns in earthquakes that allow some patterns in earthquakes that allow them to make some broad predictions.them to make some broad predictions.

The Gap HypothesisThe Gap HypothesisThe The gap hypothesis gap hypothesis states that sections of states that sections of

active faults that have had relatively few active faults that have had relatively few earthquakes are likely to be the sites of stronger earthquakes are likely to be the sites of stronger earthquakes in the future. The areas along a earthquakes in the future. The areas along a fault where relatively few earthquakes have fault where relatively few earthquakes have occurred are called occurred are called seismic gaps.seismic gaps.

The gaps hypothesis helped seismologist The gaps hypothesis helped seismologist forecast the approximate time, strength and forecast the approximate time, strength and location of the 1989 Loma Prieta earthquake in location of the 1989 Loma Prieta earthquake in the San Francisco Bay area.the San Francisco Bay area.

Are You Prepared for an Are You Prepared for an Earthquake?Earthquake?

If you live in an earthquake-prone area or ever If you live in an earthquake-prone area or ever plan to visit one, there are many things you can plan to visit one, there are many things you can do to protect yourself and your property from do to protect yourself and your property from earthquakes. Plan ahead so you will know what earthquakes. Plan ahead so you will know what to do before, during, and after an earthquake. to do before, during, and after an earthquake. Stick to your plan as closely as possible.Stick to your plan as closely as possible.

When The Shaking Starts… … The best thing The best thing to do if you are indoor is to crouch or lie face to do if you are indoor is to crouch or lie face down under a table or desk in the center of the down under a table or desk in the center of the room.room.

WEIRD SCIENCEWEIRD SCIENCE

Engineers have devised giant shock Engineers have devised giant shock absorbers for buildings. The shock absorbers for buildings. The shock absorbers contain a ferrofluid solution absorbers contain a ferrofluid solution that becomes rigid in a magnetic field. that becomes rigid in a magnetic field. When an earthquake occurs, a When an earthquake occurs, a computer controls the electromagnets computer controls the electromagnets in the shock absorbers to dampen the in the shock absorbers to dampen the vibrations!vibrations!

QUIZQUIZ1)1) What is gap hypothesis?What is gap hypothesis?2)2) Why should you lie under a table or desk Why should you lie under a table or desk

during an earthquake?during an earthquake?3)3) What are aftershocks?What are aftershocks?

1)The gap hypothesis states that sections of active faults that have had relatively few earthquakes are likely to be the sites of stronger earthquakes in the future.

2)The table or desk might prevent falling objects from hitting you and causing injury.

3)They are weaker earthquakes that follow stronger earthquakes.

After the Shaking StopsAfter the Shaking Stops

Being in an earthquake is a startling Being in an earthquake is a startling experience. Afterward, you should not be experience. Afterward, you should not be surprised to find yourself and others surprised to find yourself and others puzzled about what happened. You puzzled about what happened. You should try to calm down, get your should try to calm down, get your bearings, and remove yourself from bearings, and remove yourself from immediate danger, such as downed power immediate danger, such as downed power lines, broken glass, and fire hazards. Be lines, broken glass, and fire hazards. Be aware that there may be aftershocks. aware that there may be aftershocks. Recall your earthquake plan, and follow it Recall your earthquake plan, and follow it through.through.

Earthquake Discoveries Near Earthquake Discoveries Near and Farand Far

The study of earthquakes has led to The study of earthquakes has led to many important discoveries about the many important discoveries about the Earth’s interior. Seismologists learn Earth’s interior. Seismologists learn about the Earth’s interior by observing about the Earth’s interior by observing how seismic waves travel through the how seismic waves travel through the Earth. Likewise, seismic waves on Earth. Likewise, seismic waves on other cosmic bodies allow other cosmic bodies allow seismologists to study the interiors of seismologists to study the interiors of those bodies.those bodies.

EarthquakesEarthquakesTheThe MohoMoho is a place within the Earth where the is a place within the Earth where the

speed of seismic waves increases sharply. It speed of seismic waves increases sharply. It marks the boundary between the Earth’s crust marks the boundary between the Earth’s crust and mantle.and mantle.

The The shadow zoneshadow zone is an area on the Earth’s is an area on the Earth’s Surface where no direct seismic waves from a Surface where no direct seismic waves from a particular earthquake can be detected. This particular earthquake can be detected. This discovery suggested that the Earth has a liquid discovery suggested that the Earth has a liquid core.core.

The solid The solid inner coreinner core was discovered in 1936. was discovered in 1936. Before this discovery, seismologists thought that Before this discovery, seismologists thought that the Earth’s entire core was liquid.the Earth’s entire core was liquid.

The MoonThe Moon

In July 1969, humans set foot on the moon In July 1969, humans set foot on the moon for the first time. They brought with them for the first time. They brought with them a seismograph. Not knowing if the moon a seismograph. Not knowing if the moon was seismically active, they left nothing to was seismically active, they left nothing to chance – they purposely crashed their chance – they purposely crashed their landing vehicle back to the moon’s surface landing vehicle back to the moon’s surface after they left to create artificial seismic after they left to create artificial seismic waves. What happened after that left waves. What happened after that left seismologists astonished.seismologists astonished.

IS THAT A FACT!!IS THAT A FACT!! ““MoonMoonquakesquakes” fall into three ” fall into three

categories: deep quakes, which result categories: deep quakes, which result from the gravitational pull of the from the gravitational pull of the Earth; shallow quakes, which may be Earth; shallow quakes, which may be caused by the heating and cooling of caused by the heating and cooling of the moon’s surface; and quakes the moon’s surface; and quakes caused by collision of objects with the caused by collision of objects with the moon’s surface.moon’s surface.

Ohio EarthquakeOhio Earthquake