Earthquake Safety Information For Bangladesh
In this chapter on Earthquakes, I will discuss details of “Earthquake Safety Information For Bangladesh and What Causes an Earthquake in details which are applicable to Bangladesh Earthquake Risk”
Sections
- How and Where Earthquakes Happen
- Studying Earthquakes
- Earthquakes and Society
What You’ll Learn
How scientists measure earthquakes.
Why It’s Relevant Understanding how, where, and why earthquakes happen can help scientists and engineers reduce earthquake damage and save lives.
Studying earthquakes also helps scientists understand the Earth’s interior.
Fig. This expressway in Kobe, Japan, was toppled by the ground shaking of an earthquake that lasted 20 seconds and had a moment magnitude of 6.9.
How and Where Earthquakes Happen
Earthquakes are one of the most destructive natural disasters. A single earthquake can kill thousands of people and cause millions of dollars in damage. Earthquakes are defined as movements of the ground that are caused by a sudden release of energy when rocks along a fault move. Earthquakes usually occur when rocks under stress suddenly shift along a fault. A fault is a break in a body of rock along which one block slides relative to another.
Why Earthquakes Happen
The rocks along both sides of a fault are commonly pressed together tightly. Although the rocks may be under stress, friction prevents them from moving past each other. In this immobile state, a fault is said to be locked. Parts of a fault remain locked until the stress becomes so great that the rocks suddenly grind past each other. This slippage causes the trembling and vibrations of an earthquake.
OBJECTIVES
▸ Describe elastic rebound.
▸ Compare body waves and surface waves.
▸ Explain how the structure of Earth’s interior affects seismic waves.
▸ Explain why earthquakes generally occur at plate boundaries.
Elastic Rebound
Geologists think that earthquakes are a result of elastic rebound. Elastic rebound is the sudden return of elastically deformed rock to its undeformed shape. In this process, the rocks on each side of a fault are moving slowly. If the fault is locked, the stress in the rocks increases. When the rocks are stressed past the point at which they can maintain their integrity, they fracture. The rocks then separate at their weakest point and rebound or spring back to their original shape. This process is shown in Figure 1.
Fig. Elastic Rebound that Shows (a) Two blocks of crust pressed against each other at a fault are under stress but do not move because friction holds them in place. (b) As stress builds up at the fault, the crust deforms. (c) The rock fractures and then snaps back into its original shape, which causes an earthquake.
KEY TERMS
Earthquake elastic rebound
Focus
Epicenter
Body wave
Surface wave
P wave
S wave
Shadow zone
Fault zone
Fault
What is an earthquake?
An earthquake is a movement or trembling of the ground that is caused by a sudden release of energy when rocks along a fault move
What is elastic rebound?
The elastic rebound is the sudden return of elastically deformed rock to its undeformed shape.
Fig. The epicenter of an earthquake is the point on the surface directly above the focus.
What is Focus?
The focus is the location within Earth along a fault at which the first motion of an earthquake occurs.
What is the epicenter?
The epicenter is the point on Earth’s surface directly above an earthquake’s starting point, or focus.
What is a body wave?
The Body wave in geology, is a seismic wave that travels through the body of a medium.
What is a surface wave in geology?
The surface wave in geology is a seismic wave that travels along the surface of a medium and that has a stronger effect near the surface of the medium than it has in the interior
Anatomy of an Earthquake
The location within Earth along a fault at which the first motion of an earthquake occurs is called the focus (plural, foci). The point on Earth’s surface directly above the focus is called the epicenter (EP i SENT uhr), as shown in Figure 2.
Although the focus depths of earthquakes vary, about 90% of continental earthquakes have a shallow focus. Earthquakes that have shallow foci take place within 70 km of Earth’s surface. Earthquakes that have intermediate foci occur at depths between 70 km and 300 km. Earthquakes that have deep foci take place at depths between 300 km and 650 km. Earthquakes that have deep foci usually occur in subduction zones and occur farther from the plate boundary than shallower earthquakes do.
By the time the vibrations from an earthquake that has an intermediate or deep focus reach the surface, much of their energy has dissipated. For this reason, the earthquakes that usually cause the most damage usually have shallow foci.
Seismic Waves
As rocks along a fault slip into new positions, the rocks release energy in the form of vibrations called seismic waves. These waves travel outward in all directions from the focus through the surrounding rock. This wave action is similar to what happens when you drop a stone into a pool of still water and circular waves ripple outward from the center.
Earthquakes generally produce two main types of waves. Body waves are waves that travel through the body of a medium. Surface waves travel along the surface of a body rather than through the middle. Each type of wave travels at a different speed and causes different movements in Earth’s crust.
Body Waves
Body waves can be placed into two main categories: P waves and S waves. P waves, also called primary waves or compression waves, are the fastest seismic waves and are always the first waves of an earthquake to be detected.
P waves cause particles of rock to move in a back-and-forth direction that is parallel to the direction in which the waves are traveling, as shown in Figure 3. P waves can move through solids, liquids, and gases. The more rigid the material is, the faster the P waves travel through it.
