# 11/30/16 EARTHQUAKES ELASTIC LIMIT FAULT FORCE AND PLATES WHAT DO YOU NOTICE?

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1 ELASTIC LIMIT EARTHQUAKES Bend sitck but do not break it. What do you notice? No bend until it breaks. Describe the energy and forces at work. (Kinetic, potential etc) 8 TH GRADE FAULT FORCE AND PLATES Rocks have an elastic limit. Forces in Earth bend or stretch rocks. Rocks can bend and stretch up to a point. But once a rock s elastic limit is passed, the rock breaks. When rocks break in this way, they move along surfaces, or faults. A fault is the surface along which rocks move when they pass their elastic limit and break. A tremendous amount of force is required to overcome the strength of rocks and to cause movement along a fault. Rock on one side of a fault may move up, down, or sideways in relation to rock on the other side of the fault. The surface of Earth is in constant motion because of forces inside the planet. These forces cause sections of Earth s surface, called plates, to move. WHAT DO YOU NOTICE? The red dots are where earthquakes have been recorded. The movement of plates puts stress on rocks near the edges of the plates. Most earthquakes occur near plate boundaries. To relieve this stress, rocks bend, stretch, or compress 1

2 THINK OF THE STICK To relieve this stress, rocks bend, stretch, or compress. If the force is great enough, the rocks will break. An earthquake is the vibrations, or shaking, produced when rocks break along a fault. As rocks along a fault move past each other, the rough surfaces catch, or grind, to a temporary halt. However, the forces inside Earth are so strong, they do not let the rocks stop moving. The forces keep driving the rocks, and stress builds up along the fault. When the rocks are stressed beyond their elastic limit, they break, move along the fault, and then return to their original shapes. An earthquake results??? Earthquakes may be small vibrations that no one notices. Or they may be enormous vibrations that cause a great deal of damage. Why do earthquakes rarely occur at the middle of plates? Most stress is at boundaries where plates meet No matter what their strength, most earthquakes result from rocks moving over, under, or past each other along fault surfaces. A REVIEW OF FORCES Earthquakes may be small vibrations that no one notices. Describe three types of forces that can act on rocks? Or they may be enormous vibrations that cause a great deal of damage. No matter what their strength, most earthquakes result from rocks moving over, under, or past each other along fault surfaces. 2

3 FORCES ON ROCKS A FAULTY REVIEW ;) Three types of forces act on rocks tension, compression, and shear. Describe the three types of faults. Tension is the force that pulls rocks apart. Compression is the force that squeezes rocks together. Shear is the force that causes rocks on either side of a fault to slide past each other. TENSION = NORMAL NORMAL Forces of tension inside Earth cause rocks to be pulled apart. When rocks are stretched by these forces, a normal fault can form. Along a normal fault, rocks are pulled apart, and the rock above the fault moves downward in relation to rock below the fault. COMPRESSION = REVERSE REVERSE FAULT Some faults occur when forces of compression squeeze rocks together. In a reverse fault, rock breaks from forces of compression pushing on it from opposite directions. The force of compression along a reverse fault pushes the rock that is above the fault up and over the rock that is below the fault. 3

4 SLIDE BY STRIKE SLIP FAULT In a strike-slip fault, rocks move past each other without much movement upward or downward. The figure below shows that shear forces push rocks past each other on either side of the strike-slip fault surface. The San Andreas Fault in California is a strike-slip fault that extends for more than 1,100 km through the state. The San Andreas Fault is the boundary between two of Earth s plates that are moving sideways past each other. OUTLINE A. When rocks break they move along faults. 1. Applied forces cause rocks to undergo elastic deformation. 2. When elastic limits are passed, rocks break. 3. Rock on one side of a fault can move up, down, or sideways in relation to rock on the other side of the fault. B. Faults occur because forces inside the Earth cause Earth s plates to move placing stress on or near the plate edge. 1. Rocks will bend, compress, stretch, and possibly break. 2. Earthquake vibrations produced by breaking rock a. Rocks break, move along the fault, return to original shapes b. Rock on one side of a fault can move over, under, or past each other along fault lines. C. Three types of forces act on rocks tension, compression, and shear. 1. Tension forces; normal fault caused by rock above the fault moving downward in relation to the rock below the fault 2. Reverse fault compression forces squeeze rock above the fault up and over the rock below the fault. 3. Create by shear forces; strike-slip fault rocks on either side of the fault move past each other without much upward or downward motion. HOLD THE ROPE What three types of forces act on rocks to create faults? Watch the rope Tension, compression, and shear Which way is the force (motion) traveling? Which way is the rope moving in relation to the force? 4

