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Chapter 6: Earthquakes and Plate Boundaries Ch 6 Journal: Have you ever felt an earthquake? Or, was there ever a time when you felt unsteady, like the ground was going to slip out from under you? Write a description of your experience. Lesson 6.1- What is an earthquake? 1. An earthquake is the rupture and sudden movement of rocks along a fault. a. A fault is a fracture surface along which rocks can slip. 2. Energy stored as a change in shape is called elastic strain. a. Heat and matter move through Earth’s mantle by convection. Some heat energy from Earth’s interior is transformed into kinetic energy. b. Some of the kinetic energy in Earth’s mantle creates elastic strain in rocks. 3. Elastic strain builds up in rocks along faults as the rocks move past each other. When rocks cannot stretch to add more elastic strain, they release energy by breaking and slipping. a. When rocks are so strained that they can no longer stretch or flow a fault ruptures. b. A stretched rubber band best models elastic strain. 4. When rocks along a fault suddenly break or slip, elastic strain is released. Complex waves radiate in all directions, carrying some of the released energy. a. The energy flow leading to an earthquake would be heat energy, kinetic energy, elastic energy, to seismic waves. 5. Plate boundaries are often fault zones that are 40-200 km wide, rather than a single fault. 6. The focus of an earthquake is the place on a fault where the rupture and movement of the earthquake begins. B. Plate Boundaries and Earthquakes 1. Most, but not all, earthquakes occur at plate boundaries. The three different types of plate boundaries have different usual patterns of earthquakes. a. Most earthquakes at divergent plate boundaries occur at relatively shallow depths and are relatively weak earthquakes. b. At transform plate boundaries, most earthquakes occur on strike-slip faults and at relatively shallow depths. Earthquakes with a shallow focus can produce severe shaking, and so dangerous earthquakes can occur at transform plate boundaries. c. Earthquakes at convergent plate boundaries tend to be the deepest earthquakes. They have been some of the most destructive earthquakes in human history. 2. A small percentage of earthquakes occur away from plate boundaries. These earthquakes can be destructive because people are often unprepared for them. Discussion Question: Not all the energy stored as elastic strain by rocks is carried away from a fault by complex waves when an earthquake occurs. What happens to the rest of the energy? The total amount of energy before and after the earthquake must be the same, but energy will be transferred from one place or form to another. A large amount of energy is carried away by complex waves through the ground, but some energy is transferred in other forms. The noise of rocks breaking is caused by sound energy, some energy is used to break rocks themselves, and other energy is used to heat areas around the breaking rocks slightly. Earthquake video (20 min) Earthquake video (20 min) Lesson 6.2- Earthquakes and Seismic Waves A. What are seismic waves? 1. Seismic waves are waves of energy produced at the focus of an earthquake as elastic strain is released. a. Seismic waves travel outward in all directions from the focus of an earthquake. b. The amount of energy carried by seismic waves decreases as the waves move away from the focus. This is because rocks absorb some energy as the seismic waves pass through them. 2. The epicenter of an earthquake is the point on Earth’s surface directly above the focus of the earthquake. B. Types of Seismic Waves 1. There are three different types of seismic waves: primary waves, secondary waves, and surface waves. a. Primary waves are compressional waves that move rocks and other matter parallel to the direction the wave is traveling. Primary waves are the fastest seismic waves. b. Secondary waves are shear waves that move rocks and other matter back and forth perpendicular to the direction the waves are traveling. Secondary waves travel about 60 percent as fast as P-waves. c. A spring toy illustrates the motion of an S-wave. d. Some of the energy from P-waves and S-waves that reach Earth’s surface can be trapped in the upper areas of Earth’s crust to form surface waves. Surface waves are the slowest type of seismic wave. e. Surface waves usually cause stronger shaking than P-waves or S-waves, and they are often the most destructive type of seismic wave. C. Using Seismic Wave Data 1. A seismologist is a scientist who studies earthquakes. 2. The farther you are from the focus of an earthquake, the farther S- waves will be behind the P- waves. 3. Seismic waves change speed and direction when the material through which they are traveling changes. 4. Observing the paths of seismic waves can help scientists to understand what kind of materials make up Earth’s interior. 5. The shadow zone is a portion of Earth that does not receive any seismic waves from a particular earthquake. 6. Because S-waves cannot move through liquid and P-waves would be bent by a liquid, scientists think Earth’s outer core is liquid and causes the shadow zone. 7. If a seismograph is located a great distance from the epicenter, the P-waves would arrive earlier than the S-waves. Discussion Question: Where is the first place on Earth’s surface that an earthquake will be felt, and why? If an earthquake is noticeable on Earth’s surface at all, it will always be felt first at the epicenter. This is because the epicenter is directly above the focus, which means it is the shortest straight-line distance from the focus of the earthquake to the surface. Even though P- and S- waves move out from the focus in all directions, each type of wave moves at the same speed. Because they have to travel the shortest distance to reach the epicenter, P-waves will be felt there first. Nature’s Fury video (44 min) Lesson 6.3- Measuring Earthquakes A. How are Earthquakes measured? 