S waves, also called secondary waves or shear waves, are the second-fastest seismic waves and arrive at detection sites after P waves. S waves cause particles of rock to move in a side-to-side direction that is perpendicular to the direction in which the waves are traveling. Unlike P waves, however, S waves can travel through only solid material.
P wave is a primary wave, or compression wave; a seismic wave that causes particles of rock to move in a back-and-forth direction parallel to the direction in which the wave is traveling: P waves are the fastest seismic waves and can travel through solid, liquids, and gases
S wave is a secondary wave, or shear wave; a seismic wave that causes particles of rock to move in a side-to-side direction perpendicular to the direction in which the wave is traveling: S waves are the second-fastest seismic waves and can travel only through solids
Surface Waves
Surface waves form from motion along a shallow fault or from the conversion of energy when P waves and S waves reach Earth’s surface. Although surface waves are the slowest-moving waves, they may cause the greatest damage during an earthquake.
The two types of surface waves are Love waves and Rayleigh waves. Love waves cause the rock to move side-to-side and perpendicular to the direction in which the waves are traveling. Rayleigh waves cause the ground to move with an elliptical, rolling motion.
Reading Check:
Describe the two types of surface waves.
Connection to ENGINEERING:
Seismic Reflection Surveying:
Many surveying companies have discovered the usefulness of seismic waves in mapping underground features. These features can be used to identify possible mineral deposits and oil or natural gas reservoirs.
In seismic surveying, a seismic shock is generated by using explosives, air guns, or a mechanical thumper. The seismic waves produced by the shock travel through the ground and reflect off bedding planes or other features below the surface. The reflected waves travel back to the surface, where they are recorded by an array of geophones. Geophones are instruments that convert the motion of a seismic wave into an electrical signal.
The geophones are set up in a straight line to collect data that are used to construct a two-dimensional profile of the underground layers. Scientists can also arrange the geophones in more complex patterns to create three-dimensional images of the layers. Because each underground layer reflects waves at a different time, scientists can plot all of the data to construct an accurate picture of the underground layers.
Seismic reflection can be used to study phenomena a variety of scales. It may be used to study just the top few tens of meters of soil and rock, or it may be used to study the structure of the deep crust. In particular, this technology has been adapted for use by oil and natural gas exploration companies to locate oil and gas reservoirs.
What is a Shadow zone?
The shadow zone is an area on Earth’s surface where no direct seismic waves from a particular earthquake can be detected
Figure 4▸ P waves and S waves behave differently as they pass through different structural layers of Earth.
Seismic Waves and Earth’s Interior
Seismic waves are useful to scientists in exploring Earth’s interior. The composition of the material through which P waves and S waves travel affects the speed and direction of the waves. For example, P waves travel fastest through materials that are very rigid and are not easily compressed. By studying the speed and direction of seismic waves, scientists can learn more about the makeup and structure of Earth’s interior.
Earth’s Internal Layers
In 1909, Andrija Mohorovičić (MOH hoh ROH vuh CHICH), a Croatian scientist, discovered that the speed of seismic waves increases abruptly at about 30 km beneath the surface of continents. This increase in speed takes place because the mantle is denser than the crust. The location at which the speed of the waves increases marks the boundary between the crust and the mantle.
The depth of this boundary varies from about 10 km. below the oceans to about 30 km below the continents. By studying the speed of seismic waves, scientists have been able to locate boundaries between other internal layers of Earth. The three main compositional layers of Earth are the crust, the mantle, and the core. Earth is also composed of five mechanical layers-the lithosphere, the asthenosphere, the mesosphere, the outer core, and the inner core.
Shadow Zones
Recordings of seismic waves around the world reveal shadow zones. Shadow zones are locations on Earth’s surface where no waves from a particular earthquake can be detected. Shadow zones exist because the materials that makeup Earth’s interior are not uniform in rigidity. When seismic waves travel through materials of differing rigidi- ties, the speed of the waves changes. The waves will also bend and change direction as they pass through different materials.
As shown in Figure 4, a large S-wave shadow zone covers the side of the Earth that is opposite an earthquake. S waves do not reach the S-wave shadow zone because they cannot pass through the liquid outer core. Although P waves can travel through all of the layers, the speed and direction of the waves change as the waves pass through each layer. The waves bend in such a way that a P-wave shadow zone forms.
Reading Check
What causes the speed of a seismic wave to change?
Earthquakes and Plate Tectonics
Earthquakes are the result of stresses in Earth’s lithosphere. Most earthquakes occur in three main tectonic environments, as shown in Figure. These settings are generally located at or near tectonic plate boundaries, where stress on the rock is greatest.
Convergent Oceanic Environments
At convergent plate boundaries, plates move toward each other and collide. The denser plate subducts or sinks into the asthenosphere below the other plate. As the plates move, the overriding plate scrapes across the top of the subducting plate, and earthquakes occur. Convergent oceanic boundaries can occur between two oceanic plates or between one oceanic plate and one continental plate.
