Earthquakes and Volcanoes

Transcription

1 chapter 38 Earthquakes and Volcanoes section 1 Earthquakes Before You Read Have you ever experienced an earthquake or perhaps seen an earthquake in a movie or on TV? On the lines below, describe an earthquake. What You ll Learn how energy from an earthquake reaches points on Earth s surface about primary, secondary, and surface waves produced by earthquakes how to recognize earthquake hazards and prepare for them Read to Learn What causes earthquakes? Have you ever bent a stick? If so, you probably noticed that it changes shape while you are bending it and springs back when you stop. But if you don t stop bending the stick, it changes permanently. If its elastic limit is passed, the stick may break. As it breaks, you can feel vibrations in the stick. What is elastic rebound? Rocks are like other solid materials. If enough force pulls or pushes on them, they will change shape. They may even break. After breaking, the ends of the broken pieces may snap back. This snapping back is called elastic rebound. Inside Earth, pushing and pulling forces cause rocks to change shape slowly over time. As they are strained, potential energy builds up in them. This energy is released suddenly when the rocks finally break or move. The breaking and the movement that follows causes vibrations that move through rock. If they are strong enough, the vibrations are felt as earthquakes. An earthquake is a movement of the ground that occurs when rocks inside Earth pass their elastic limit, break suddenly, and experience elastic rebound. Underline As you read this section, underline the key terms and main ideas in each paragraph to help you understand new information. A Organize Information Construct a three-tab Foldable as shown. Under the tabs, write information about faults. Reading Essentials 101

2 Types of Faults Normal fault Reverse fault Strike-slip fault Picture This 1. Determine Trace over the arrows in each figure. What do the arrows show? What is a fault? When part of a rock breaks, rocks on either side move as a result of elastic rebound. The surface where rocks break and move is called a fault. Rocks can break in different ways, depending on the forces that cause the break. The figures above show three different faults, a normal fault, a reverse fault, and a strike-slip fault. What are the types of faults? Sometimes rocks are pulled apart because of tension forces. This is what occurs in a normal fault, as shown in the normal fault figure. Normal faults form where tension forces pull rocks apart, and the rock above the fault moves down. Sometimes rocks are pushed together, or compressed. Reverse faults are caused by compression. This is shown in the reverse fault figure. When the two rocks push together, rock above the fault is pushed up. Sometimes sections of rock move past one another in opposite directions along Earth s surface as shown in the strike-slip fault figure. This is called shearing. Strike-slip faults are caused by shear forces. 102 Earthquakes and Volcanoes

3 Making Waves Do you recall the last time you shouted for a friend to save you a seat on the bus? When you called out, energy traveled through the air in the form of sound waves. These sound waves were released by your vocal cords and were affected by your tongue and mouth. The sound waves traveled from your mouth outward through the air to your friend. Your friend identified the familiar sound of your voice as belonging to you. Sound is a form of energy that travels through the air in waves. Earthquakes also release energy in waves. Earthquake waves move through material inside Earth and along Earth s surface. Waves from an earthquake are called seismic waves. How does energy from an earthquake reach Earth s surface? Movement along a fault releases energy causing vibrations. When this energy is released, it moves away from the fault in the form of seismic waves. The point deep inside Earth where energy is released causing an earthquake is a focus. Some of the energy from the earthquake travels straight up to Earth s surface where it can be felt. The epicenter is the point on Earth s surface directly above the earthquake focus. This release of energy can be seen in the figure below Fault Focus 2. Identify What are waves from an earthquake called? Picture This 3. Label Add the labels epicenter and seismic wave to the figure. Reading Essentials 103

4 How do seismic waves travel? When seismic waves leave the focus of an earthquake, some travel through Earth s interior, and others travel along the surface. Primary Waves The seismic waves that travel fastest through rock material are primary waves, or P-waves. Primary waves cause the material to move from side to side, in the same direction that the wave is moving. Secondary waves Other seismic waves that travel through Earth s interior are called secondary waves. Secondary waves, or S-waves, do not move as fast as P-waves. As they move through rock material, they cause the material to vibrate at right angles to the direction of the wave. 4. Determine What two types of seismic waves travel through Earth s interior? 5. Describe What do seismologists do? 104 Earthquakes and Volcanoes Surface waves Seismic waves that travel along Earth s surface are called surface waves. They are the largest and slowest type of seismic wave. They cause more damage than other types of seismic waves. Surface waves move in different ways. When some surface waves travel over Earth s surface, they may move the rock and soil in a backward rolling motion. They move across land like waves of water. Some surface waves shake or sway the rock and soil from side to side. Learning from Earthquakes On your way to lunch, suppose you walk twice as fast as your friend does. What would happen to the distance between the two of you? The distance between you would become greater the farther you walk, and you would arrive first. When scientists study seismic waves, they apply the same principle. Scientists look at the different speeds of seismic waves and at the different arrival times to figure out the exact location of an earthquake s epicenter. How are earthquakes measured? Scientists who study earthquakes are called seismologists. They use instruments called seismographs to record seismic waves. One type of seismograph has a drum that holds a roll of paper on a frame. A pendulum with an attached pen hangs over the frame. When seismic waves reach the station, the drum vibrates. The pen on the pendulum traces a record of the vibration. The height of the lines traced on the paper measures the magnitude of the earthquake. Magnitude is the measure of energy released by an earthquake.

