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Name CHAPTER 7 Class Date Earthquakes SECTION 1 What Are Earthquakes? California Science Standards BEFORE YOU READ After you read this section, you should be able to answer these questions: 6.1.a, 6.1.d, 6.1.e, 6.3.a • Where do most earthquakes happen? • What makes an earthquake happen? • What are seismic waves? What Is an Earthquake? Have you ever been in an earthquake? An earthquake is a movement or shaking of the ground. Earthquakes happen when huge pieces of the Earth’s crust move suddenly and give off energy. This energy travels through the ground and makes it move. STUDY TIP Outline As you read, create an outline of this section. Use the headings in the section to guide you. Where Do Most Earthquakes Happen? Most earthquakes happen at places where two tectonic plates touch. Tectonic plates are always moving. Where two plates touch, they may be moving toward each other, moving away from each other, or sliding past each other. The movements of the plates can break the rocky crust in many places. A place where the crust is broken is called a fault. Earthquakes happen when rock breaks and slides along a fault. A place in the crust where a lot of faults are found is called a fault zone. Most fault zones, such as the San Andreas Fault Zone, are found at plate boundaries. But some fault zones are in the middle of plates. READING CHECK 1. Define What is a fault? Earthquakes and Plate Boundaries • Recorded earthquakes TAKE A LOOK 2. Infer Use the earthquake locations to help you figure out where the tectonic plate boundaries are on the map. Use a colored pen or marker to draw tectonic plate boundaries on the map. Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader and Study Guide 121 Earthquakes Name Class SECTION 1 Date What Are Earthquakes? continued Why Do Earthquakes Happen? As tectonic plates move, pressure builds up on the rock near the edges of the plates. When a rock is put under pressure, it deforms, or changes shape. Rock can deform in two main ways. One kind of deformation is called plastic deformation. Plastic deformation happens when rock bends and folds like clay. When the pressure is taken away, the rock stays folded. TAKE A LOOK 3. Explain How do you know that the rock layers in the figure were once under a lot of pressure? CALIFORNIA STANDARDS CHECK 6.1.e Students know major geologic events, such as earthquakes, volcanic eruptions, and mountain building, result from plate motions. Word Help: major of great importance or large scale Folded Layers of Rock A second kind of deformation is called elastic deformation. Elastic deformation happens when rock bends a little bit under pressure but returns to its original size and shape when the pressure stops. Earthquakes happen when rock breaks under pressure. When the rock breaks, it snaps back to almost the same size and shape as before the earthquake. This snap back is called elastic rebound. When the rock breaks and rebounds, it gives off energy. This energy causes the ground to shake. Elastic Rebound 4. Explain How does the movement of tectonic plates cause earthquakes? 1. Forces push rock in opposite directions. The rock deforms elastically. It does not break. 2. If enough force is placed on the rock, it breaks. The rock slips along the fault. Energy is released. Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader and Study Guide 122 Earthquakes Name SECTION 1 Class Date What Are Earthquakes? continued How Do Earthquakes Happen at Divergent Boundaries? A divergent boundary is a place where two tectonic plates are moving away from each other. As the plates pull apart, the crust stretches. The crust breaks into pieces along faults. The pieces are called fault blocks because they are blocks of rock that move along faults. Most of the crust at divergent boundaries is thin, so the earthquakes tend to be shallow. Most earthquakes at divergent boundaries are no more than 20 km deep. Earthquakes at Divergent Boundaries Ocean Tectonic plate Tectonic plate READING CHECK 5. Define What is a fault block? READING CHECK 6. Explain Why do most earthquakes at divergent boundaries happen at shallow depths? Earthquake How Do Earthquakes Happen at Convergent Boundaries? A convergent boundary is a place where two tectonic plates collide. When two plates come together, both plates may crumple up to form mountains. Or one plate can subduct, or sink, underneath the other plate and into the mantle. The earthquakes that happen at convergent boundaries can be very strong because so much pressure is produced. When one plate subducts under another, earthquakes can happen inside the sinking plate at depths of up to 700 km. Earthquakes at Convergent Boundaries READING CHECK 7. Explain Why are most earthquakes at convergent boundaries very strong? Ocean TAKE A LOOK Earthquake Tectonic plate 8. Identify Draw arrows on the figure to show the directions in which the two tectonic plates are moving. Tectonic plate Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader and Study Guide 123 Earthquakes Name SECTION 1 Class Date What Are Earthquakes? continued How Do Earthquakes Happen at Transform Boundaries? READING CHECK 9. Identify How deep can earthquakes occur at transform boundaries? A transform boundary is a place where two tectonic plates slide past each other. As the plates move past each other, pressure builds up on the rock. Eventually, the rock breaks along a fault. The rock will break only if it is brittle. Rocks far below Earth’s surface are generally not brittle. Therefore, most earthquakes at transform boundaries happen in the upper 50 km of the crust. Earthquakes at Transform Boundaries TAKE A LOOK 10. Identify Draw arrows showing the directions that the tectonic plates in the figure are moving. Tectonic plate Tectonic plate Earthquake How Does Earthquake Energy Travel? READING CHECK 11. List What are the two kinds of seismic waves? When an earthquake occurs, a lot of energy is given off. This energy travels through the Earth in the form of waves called seismic waves. There are two kinds of seismic waves. Body waves are seismic waves that travel through the inside of Earth to the surface. Surface waves are seismic waves that travel through the top part of Earth’s crust. BODY WAVES There are two kinds of body waves: P waves and S waves. P waves are the fastest kind of seismic wave. P waves cause rock to move back and forth. P waves can move through solids, liquids, and gases. Another kind of wave, an S wave, can cause the rock to move horizontally from side to side. S waves can also cause the rock to move up and down. S waves travel more slowly than P waves. They can travel only through solids. Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader and Study Guide 124 Earthquakes Name Class SECTION 1 Date What Are Earthquakes? continued SURFACE WAVES Surface waves travel along the top of Earth’s crust. Only the very top part of the crust moves when a surface wave passes. Surface waves travel much more slowly than body waves, but they cause a lot more damage. READING CHECK 12. Compare Which kind of seismic waves travel the most slowly? Contraction Expansion Contraction Direction P wave is traveling Wave 2 Wave 1 P waves are body waves that squeeze and stretch rock. Side-to-side motion Direction S wave is traveling TAKE A LOOK 13. Compare How are the motions of P waves and S waves different? S waves are body waves that can move rock from side to side. Critical Thinking Up-and-down rolling motion Direction surface wave is traveling 14. Infer What do you think is the reason that surface waves usually cause the most damage? Surface waves can move the ground up and down in a circular motion. Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader and Study Guide 125 Earthquakes Name Class Section 1 Review Date 6.1.a, 6.1.d, 6.1.e, 6.3.a SECTION VOCABULARY earthquake a movement or trembling of the ground that is caused by a sudden release of energy when rocks along a fault move elastic rebound the sudden return of elastically deformed rock to its undeformed shape seismic wave a wave of energy that travels through the Earth, away from an earthquake in all directions 1. Compare What is the difference between an earthquake and a fault? 2. Compare Give two ways that P waves are different from S waves. 3. Identify Where do most earthquakes happen? 4. Describe What causes an earthquake to happen? 5. Explain What is the main difference between body waves and surface waves? 6. Apply Concepts Why are some earthquakes stronger than others? 7. Infer Why do few earthquakes happen in Earth’s mantle? Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader and Study Guide 126 Earthquakes Earth Science Answer Key continued 6. weathering and erosion have removed the 4. The movement of the plates causes pressure overlying rock 7. Sierra Nevada, Cascades, Transverse Range, Central Range 8. Type of How it forms Examples in California 5. 6. 7. Magma forms at a convergent plate boundary. It erupts to form volcanoes or cools to form batholiths. Mount Shasta, Lassen Peak, Sierra Nevadas 8. Pieces of a subducting plate are scraped off and attached to a continent. Coast Ranges, Transverse Ranges mountain Volcanic Accreted terrane 9. 10. 11. 12. 13. 9. North American and Pacific 10. There are many faults along which plate 14. movement takes place. Review 1. a piece of a tectonic plate that is scraped off 2. 3. 4. 5. 6. to build up. When the pressure is released, an earthquake occurs. a piece of rock surrounded by faults The crust is thin at divergent boundaries. A lot of pressure builds up before the rock breaks. Arrows should be perpendicular to the plate boundary, pointing toward each other. 50 km Arrows should be parallel to the fault, pointing in opposite directions, left side away, right side toward. body waves and surface waves surface waves P waves move particles back and forth. S waves move them from side to side. Possible answer: All of their energy is released at the surface. Review 1. An earthquake is a shaking or movement of and added to another tectonic plate during convergence transform Lassen Peak and Mount Shasta are part of the Cascade Mountains. They formed above a subduction zone when melted rock erupted onto Earth’s surface. Strike-slip faults are probably most common because the San Andreas is a transform boundary. Strike-slip faults are common at transform boundaries. The southern part has more compression than the northern part. Until about 10 million years ago, most of California’s coast was located on a convergent boundary. Today, most of California’s coast is located on a transform boundary. Only a small part of it is found on a convergent boundary. 2. 3. 4. 5. 6. Chapter 7 Earthquakes SECTION 1 WHAT ARE EARTHQUAKES? 7. 1. A fault is a break in the crust. 2. Plate boundaries should follow the lines defined by earthquake locations. the Earth. A fault is a crack in the crust that rocks can slide along. That is, earthquakes happen along faults. P waves move particles back-and-forth, and S waves move them side-to-side. P waves travel more quickly than S waves. P waves travel through solids, liquids, and gases, but S waves travel only through solids. Most earthquakes happen at tectonic plate boundaries. Earthquakes happen when pressure builds up on a rock and it breaks. That is, elastic rebound of rocks causes earthquakes. Body waves travel through the interior of the Earth, but surface waves travel only on its surface. The strength of an earthquake is directly related to the amount of pressure that builds up on the rock before it breaks. Some rocks are stronger than others, so more pressure builds up before the rock breaks. When lots of pressure builds up, larger earthquakes happen. Almost all earthquakes happen when rock breaks. The rock in the crust is brittle and breaks in response to stress. Rock in the mantle flows in response to stress. 3. The rocks are folded. Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader and Study Guide Answer Key 10 Earth Science