5 SEISMIC WAVES ORIGIN/ FOCUS When a rope is shaken at one end, energy travels through the rope in the form of waves. Seismic waves are generated by earthquakes. Seismic waves travel through Earth just as waves travel through rope. When an earthquake occurs, the ground moves forward and backward, up and down, or side to side. Sometimes, earthquakes cause the surface of Earth to ripple like the waves on the ocean. Rocks move past each other along faults. Stress builds up along rock surfaces that catch on each other. The stress continues to build up until the rocks elastic limit is passed. Then the built up energy is released as seismic waves that travel outward from the earthquake focus. FOCUS! TYPES OF WAVES The focus (plural, foci) of an earthquake is the point where this energy is first released. The foci of most earthquakes are within 65 km of Earth s surface. A few earthquakes have occurred as much as 700 km beneath the surface. Earthquakes produce three different types of seismic waves primary, secondary, and surface waves. All three types of seismic waves are produced at the same time. But each type of wave behaves in a different way within Earth. SPRING LOADED SECONDARY WAVES Primary Waves Primary waves (P-waves) cause particles in rock to move back and forth in the same direction that the wave is traveling. Primary waves occur when particles in rocks are compressed and then stretch apart. Secondary waves (S-waves) move through Earth by causing particles in rock to move at right angles to the direction in which the wave is traveling. Rope!!!!! This motion sends primary waves traveling through the rock. Think of a stretched spring going back and forth (compress and stretch). 5

6 SURFACE WAVES Surface waves move particles in rock in a backward, rolling motion and also in a side-to-side, swaying motion. They are produced when earthquake energy reaches the surface. The arrows in the figure show the different directions of movement made by surface waves. EPICENTER Surface waves cause most of the damage from earthquakes. Many buildings are made with stiff materials that crack when surface waves shake the ground. The buildings fall apart or collapse when surface waves cause different parts of the building to move in different directions. Surface waves travel outward from the epicenter of an earthquake. An earthquake s epicenter is the point on Earth s surface directly above the earthquake focus. FIGURE IT OUT Different seismic waves travel through Earth at different speeds. Primary waves are the fastest; secondary waves are slower; surface waves are the slowest. They travel about half the speed of primary waves. Surface waves are the slowest seismic waves Scientists have learned to use the different speeds of seismic waves to figure out the distance to an earthquake s epicenter. When an earthquake s epicenter is far from a location, the primary wave has more time to get farther ahead of the secondary and surface waves. It reaches the monitoring center first, ahead of the other seismic waves 6

7 SEISMOGRAPH A seismograph is the instrument scientists use to measure seismic waves. A seismograph registers the waves and records the time that each wave arrived. A seismograph is made up of a rotating drum of paper and a hanging weight, or pendulum, with a pen attached to it. When seismic waves reach the seismograph, the drum vibrates, but the pendulum remains at rest. The unmoving pen traces a record of the vibrations on the moving drum of paper. The paper record of a seismic event is called a seismogram. STATIONS Seismographs differ according to whether they are intended to measure horizontal or vertical seismic motions. The figure shows two different seismographs recording two different ground movements horizontal and vertical. Scientists record and monitor earthquake activity. Each type of seismic wave reaches the seismograph station at a different time, depending on its speed. Primary waves reach the seismograph station first. Secondary waves reach the station second, and the slow surface waves reach the seismograph station last. Scientists use the time difference to figure out the distance between the seismograph station and the earthquake epicenter. For example, if a seismograph station is located 4,000 km from an earthquake epicenter, primary waves will reach the station about 6 minutes before secondary waves. 7

8 LOCATING EPICENTERS Seismic waves must be recorded at three or more stations to determine an earthquake s epicenter. To locate an epicenter, scientists draw a circle around each station s position on a map, as shown in the figure below. The size of each circle is based on how far each seismograph station is from the epicenter of the earthquake. The point at which the three circles meet is the location of the earthquake epicenter. LAYERS OF THE EARTH Scientists usually describe earthquakes based on their distance from the seismograph. Local earthquakes occur less than 100 km away. Regional events occur from 100 km to 1,400 km from the seismograph. The outermost layer of Earth is called the crust. The crust contains more silicon and aluminum, but less magnesium and iron, than the mantle. The crust and the part of the mantle just beneath it make up the lithosphere). Teleseismic events occur more than 1,400 km away. The lithosphere is broken up into a number of plates. The plates move over the asthenosphere that lies just below them. The thickness of Earth s crust varies. In some mountain regions, it is more than 60 km thick. Under some parts of the ocean, it is only 5 km thick. Earth s crust is generally less dense than the mantle beneath it. 8