1. Scientists determine the size of the large earthquake that occurred in the Indian Ocean on December 26, 2004, by measuring how much the rock moved along the fault where the earthquake started. 2. Because the earthquake occurred underwater, the movement of rock caused a huge ocean wave in the Indian Ocean. B. Recording Seismic Waves 1. A seismograph is an instrument used to record and measure movements of the ground caused by seismic waves. It measures the magnitude of an earthquake. a. Seismographs record the size, direction, and time of the movements caused by different types of seismic waves. b. Seismographs record ground motion in two orientations: horizontal, or back-and-forth, and vertical, or upand-down. 2. The record of the seismic waves created by a seismograph is called a seismogram. a. The x-axis of a seismogram represents time. b. Heights of waves on a seismogram show the relative size of ground motion caused by seismic waves. c. P-waves are the type of seismic wave that is recorded first on a seismograph. C. Locating an Epicenter 1. With readings from at least three seismographs, you can use triangulation to find the location of the epicenter of an earthquake. a. Use a seismogram to measure the number of seconds between the arrival of P-waves and the arrival of S-waves. b. Use a graph showing the time difference between P- and S-waves plotted against distance to determine how far the waves traveled. c. On a map, draw a circle with a radius equal to the distance the waves have traveled around the location of the seismograph. d. Draw the same kind of circles for additional seismograms. The location of the epicenter will be shown by the intersection of the circles. D. Measuring Earthquake Size 1. The magnitude scale is based on a seismogram’s record of the amplitude of ground motion. a. Numerical measurements of magnitude vary from about 0.0 to 9.0, but each one number represents ten times the amount of ground motion. b. Magnitude can also be used to understand the amount of energy released by an earthquake. An increase of one unit on the magnitude scale represents 30 times as much energy being released. 2. The Richter magnitude scale was the first magnitude scale, but is not as accurate as more modern magnitude scales. With the Richter scale, the magnitude values are not accurate for small or large earthquakes. 3. Today, the most commonly used scale for measuring earthquakes is the moment magnitude scale. E. Earthquake Intensity 1. Besides size, earthquakes can also be compared by their intensity, or the amount of damage they cause. 2. Intensity tends to decrease as you move from the epicenter, but it can vary depending on the types of rocks or sediments in an area. 3. The most widely used intensity scale in the U.S. is the modified Mercalli scale. Discussion Question: Would you personally rather experience an earthquake with a large magnitude but low local intensity, or a smaller-magnitude earthquake with a higher intensity? Standard Deviant video (25 min) Lesson 6.4- Earthquake Hazards and Safety A. Earthquake Hazards 1. Most injuries from an earthquake are caused by the collapse of buildings and other structures, not directly by the ground shaking. 2. Fires caused by broken gas pipes or electrical lines are the most common hazard following an earthquake. 3. A landslide is the sudden movement of soil and rocks down a slope. 4. Earthquakes sometimes cause liquefaction, a process by which shaking makes loose sediment behave like a liquid. Liquefaction near buildings can cause them to sink into the ground. 5. An ocean wave caused by an earthquake is called a seismic sea wave, or a tsunami. a. Like seismic waves, tsunamis carry energy, but they can also cause flooding and carry objects in the water they move. b. The towering wave on a tsunami forms where the wave hits shallow water. c. A warning sign just before a tsunami strikes, water along a shoreline sometimes moves quickly toward the sea, exposing areas that are usually underwater. B. Avoiding Earthquake Hazards 1. The chance of earthquake damage is greatest close to faults and also increases in areas where Earth’s surface is made of loose sediments rather than solid rocks. 2. Maps showing where the chance of earthquake damage is greatest, such as in areas along the San Andreas Fault, can help in the planning of safe ways to use land. C. Earthquakes and Structures 1. Tall buildings or structures and buildings made of brittle materials usually suffer the most damage in an earthquake. Structures made from flexible building materials, such as wood, usually suffer less damage. 2. Some buildings in areas threatened by earthquakes are supported by flexible, circular moorings placed under the buildings. These moorings act like shock absorbers for the buildings. D. Earthquake Safety 1. To be as safe as possible during and after an earthquake, it is important to prepare ahead of time. a. With your family, create, review, and practice an earthquake disaster plan. b. Make an earthquake supply kit 9with canned food, water, a battery-powered radio, a flashlight, and first aid supplies. c. Move heavy objects close to the ground, and learn how to shut off gas, water, and electricity in your home. 2. During an earthquake, stay indoors, get away from anything that could break or fall on you, and find something sturdy to hide underneath. Discussion Question: If you feel the ground shake during an earthquake, does that mean it is safe to check around your house to see if everything is all right? No. P-waves move the fastest, so if you are some distance away from the epicenter of the earthquake, s-waves and surface waves might not yet have reached you. Surface waves are the slowest waves, but they often cause the most damage. When you feel the first movement of an earthquake, it is important to move to a safe place and stay there until all the waves have passed. Even then, it is important to be careful and observant before moving to another safe place, because weakened parts of structures might not fall immediately. Natural Disasters video (55 min)