Divergent Oceanic Environments
At the divergent plate boundaries that make up the mid-ocean ridges, plates are moving away from each other. Earthquakes occur along mid-ocean ridges because the oceanic lithosphere is pulling away from both sides of each ridge. This spreading motion causes earthquakes along the ocean ridges.
Continental Environments
Earthquakes also occur at locations where two continental plates converge, diverge, or move horizontally in opposite directions. As the continental plates interact, the rock surrounding the boundary experiences stress. The stress may cause mountains to form and also cause frequent earthquakes.
Earthquakes are the result of tectonic stresses in Earth’s crust and occur in three main tectonic settings-mid-ocean ridges, subduction zones, and continental collisions.
Graphic
Organizer Spider Map
Create the Graphic Organizer entitled “Spider Map” described in the Skills Handbook section of the Appendix. Label the circle “Major earthquake zones.” Create a leg for each type of earthquake zone. Then, fill in the map with details about each type of earthquake zone.
What is a fault zone?
A fault zone is a region of numerous, closely spaced faults
Fault Zones
At some plate boundaries, there are regions of numerous, closely spaced faults called fault zones. Fault zones form at plate boundaries because of the intense stress that results when the plates separate, collide, subduct, or slide past each other. One such fault zone is the North Anatolian fault zone, which extends almost the entire length of the country of Turkey. Where the edge of the Arabian plate pushes against the Eurasian plate, the small Turkish microplate is squeezed westward. When enough stress builds up, movement occurs along one or more of the individual faults in the fault zone and sometimes causes major earthquakes.
Earthquakes Away from Plate Boundaries
Not all earthquakes result from movement along plate boundaries. The most widely felt series of earthquakes in the history of the United States did not occur near an active plate boundary. Instead, these earthquakes occurred in the middle of the continent, near New Madrid, Missouri, in 1811 and 1812. The vibrations from the earthquakes that rocked New Madrid were so strong that they caused damage as far away as South Carolina.
It was not until the late 1970s that studies of the Mississippi River region revealed an ancient fault zone deep within Earth’s crust. This zone is thought to be part of a major fault zone in the North American plate. Scientists have determined that the fault formed at least 600 million years ago and that it was later buried under many layers of sediment and rock.
Section Review
- Describe elastic rebound.
- Explain the difference between the epicenter and the focus of an earthquake.
- Compare body waves and surface waves.
- Explain how seismic waves help scientists learn about Earth’s interior.
- Explain how the structure of Earth’s interior affects seismic wave speed and direction.
- Explain why earthquakes generally take place at plate boundaries.
- Describe a fault zone, and explain how earthquakes occur along fault zones.
CRITICAL THINKING
- Applying Ideas In earthquakes that cause the most damage, at what depth would movement along a fault most likely occur?
- Identifying Patterns If a seismologic station measures P waves but no S waves from an earthquake, what can you conclude about the earthquake’s location?
- Making Inferences If an earthquake occurs in the center of Brazil, what can you infer about the geology of that area?
CONCEPT MAPPING
- Use the following terms to create a concept map: earthquake, seismic wave, body wave, surface wave, P wave, 5 wave, Rayleigh wave, and Love wave.
Studying Earthquakes
The study of earthquakes and seismic waves is called seismology. Many scientists study earthquakes because earthquakes are the best tool Earth scientists have for investigating Earth’s internal structure and dynamics. These scientists have developed special sensing equipment to record, locate, and measure earthquakes.
Recording Earthquakes
Vibrations in the ground can be detected and recorded by using an instrument called a seismograph (SIEZ MUH graf), such as the one shown in Figure 1. A modern three-component seismograph consists of three sensing devices. One device records the vertical motion of the ground. The other two devices record horizontal motion-one for east-west motion and the other for north-south motion. Seismographs record motion by tracing wave-shaped lines on paper or by translating the motion into electronic signals. The electronic signals can be recorded on magnetic tape or can be loaded directly into a computer that analyzes seismic waves. A tracing of earthquake motion that is recorded by a seismograph is called a seismogram.
Because they are the fastest-moving seismic waves, P waves are the first waves to be recorded by a seismograph. S waves travel much slower than P waves. Therefore, S waves are the second waves to be recorded by a seismograph. Surface waves, or Rayleigh and Love waves, are the slowest-moving waves. Thus, Rayleigh and Love waves are the last waves to be recorded by a seismograph.
OBJECTIVES of Studying Earthquakes
→Describe the instrument used to measure and record earthquakes. →Summarize the method scientists use to locate an epicenter.
→Describe the scales used to measure the magnitude and intensity of earthquakes.
KEY TERMS
Seismograph
Seismogram
Magnitude
Intensity
A seismograph is an instrument that records vibrations in the ground.
A seismogram is a tracing of earthquake motion that is recorded by a seismograph.