9 FIGURE IT OUT The speeds and paths of seismic waves change as they travel through materials with different densities. By studying seismic waves, scientists have concluded that different layers of Earth are made of different materials with different densities. Studying how seismic waves travel through Earth has allowed scientists to map Earth s internal structure without being there. In general, the deeper inside Earth a layer is, the denser it is and more pressure it has. Scientists also discovered that seismic waves do not travel through some regions of Earth. Seismic waves are not detected in the region of Earth called the shadow zone ( ). The shadow zone occurs because secondary waves cannot travel through liquid, so they stop moving when they reach the liquid outer core. Primary waves are bent, but not stopped, by the liquid outer core. From these findings, scientists concluded that the liquid outer core and the mantle are made of different materials with different densities Seismic waves change speed when they travel through different layers of Earth. Seismic waves speed up when they pass through the bottom of the crust and enter the upper mantle. This boundary is called the Mohorovicic discontinuity or Moho. Seismic waves also change speed as they travel through different parts of the mantle. Seismic waves change speed at this boundary because the crust and the upper mantle have different densities. 9

10 For example, waves slow down when they reach the asthenosphere. They speed up again when they move through the more solid part of the mantle below the atmosphere. Earth s core is divided into two regions based on how seismic waves travel through it. Secondary waves cannot travel through the liquid outer core. Primary waves slow down when they reach the outer core, but speed up again when they reach the solid inner core OUTLINE A. Seismic waves waves generated by an earthquake, can move the ground forward and backward, up and down, and side to side. 1. Focus an earthquake s point of energy release 2. Primary waves (P-waves) cause particles in rocks to move back and forth in the same direction that the wave is traveling 3. Secondary waves (S-waves) cause particles in rock to move at right angles to the direction of wave travel 4. Surface waves move rock particles in a backward, rolling motion and a sideways swaying motion 5. The point on the Earth s surface directly above the earthquake focus is called the epicenter. B. The different speeds of seismic waves allow scientists to determine the epicenter. 1. Primary waves move fastest. 2. Secondary waves follow. 3. Surface waves move slowest and arrive at the seismograph station last. 4. Seismograph measures seismic waves a. Consists of a rotating drum of paper and a pendulum with an attached pen. b. The paper record of a seismic event is called a seismogram. C. Earth s structure consists of an inner, mostly iron, solid core surrounded by a mostly iron liquid outer core surrounded by the mantle. 1. The crust is Earth s outer layer, about 5 to 60 km thick. 10

11 ???? 2. A seismic wave s speed and direction change as the wave moves through different layers with densities. a. Density generally increase with depth as pressures increase. b. Shadow zones do not receive seismic waves because the waves are bent or stopped by materials of different density. 3. Changes in seismic wave speed allowed detection of boundaries between Earth s layers. Name the three seismic wave types in order of fastest to slowest. Primary waves, secondary waves, and surface waves EARTHQUAKE ACTIVITY SEIS IT UP Earthquakes are natural events. They provide scientists with information about Earth. Unfortunately, they also do a great deal of damage to property and to people. On average, 10,000 people are killed in earthquakes every year. It is important for scientists to learn as much as they can about earthquakes to help reduce their impact on society Scientists who study earthquakes and seismic waves are seismologists. As you have read, the instrument that is used to record primary, secondary, and surface waves is called a seismograph. Seismologists use records from seismographs to study earthquakes. There are seismograph stations all over the world that help determine where earthquake epicenters are located SEIS THE DAY This is a seismogram, created by a seismograph; which will be read by a seismologist! The height of the lines on a seismogram is a measure of energy the earthquake released. The magnitude of an earthquake is the measure of energy released. The taller the lines on the seismogram, the greater the magnitude of the earthquake 11

12 RICHTER SCALE Richter magnitude scale is used to describe the magnitude of earthquakes. For each increase of 1.0 on the Richter scale, the height of the line on a seismogram is ten times taller. The Richter scale has no upper limit. However, scientists think that a value of about 9.5 would be the maximum strength an earthquake could register. 1 0 For example, the line on a seismogram that shows a magnitude 7.0 earthquake is ten times taller than the line on a seismogram that shows a magnitude 6.0 earthquake 1 TIC-TIC- BOOM! 32 TIMES!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! The difference in the energy released by earthquakes of different magnitudes is even greater. About 32 times as much energy is released for every increase of 1.0 on the Richter scale. An earthquake with a Richter scale magnitude of 8.5 releases about 32 times more energy than an earthquake with a magnitude of 7.5 FREQUENCY Thousands of earthquakes occur on Earth every day. Most are so small that no one is aware of them. Earthquakes with a magnitude of 3.0 or below are generally not noticed and cause no damage. Earthquakes with a magnitude between 3.0 and 4.9 can usually be felt by humans. There are about 55,000 earthquakes each year that are felt, but that cause little or no damage. 12

