Ch 12-Earthquakes • Objectives • 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 • Describe the instrument used to measure and record earthquakes • Summarize the method scientists use to locate an eipcenter • Describe the scales used to measure the magnitude and intensity of earthquakes • Discuss the relationship between tsunamis and earthquakes • Describe possible effects of major earthquake on buildings • List safety techniques to prevent injury caused by earthquake activity • Identify methods scientists use to forecast earthquake risks
Earthquakes • http://player.discoveryeducation.com/index.cfm?guidAssetId=33A7BE0A-041F-4B73-AF43-56561233E209&blnFromSearch=1&productcode=US • Movement or trembling of the ground that is caused by sudden release of energy when rocks along a fault move • What happens? • Rocks under stress suddenly shift along a fault. What is a fault? • Break in body of rock along which one block slides relative to another • Why do earthquakes happen?
What do you think elastic rebound means? • Sudden return of elastically deformed rock to its undeformed shape • Anatomy of Earthquake • Focus-location w/in Earth along fault at which first motion of earthquake occurs • Epicenter-point on Earth’s surface directly above an earthquake’s starting pt • Earthquakes that have most damage=shallow foci • Seismic Waves • As rocks slip, rocks release energy in form of vibrations, travel in all directions • 2 types-body and surface waves • Concepts in Motion
Seismic Waves • Body Waves-travel through the body of a medium • P waves-travel fastest and 1st , causes particles to move back and forth parallel to direction that wave is traveling. Can travel through solids, liquids, and gases • S waves-shear waves, 2nd fastest, cause particles to move side to side, perpendicular to direction of wave. Only travel through solid material
Surface Waves • Form from motion along shallow fault or from the conversion of energy when P waves and S waves reach Earth’s surface • Slowest, most damage • 2 types-Love and Rayleigh • Love-rock side to side and perpendicular to waves • Rayleigh-ground to move w/ elliptical, rolling motion
Seismic Waves and Earth’s Interior • What are Earth’s internal layers? • What are the mechanical layers? • Lithosphere, Asthenosphere, mesosphere, outer core, inner core • Composition of material determines the speed and direction of the waves • What are shadow zones? • Area on Earth’s surface where no direct seismic waves from particular earthquake can be detected. Why are there shadow zones?
Earthquakes and Plate Tectonics • What causes earthquakes? • Stresses in the Earth’s lithosphere • Earthquakes happen at 3 main tectonic environments • Convergent Oceanic Environment-plates move toward each other and collide • 2 oceanic plates or oceanic and continental • Divergent Oceanic Environment-plates moving away from each other • Mid ocean ridges • Continental Environment-2 continental plates converge, diverge or move horizontally in opposite directions • Stress may cause frequent earthquakes or mts to form
Fault Zones • What is a fault zone? Region of numerous, closely spaced faults • Form at plate boundaries. Why? • Intense stress that results when plates separate • Not all earthquakes occur from movement along plate boundaries • New Madrid, Missouri • Mississippi River region
Sec 2-Studying Earthquakes • Seismology-study of earthquakes and seismic waves • What instrument is used to measure and record earthquakes? • Seismograph-instrument that records vibrations in ground, record motion by tracing wave shaped lines on paper or by translating the motion into electronic signals • What waves are recorded first?
Locating an Epicenter • Analyze arrival times of P waves and S waves Longer lag time between waves, the farther the earthquake is • Use computers- to perform triangulations based on data from seismograph stations • http://glencoe.com/sec/science/earthscience/2007/concept_motion/animated_art/PandSwaves19_9.swf • Earthquake measurement • Study magnitude and intensity and amount of damage • Magnitude-measure of strength of an earthquake • Measure the amount of ground motion caused by earthquakes • Richter scale • Moment magnitude-measurement of earthquake strength based on size of area of fault that moves • More accurate for large earthquakes
Intensity-amount of damage caused by an earthquake • Mercalli scale-provides description of effects of each earthquake • Intensity depends on earthquake’s magnitude, distance between epicenter and affected area, local geology, earthquake’s duration and human infrastructure
Sec 3-Earthquakes and Society • http://player.discoveryeducation.com/index.cfm?guidAssetId=A403BF3E-BB17-4BD6-BF43-B2E3BA6CDBAC&blnFromSearch=1&productcode=US • http://glencoe.com/sec/science/earthscience/2007/concept_motion/animated_art/Tsunami19_23.swf • What is a tsunamis? • Giant ocean wave that forms after volcanic eruption, submarine earthquake, or landslide • Waves increase in height as they near shore • Destruction of earthquakes • Tall buildings, buildings build on loose soil and rock • How to prevent injury by earthquake activity • Be prepared, supply of food and water, flashlights, batteries, portable radio • Stay calm, move to safer position, stay away from windows and heavy furniture • Be cautious, check for fire or other hazards, wear shoes, avoid downed power lines
Earthquake Warnings and Forecasts • Study past earthquakes, nothing reliable to predict when and where • Studying Earth’s crust to detect changes • Seismic gaps-area along fault where relatively few earthquakes have occurred recently • Foreshocks-little earthquakes that precede actual earthquakes • Changes in rock • Use sensors to detect slight tilting of ground and to identify the strain and cracks in rocks caused by stress that builds up in fault zones • http://player.discoveryeducation.com/index.cfm?guidAssetId=4305A485-5458-4853-ACF6-18FE58ECCD0B&blnFromSearch=1&productcode=US