Locating an Earthquake
To determine the distance to an epicenter, scientists analyze the arrival times of the P waves and the S waves. The longer the lag time between the arrival of the P waves and the arrival of the S waves, the farther away the earthquake occurred. To determine how far an earthquake is from a given seismograph station, scientists consult a lag-time graph. This graph translates the difference in arrival times of the P waves and S waves into the distance from the epicenter to each station. The start time of the earthquake can also be determined by using this graph.
To locate the epicenter of the earthquake, scientists use computers to perform complex triangulations based on information from several seismograph stations. Before computers were widely available, scientists performed this calculation in a simpler and more imprecise way. On a map, they drew circles around at least three seismograph stations that recorded vibrations from the earthquake. The radius of each circle was equal to the distance from that station to the earthquake’s epicenter. The point at which all of the circles intersected indicated the location of the epicenter of the earthquake.
→ Quick LAB
Seismographic Record
Procedure
- Line a shoe box with a plastic bag. Fill the box to the rim with sand. Put on the lid.
- Mark an X near the center of the lid.
- Fasten a felt-tip pen to the lid of the box with a tight rubber band so that the pen extends slightly beyond the edge of the box.
- Have a partner hold a pad of paper so that the paper touches the pen.
- Hold a ball over the X at a height of 30 cm. As your partner slowly moves the paper horizontally past the pen, drop the ball on the X. 6. Label the resulting line with the type of material in the box.
- Replace about 2/3 of the sand with crumpled newspaper. Put on the lid, and fasten the pen to the lid with the rubber band.
- Repeat steps 4-6.
Analysis
- What do the lines on the paper represent?
- What do the sand and newspaper represent?
- Compare the lines made in steps 4-6 with those made in step 8. Which material vibrated more when the ball was dropped on it? Explain why one material might vibrate more than the other.
- How might different types of crustal material affect seismic waves that pass through it?
- How might the distance of the epicenter of an earthquake from a seismograph affect the reading of a seismograph?
Earthquake Measurement
Scientists who study earthquakes are interested in the amount of energy released by an earthquake. Scientists also study the amount of damage done by the earthquake. These properties are studied by measuring magnitude and intensity.
Magnitude
The measure of the strength of an earthquake is called magnitude. Magnitude is determined by measuring the amount of ground motion caused by an earthquake. Seismologists express magnitude by using a magnitude scale, such as the Richter scale or the moment magnitude scale.
The Richter scale measures the ground motion from an earthquake to find the earthquake’s strength. While the Richter scale was widely used for most of the 20th century, scientists now pre-face the moment magnitude scale. The moment magnitude is a measurement of earthquake strength based on the size of the area of the fault that moves, the average distance that the fault blocks move, and the rigidity of the rocks in the fault zone. Although the moment magnitude and the Richter scales provide similar values for small earthquakes, the moment magnitude scale is more accurate for large earthquakes.
The moment magnitude of an earthquake is expressed by a number. The larger the number, the stronger the earthquake. The largest earthquake that has been recorded (in Chile) registered a moment magnitude of 9.5. The earthquake in Pakistan in 2005 that caused the damage shown in Figure 2 had a moment magnitude of 7.6. Earthquakes that have moment magnitudes of less than 2.5 usually are not felt by people.
Reading Check
What is the difference between the Richter scale and the moment magnitude scale? magnitude a measure of the strength of an earthquake
MATH PRACTICE
Magnitudes On both the moment magnitude scale and the Richter scale, the energy of an earthquake increases by a factor of about 30 for each increment on the scale. Thus, a magnitude 4 earthquake releases 30 times as much energy as a magnitude 3 earthquake does. How much more energy does a magnitude 6 earthquake release than a magnitude 3 earthquake does?
Modified Mercalli Intensity Scale
Intensity→ Description
I→ is not felt except by very few under especially favorable conditions
II→ is felt by only a few people at rest; delicately suspended items may swing
III→ is felt by most people indoors; vibration is similar to the passing of a large truck
IV→ is felt by many people; dishes and windows rattle; the sensation is similar to a building being struck
V→ is felt by nearly everyone; some objects are broken; and unstable objects are overturned
VI→ is felt by all people; some heavy objects are moved; causing very slight damage to structures
VII→ causes slight to moderate damage to ordinary buildings; some chimneys are broken
VIII→ causes considerable damage (including partial collapse) to ordinary buildings
IX→ causes considerable damage (including partial collapse) to earthquake-resistant buildings
X→ destroys some to most structures, including foundations; rails are bent
XI→ causes few structures, if any, to remain standing; bridges are destroyed and rails are bent
XII→ causes total destruction; distorts lines of sight; objects are thrown into the air
What is the Intensity of an earthquake?
The intensity in Earth science, the amount of damage caused by an earthquake
Intensity
Before the development of magnitude scales, the size of an earthquake was determined based on the earthquake’s effects. A measure of the effects of an earthquake is the earthquake’s intensity. The modified Mercalli scale, shown in Table 1, expresses intensity in Roman numerals from I to XII and describes the effects of each earthquake intensity. The highest-intensity earthquake is designated by the Roman numeral XII and is described as destruction. The intensity of an earthquake depends on the earthquake’s magnitude, the distance between the epicenter and the affected area, the local geology, the earthquake’s duration, and human infrastructure.