5 Travel time (min) First S wave S-wave curve First P wave P-wave curve 1,000 2,000 3,000 4,000 Distance to epicenter (km) 5 min Picture This 6. Interpret Look at the seismograph reading. How far away was the earthquake epicenter from this location? How many minutes apart did the waves arrive? How is an epicenter located? The epicenter of an earthquake is the point on the surface of Earth directly above the focus. Far away from the epicenter, the P-waves and S-waves arrive at different times. But close to the epicenter, the waves arrive at almost the same time. This difference in arrival time is shown in the graph above. Once scientists know the P-wave and S-wave arrival times for at least three seismograph stations, they can figure out the location of an earthquake s epicenter. They draw circles on a map. Each circle shows the distance from a seismograph station to the earthquake. The point where three or more circles intersect is the location of the epicenter. Look at the map below. The three seismographic stations are in Tokyo, Berkeley, and Anchorage. The epicenter is the point where the three circles intersect. B Picture This 7. Interpret On the map, which seismograph station is closest to the epicenter of the earthquake? Reading Essentials 105

6 Applying Math 8. Calculate How much more energy is released from an earthquake with a magnitude of 8 than from an earthquake with a magnitude of 6? Show your work in the box. 9. Identify What are powerful seismic sea waves called? How strong are earthquakes? Some earthquakes are not felt on the surface of Earth. People do not even know these small earthquakes are happening. Larger earthquakes, on the other hand, can cause major damage. What is the Richter scale? Richter (RIHK tur) magnitude is based on the measurements of heights of seismic waves as they are recorded on seismographs. Scientists use this information to determine the Richter magnitude of an earthquake. Richter magnitude describes how much energy an earthquake releases. Very weak earthquakes have low magnitudes like 1.0 on the Richter scale. Strong earthquakes have high magnitudes in the range of 6 to 7. For every increase of 1.0 on the Richter scale, an earthquake actually releases 32 times more energy. This means that an earthquake with a magnitude of 7.5 releases 32 times more energy than an earthquake with a magnitude of 6.5. How is earthquake intensity measured? Another way to measure earthquakes is by the modified Mercalli intensity scale. This scale measures the intensity of an earthquake. Intensity is a measure of the amount of damage to structures and to rocks and soil in a specific area. The amount of damage depends on how strong the earthquake is, kinds of structures in an area, distance from the earthquake s epicenter, and the nature of the surface material. The Mercalli scale uses Roman numerals I through XII. An earthquake with an intensity of I would be felt by few people. An intensity-vi earthquake would be felt by everyone. An intensity-xii earthquake would cause major damage to Earth s surface and to human-built structures. What are tsunamis? When an earthquake occurs on the ocean floor, powerful waves are produced. These waves travel outward from the earthquake in all directions. A powerful seismic sea wave is called a tsunami. Tsunamis traveling in open ocean water are low and fast moving. But tsunamis change as they approach land. The speed of the tsunami slows and the height of the wave increases. Huge tsunami waves can be up to 30 m high. These huge waves can cause large amounts of damage along coastal areas. 106 Earthquakes and Volcanoes

7 Earthquake Safety You have read about the destruction earthquakes can cause. If the area you are in has had earthquakes before, chances are it will again. You can prepare for that. What measures can you take to be safer? There are things you can do to make your home safe. Put heavy things on lower shelves so they won t fall on you. Make sure your gas hot-water heater and appliances are well secured. There are now sensors that can be placed on gas lines. The sensors shut off the gas when the vibrations of an earthquake are felt. This helps prevent fires. During an earthquake, stay away from windows and anything that could fall on you. Sometimes the safest thing is to get outside. But then you must watch for fallen power lines and fire hazards. How do people build seismic-safe structures? Seismic-safe structures are buildings that can stand up against the vibrations caused by most earthquakes. It is not possible to prevent earthquakes, but structures can be built that withstand them. California has many regulations for making sure that buildings are seismic-safe. Many high-rise office buildings now stand on huge steel-and-rubber supports. This can help buildings to ride out the vibrations of an earthquake. Underground water and gas pipes are replaced with pipes that will bend during an earthquake. This can help prevent broken gas lines and therefore prevent damage from fires. Highways and bridges are built with spiral rods in the concrete columns. These spiral rods reinforce the structure, and keep it from collapsing. Can earthquakes be predicted? Right now, it is impossible to predict the exact time and place an earthquake will occur. However, scientists do know which locations are more likely to have earthquakes. In these places, they monitor the movement along faults to see when an earthquake might occur. They also watch the groundwater levels and electrical properties in rocks. It is possible to predict that an earthquake of a certain magnitude will probably occur in a certain location within the next 30 to 100 years. But it is difficult to predict an earthquake more exactly than that. 10. Explain What is one thing you can do to make your home safer during an earthquake? 11. Describe one thing that can be done to make buildings and structures more seismic-safe. Reading Essentials 107

8 After You Read Mini Glossary earthquake: movement of the ground that occurs when rocks inside Earth pass their elastic limit, break suddenly, and experience rebound epicenter: point on Earth s surface directly above the earthquake s focus fault: fracture that occurs where rocks break which results in movement of opposing sides focus: point deep inside Earth where energy is released, causing an earthquake magnitude: measure of energy released by an earthquake seismic-safe: describes the ability of structures to stand up against vibrations caused by an earthquake seismic waves: earthquake waves including primary waves, secondary waves, and surface waves seismograph: instrument used to record seismic waves tsunami: powerful seismic sea wave that begins over an ocean-floor earthquake 1. Review the terms and their definitions in the Mini Glossary. Write two sentences explaining how earthquakes are measured. Use at least three glossary words. 2. Fill in the boxes with the correct terms. waves P-waves Travel fastest through rock waves waves S-waves Travel slower through rock waves Slowest waves Largest waves Cause most destruction Travel on Earth s surface 3. You underlined key terms and main ideas in each paragraph in this section. Did this strategy help you learn more about earthquakes? Would you use it again? End of Section Visit blue.msscience.com to access your textbook, interactive games, and projects to help you learn more about earthquakes. 108 Earthquakes and Volcanoes