13 INTENSITY Most of the earthquakes you hear about are large ones that cause great damage. Destructive earthquakes have a magnitude of 5.0 or more on the Richter scale. The earthquakes that cause the most damage and take the most lives have a magnitude of 6.8 or higher. Earthquakes also can be described by the amount of damage they cause. The modified Mercalli intensity scale describes earthquake intensity based on the amount of damage an earthquake does to rock formations, buildings, and other structures at a specific location. The amount of damage an earthquake does depends on its strength, the kind of material at Earth s surface, how structures are designed, and how far a location is from the epicenter of the earthquake. On the Mercalli scale, an intensity-i earthquake would be felt by few people and do no damage. An intensity-iv earthquake would be felt by everyone indoors, and it might do some damage to buildings. Everyone would feel an intensity-ix earthquake, which would cause serious damage to buildings and open cracks in the ground. An intensity-xii earthquake would result in total destruction of structures The magnitude earthquake in Northridge, California, was listed at an intensity of IX because of the damage it caused. LIQUEFACTION TSUNAMIS The intense shaking from an earthquake can cause wet soil to act like a liquid. Liquefaction occurs when wet soil acts more like a liquid during an earthquake. When liquefaction occurs in soil under buildings, the buildings can sink into the soil and collapse Some earthquakes occur under the ocean. These earthquakes cause a sudden movement of the ocean floor, which pushes against the water. The powerful wave that results can travel thousands of kilometers across the ocean in all directions, and it may finally crash into a coast. 13

14 TSUNAMI Tsumamis are seismic sea waves caused by undersea earthquakes. On the open ocean, tsunamis may not be noticed but when they reach land, tsunamis may form a wall of water up to 30 m high. Most tsunamis occur around the Pacific Ocean. The Pacific Tsunami Warning Center in Hawaii monitors undersea earthquakes. People are warned if a tsunami is likely to occur so they have time to leave the danger area. SAFETY RUBBER MOORING Today buildings can be built to resist earthquake damage. In California, some new buildings are supported by flexible moorings made of rubber and steel. The rubber acts like a cushion to absorb the wave motion of an earthquake(up to 8.3 on the Richter scale). In older buildings, steel rods can be installed to make the walls stronger. There are some steps you can take before an earthquake to make your home safer. Move heavy objects from high shelves to lower shelves. Make sure water heaters and gas appliances are held securely in place. New sensors can now be installed on gas lines. These sensors automatically shut off the gas when earthquake vibrations are felt. Seek shelter in a doorway or under a study table or desk, move away from windows or objects that can fall on you 14

15 OUTLINE A. Although earthquakes are natural geologic events, they kill many people and cause a lot of damage. 1. Seismologists scientists who study earthquakes 2. Magnitude measure of energy released by an earthquake; determined by the Richter scale and based on the height of the lines on a seismogram a. The Richter scale has no upper limit. b. Most earthquakes have magnitudes too low to be felt by humans 3.0 to 4.9 on the Richter scale. 3. The modified Mercalli intensity scale describes earthquake intensity based on structural and geologic damage. 4. Liquefaction shaking from an earthquake can make wet soil act like a liquid. 5. Ocean waves caused by earthquakes are called tsunamis. a. Caused when a sudden movement of the ocean floor pushes against the water b. Can travel thousands of kilometers in all directions B. Earthquakes cannot be reliably predicted. 1. Knowing how and where to plan for earthquakes can help prevent death and damage. 2. Buildings can be constructed to withstand seismic vibrations. a. Flexible, circular moorings are being placed under buildings; made of alternating layers of rubber and steel. b. The rubber acts like a cushion to absorb earthquake waves. 3. Homes can be protected by careful placement of heavy objects and securing gas appliances. 4. During an earthquake, crawl under a sturdy table or desk; outdoors; stay away from buildings and power lines. 5. After an earthquake, check for water or gas line damage; leave immediately if a gas smell is present. DISCUSSION QUESTION: What two scales measure earthquake magnitude and intensity? Richter scale measures magnitude; modified Mercalli intensity scale measures intensity 15

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