Section Review
- Describe the instrument that is used to record seismic waves.
- Compare a seismograph and a seismogram. 3. Summarize the method that scientists used to identify the location of an earthquake before computers became widely used.
- Describe the scales that scientists use to measure the magnitude of an earthquake.
- Explain the difference between the magnitude and intensity of an earthquake.
CRITICAL THINKING
- Analyzing Methods Explain why it would be difficult for scientists to locate the epicenter of an earthquake if they have seismic wave information from only two locations.
- Evaluating Data Explain why an earthquake with a moderate magnitude might have a high intensity.
CONCEPT MAPPING
- Use the following terms to create a concept map: seismograph, seismogram, epicenter, P wave, S wave, magnitude, and intensity.
Earthquake Safety Information and Society
Movement of the ground during an earthquake seldom directly causes many deaths or injuries. Instead, most injuries result from the collapse of buildings and other structures or falling objects and flying glass. Other dangers include landslides, fires, explosions caused by broken electric and gas lines, and floodwaters released from collapsing dams.
Tsunamis
An earthquake whose epicenter is on the ocean floor may cause a giant ocean wave called a tsunami (tsoo NAH mee), which may cause serious destruction if it crashes into the land. A tsunami may begin to form when a sudden drop or rise in the ocean floor occurs because of faulting associated with undersea earthquakes. The drop or rise of the ocean floor causes a large mass of seawater to also drop or rise. This mass of water moves up and down as it adjusts to the change in sea level. This movement sets into motion a series of long, low waves that increase in height as they near the shore. These waves are tsunamis.
On December 26, 2004, a tsunami was triggered by an earthquake that struck beneath the Indian Ocean. The tsunami devastated coastal areas of Indonesia, India, Sri Lanka, and Thailand, leaving more than 200,000 people dead or missing.
Destruction of Buildings and Property
Most buildings are not designed to withstand the swaying motion caused by earthquakes. Buildings whose walls are weak may collapse completely. Very tall buildings may sway so violently that they tip over and fall onto lower neighboring structures, as shown in Figure.
The type of ground beneath a building can affect how the building responds to seismic waves. A building constructed on loose soil and rock is much more likely to be damaged during an earthquake than a building constructed on solid ground. During an earthquake, the loose soil and rock can vibrate like jelly. Buildings constructed on top of this kind of ground experience exaggerated motion and sway violently.
OBJECTIVES
➤ Discuss the relationship between tsunamis and earthquakes. Describe two possible effects of a major earthquake on buildings. List three safety techniques to prevent injury caused by earthquake activity.
▸ Identify four methods scientists use to forecast earthquake risks.
KEY TERMS
Tsunami
Seismic gap
What is a Tsunami?
Tsunami is a giant ocean wave that forms after a volcanic eruption, submarine earthquake, or landslide
Quick LAB Earthquake-Safe Buildings
Procedure
- On a tabletop, build one structure by stacking building blocks on top of each other.
- Pound gently on the side of the table. Record what happens to the structure.
- Using rubber bands, wrap sets of three blocks together. Build a second structure by using these blocks.
- Repeat step 2. Analysis
- Which of your structures was more resistant to damage caused by the “earthquake”? 2. How could this model relate to building real structures, such as elevated highways?
Earthquake Safety Information
A destructive earthquake may take place in any region of Bangladesh. However, destructive earthquakes are more likely to occur in certain geographic areas, such as Dhaka City or Chittagong area. People who live near active faults should be ready to follow a few simple earthquake safety rules. These safety rules may help prevent death, injury, and property damage.
Earthquake Safety Information Before an Earthquake
Before an earthquake occurs, be prepared. Keep on hand a supply of canned food, bottled water, flashlights, batteries, and a portable radio. Some safety material is shown in Figure. Plan what you will do if an earthquake strikes while you are at home, in school, or a car. Discuss these plans with your family. Learn how to turn off the gas, water, and electricity in your home.
Earthquake Safety Information During an Earthquake
When an earthquake occurs, stay calm. During the few seconds between tremors, you can move to a safer position. If you are indoors, protect yourself from falling debris by standing in a doorway or crouching under a desk or table. Stay away from windows, heavy furniture, and other objects that might topple over. If you are in school, follow the instructions given by your teacher or principal. If you are in a car, stop in a place that is away from tall buildings, tunnels, power lines, or bridges. Then, remain in the car until the tremors cease.
Earthquake Safety Information After an Earthquake
After an earthquake, be cautious. Check for fire and other hazards. Always wear shoes when walking near broken glass, and avoid downed power lines and objects touched by downed wires.
Earthquake Warnings and Forecasts
Humans have long dreamed of being able to predict earthquakes. Accurate earthquake predictions could help prevent injuries and deaths that result from earthquakes.
Today, scientists study past earthquakes to predict where future earthquakes are most likely to occur. Using records of past earthquakes, scientists can make approximate forecasts of future earthquake risks. However, there is currently no reliable way to predict exactly when or where an earthquake will occur. Even the best forecasts may be off by several years.
To make more accurate forecasts, scientists are trying to detect changes in Earth’s crust that can signal an earthquake. Faults near many population centers have been located and mapped. Instruments placed along these faults measure small changes in rock movement around the faults and can detect an increase in stress. Currently, however, these methods cannot provide reliable or accurate predictions of earthquakes.
Seismic Gaps
Scientists have identified zones of low earthquake activity, or seismic gaps, along some faults. A seismic gap is an area along a fault where relatively few earthquakes have occurred recently but where strong earthquakes occurred in the past. Some scientists think that seismic gaps are likely locations of future earthquakes. Several gaps that exist along the San Andreas Fault zone may be sites of major earthquakes in the future. One of these locations, Loma Prieta, California, is shown in Figure 3.
Reading Check
Why do scientists think that seismic gaps are areas where future earthquakes are likely to occur?
What is a Seismic Gap?
A seismic gap is an area along a fault where relatively few earthquakes have occurred recently but where strong earthquakes are known to have occurred in the past.
Figure: Each red dot in the cross-section of the San Andreas Fault represents an earthquake or aftershock before the 1989 Loma Prieta earthquake. Note how seismic gap 2 was filled by the 1989 earthquake and its aftershocks, which are represented by the blue dots. Before 1989 Earthquake
Foreshocks
Some earthquakes are preceded by little earthquakes called foreshocks. Foreshocks can precede an earthquake by a few seconds or a few weeks. In 1975, geophysiologists in China recorded foreshocks near the city of Haicheng, which had a history of earthquakes. The city was evacuated the day before a major earthquake. The earthquake caused widespread destruction, but few lives were lost thanks to the warning. However, the Haicheng earthquake is the only example of a successful prediction made by using this method.
Changes in Rocks
Scientists use a variety of sensors to detect slight tilting of the ground and to identify the strain and cracks in rocks caused by the stress that builds up in fault zones. When these cracks in the rocks are filled with water, the magnetic and electrical properties of the rocks may change. Scientists also monitor natural gas seepage from rocks that are strained or fractured from seismic activity. Scientists hope that they will one day be able to use these signals to predict earthquakes.
Reliability of Earthquake Forecasts
Unfortunately, not all earthquakes have foreshocks or other pre-cursors. Earthquake prediction is mostly unreliable. However, scientists have been able to determine areas that have a high earthquake hazard level, as shown in Figure 4. Scientists continue to study seismic activity so that they may one day make accurate forecasts and save more lives.
Section Review
- Discuss the relationship between tsunamis and earthquakes.
- Describe two possible effects of a major earthquake on buildings.
- List three safety rules to follow when an earthquake strikes.
- Describe how identifying seismic gaps may help scientists predict earthquakes.
- Identify changes in rocks that may signal earthquakes.
CRITICAL THINKING
- Applying Concepts What type of building construction and location regulations should be included in the building code of a city that is located near an active fault?
- Applying Concepts You are a scientist assigned to study an area that has a high earthquake-
hazard level. Describe a program that you could set up to predict potential earthquakes.
CONCEPT MAPPING
- Use the following terms to create a concept map: earthquake, earthquake-hazard level, damage, tsunami, safety, and prediction.
Chapter Highlights
Sections 1: How and Where Earthquakes Happen
→ Earthquake
→ Elastic rebound
→ Focus
→ Epicenter
→ Body wave
→ Surface wave
→ P wave
→ S wave
→ Shadow zone
→ Fault zone
Key Concepts in the section 1 of How and Where Earthquakes Happen:
→ In the process of elastic rebound, stress builds in rocks along a fault until they break and spring back to their original shape.
→ There are two major types of seismic waves: body waves and surface waves.
→ Different seismic waves act differently depending on the material of Earth’s interior through which they pass.
→ Most earthquakes occur near tectonic plate boundaries.
Section 2: Studying Earthquakes
→ Seismograph
→ Seismogram
→ Magnitude
→ Intensity
Key Concepts in the Section 2: Studying Earthquakes
→ Scientists use seismographs to record earthquake vibrations.
→ The difference in the times that P waves and S waves take to arrive at a seismograph station helps scientists locate the epicenter of an earthquake.
→ Earthquake magnitude scales describe the strength of an earthquake. Intensity is a measure of the effects of an earthquake.
Section 3: Earthquakes and Society
→ Tsunami
→ Seismic gap
Key Concepts in the Section 3: Earthquakes and Society
→ Most earthquake damage is caused by the collapse of buildings and other structures. ▸ Tsunamis often are caused by ocean-floor earthquakes.
→ People who follow safety guidelines are less likely to be harmed by an earthquake.
→ Seismic gaps, tilting ground, and variations in rock properties are some of the changes in Earth’s crust that scientists use when trying to predict earthquakes.
Chapter Review
Use each of the following terms in a separate sentence.
- Elastic rebound
- Fault zone
- Seismic gap
For each pair of terms, explain how the meanings of the terms differ.
- Focus and epicenter
- Body wave and surface wave
- P wave and S wave
- Seismograph and seismogram
- Intensity and magnitude
Understanding Key Concepts
- Vibrations on Earth that are caused by the sudden movement of rock are called
- Epicenters.
- Earthquakes.
- Faults.
- Tsunamis.
- In the process of elastic rebound, as a rock becomes stressed, it first
- Deforms.
- Melts.
- Breaks.
- Shrinks.
- Earthquakes that cause severe damage are likely to have what characteristics?
- A deep focus
- An intermediate focus
- A shallow focus
- A deep epicenter
- Most earthquakes occur
- In mountains.
- Along major rivers.
- At plate boundaries.
- In the middle of tectonic plates.
- P waves travel
- Only through solids.
- Only through liquids and gases.
- Through solids, liquids, and gases.
- Only through liquids.
- S waves cannot pass through
- Solids.
- The mantle.
- Earth’s outer core.
- The asthenosphere.
- Most injuries during earthquakes are caused by
- The collapse of buildings.
- Cracks in Earth’s surface.
- The vibration of S waves.
- The vibration of P waves.
- Which of the following is not a method used to forecast earthquake risks?
- Identifying seismic gaps
- Determining moment magnitude
- Recording foreshocks
- Detecting changes in the rock
Short Answer
- How do seismic waves help scientists understand Earth’s interior?
- Why is the S-wave shadow zone larger than the P-wave shadow zone?
- How do scientists determine the location of an earthquake’s epicenter?
- Why do scientists prefer the moment magnitude scale to the Richter scale?
- How might tall buildings respond during a major earthquake?
- What should you do if you are in a car when an earthquake happens?
- List three changes in rock that may one day be used to help forecast earthquakes.
Critical Thinking
- Understanding Relationships Why might surface waves cause the greatest damage during an earthquake?
- Determining Cause and Effect Two cities are struck by the same earthquake. The cities are the same size, are built on the same type of ground, and have the same types of buildings. The city in which the earthquake produced a maximum intensity of VI on the Mercalli scale suffered $1 million in damage. The city in which the earthquake produced a maximum intensity of VIII on the Mercalli scale suffered $50 million in damage. What might account for this great difference in the costs of the damage?
- Recognizing Relationships Would an earthquake in the Rocky Mountains in Colorado be likely to form a tsunami? Explain your answer.
Concept Mapping
- Use the following terms to create a concept map: earthquake, elastic rebound, surface wave, body wave, seismic wave, tsunami, seismograph, magnitude, intensity, moment magnitude scale, and Richter scale.
Math Skills
- Making Calculations If a P wave traveled 6.1 km/s, how long would the P wave take to travel 800 km?
- Using Equations An earthquake with a mag- nitude of 3 releases 30 times more energy than does an earthquake with a magnitude of 2. How much more energy does an earth- quake with a magnitude of 8 release than an earthquake with a magnitude of 6 does?
- Making Calculations Of the approximately 420,000 earthquakes recorded each year, about 140 have a magnitude greater than 6. What percentage of all earthquakes have a magnitude greater than 6?
Writing Skills
- Writing from Research Find out how and why the worldwide network of seismograph stations was formed. Also, find out how all the stations in the network work together. Prepare a report about your findings.
- Communicating Main Ideas Find out which earthquake registered the highest intensity in history. Write a brief report that de- scribes the effects of this earthquake.
Interpreting Graphics
The graph below shows three seismograms from a single earthquake. Use the graph to answer the questions that follow.
- How far from the epicenter is seismograph B?
- How far from the epicenter is Seismograph C?
- Which seismograph is farthest from the epicenter?
- Why is there an 8-minute interval between P waves and S waves in seismogram B but an 11-minute interval between P waves and S waves in seismogram C?
Suggestions for Final Exam Preparation
Part A
Directions (1-6): For each question, write the number of the correct answer on a separate sheet of paper.
1 Energy waves that produce an earthquake begin at what location on or within Earth?
(1) epicenter
(2) seismic gap
(3) focus
(4) shadow zone
2 The fastest-moving seismic waves produced by an earthquake are called
(1) P waves
(2) S waves
(3) Raleigh waves
(4) surface waves
3 Seismic waves are detected and recorded by using an instrument called a
(1) Richter scale
(2) Mercalli scale
(3) seismograph
(4) seismogram
4 Most earthquake-related injuries are caused by
(1) tsunamis
(2) collapsing buildings
(3) rolling ground movements
(4) sudden cracks in the ground
5 The magnitude of an earthquake can be expressed numerically by using
(1) only the Richter scale
(2) only the Mercalli scale
(3) both the Mercalli scale and the moment magnitude scale
(4) both the Richter scale and the moment magnitude scale
6 Which of the following is the least likely to cause deaths during an earthquake?
(1) floodwaters from collapsing dams
(2) falling objects and flying glass
(3) fires from broken electric and gas lines
(4) Ground movement
Part B-1
Directions (7-9): For each question, write the number of the correct answer on a separate sheet of paper.
Base your answers to questions 7 through 9 on the diagram below, which shows how the epicenter of an earthquake is located by using data from three seismic stations.
Finding the Epicenter of an Earthquake
Station A
Station B
Station C
7 Which of the following points on the map is located at the epicenter of the earthquake?
(1) AB
(2) ABC
(3) BC
8 According to the information in the diagram, which station is located the farthest from the epicenter of the earthquake?
(1) station A
(2) station B
(3) station C
(4) All stations are located at the same distance from the epicenter.
9 Which of the following types of data recorded by the seismograph at each station allows scientists to determine the distance from the station to the epicenter?
(1) vertical ground motion
(2) horizontal ground motion that moves from the east to the west
(3) horizontal ground motion that moves from the north to the south
(4) arrival times of incoming seismic waves
Part B-2
Directions (1 and 2): For each question below, record the correct answer on a separate sheet of paper.
Base your answers to questions 1 and 12 on the diagram below, which shows a recording of data by a seismograph.
1. What types of seismic waves are indicated by the points on the seismogram marked by the letters A and B? How can you determine this?
2. What type of seismic waves are indicated by the point on the seismogram marked by the letter C? How are these waves connected to the smaller waves that preceded them?
Part C
Directions (3): For each question below, record the correct answer on a sheet of paper. Some questions may require extended written responses. Objects in the illustration below are not drawn to scale.
3. What safety hazards can you identify in this scene? What safety advice would you give to someone approaching the scene above? How should people prepare for dealing with such post-earthquake safety hazards?
USING SCIENTIFIC METHODS
Analyze P waves and S waves to determine the distance from a city to the epicenter of an earthquake.
> Determine the location of an earthquake epicenter by using the distance from three different cities to the epicenter of an earthquake.
Materials
- Calculator
- Drawing compass
- Ruler
Skills Practice Lab
Finding an Epicenter
An earthquake releases energy that travels through Earth in all directions. This energy is in the form of waves. Two kinds of seismic waves are P waves and S waves. P waves travel faster than S waves and are the first to be recorded at a seismograph station. The S waves arrive after the P waves. The time difference between the arrival of the P waves and the S waves increases as the waves travel farther from their origin. This difference in arrival time, called lag time, can be used to find the distance to the epicenter of the earthquake. Once the distance from three different locations is determined, scientists can find the approximate location of the epicenter.
PROCEDURE
The average speed of P waves is 6.1 km/s. The average speed of S waves is 4.1 km/s. Calculate the lag time between the arrival of P waves and S waves over a distance of 100 km.
The graph below shows seismic records made in three cities following an earthquake. These traces begin at the left. The arrows indicate the arrival of the P waves. The beginning of the next wave on each seismograph record indicates the arrival of the S wave. Use the time scale to find the lag time between the P wave and the S waves for each city. Draw a table similar to Table.
Record the lag time for each city in the table.
Use the lag times found in Step 2 and the lag time per 100 km found in Step 1 to calculate the distance from each city to the epicenter of the earthquake by using the equation below.
distance = measured lag time (s) x 100 km lag time for 100 km
Record distances in the table.
Copy the map at right, which shows the location of the three cities. Using the map scale on your copy of the map, adjust the compass so that the radius of the circle with Austin at the center is equal to the calculation for Austin in step 2. Put the point of the compass on Austin. H Draw a circle on your copy of the map.
Repeat step 6 for Bismarck and for Portland.
The epicenter of the earthquake is located near the point at which the three circles intersect.
ANALYSIS AND CONCLUSION
Evaluating Data Describe the location of the earthquake’s epicenter. To which city is the location of the earthquake’s epicenter closest?
Analyzing Processes Why must measurements from three locations be used to find the epicenter of an earthquake?
Map Skills Activity
This map shows the earthquake hazard levels for Europe, Asia, Africa, and Australia. Use the map to answer the questions below.
- Using a Key Which areas of the map have a very high earthquake-hazard level?
- Using a Key Determine which areas of the map have very low earthquake-hazard levels.
- Inferring Relationships Most earthquakes take place near tectonic plate boundaries. Based on the hazard levels, describe the areas of the map where you think tectonic plate boundaries are located.
- Analyzing Relationships In Asia, just below 60° north latitude, some areas have high earthquake-hazard levels but no plate boundaries. Explain why these areas might experience earthquakes.
- Forming a Hypothesis There is a tectonic plate boundary between Africa and Saudi Arabia. However, the earthquake hazard level in that region is low. Explain the low earthquake-hazard level.
- Analyzing Relationships A divergent plate boundary began to tear apart the continent of Africa about 30 million years ago. Where on the continent of Africa would you expect to find landforms created by this boundary? Explain your answer.
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