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Earthquakes Types of Seismic Waves Seismic waves carry energy from an earthquake away from the focus, through Earth’s interior, and across the surface. -Earthquakes . *Focus: Point beneath the earth’s surface where the earth moves *Epicenter: The point on the surface directly above the focus. It is where the strength of the earthquake is strongest. Earthquakes Types of Seismic Waves P waves are seismic waves that compress and expand the ground like an accordion. S waves are seismic waves that vibrate from side to side as well as up and down. Earthquakes - Types of Seismic Waves Surface waves move more slowly than P waves and S waves, but they produce the most severe ground movements. Earthquakes - Earthquakes and Seismic Waves Seismic Waves Activity Phschool.com Webcode: cfp-1022 http://www.phschool.com/webcodes10/index.cfm?wcprefix=c fp&wcsuffix=1022&area=view&x=8&y=7ss Earthquakes - Forces in Earth’s Crust Types of Stress The stress force called tension pulls on the crust, stretching rock so that it becomes thinner in the middle. Earthquakes - Forces in Earth’s Crust Types of Stress The stress force called compression squeezes rock until it folds or breaks. Earthquakes - Forces in Earth’s Crust Types of Stress Stress that pushes a mass of rock in two opposite directions is called shearing. Earthquakes - Forces in Earth’s Crust Kinds of Faults Tension in Earth’s crust pulls rock apart, causing normal faults. Earthquakes - Forces in Earth’s Crust Kinds of Faults A reverse fault has the same structure as a normal fault, but the blocks move in the opposite direction. Earthquakes - Forces in Earth’s Crust Kinds of Faults In a strike-slip fault, the rocks on either side of the fault slip past each other sideways, with little up and down motion. Earthquakes - Forces in Earth’s Crust Changing Earth’s Surface Over millions of years, the forces of plate movement can change a flat plain into landforms such as anticlines and synclines, folded mountains, fault-block mountains, and plateaus. Earthquakes - Forces in Earth’s Crust Changing Earth’s Surface Over millions of years, the forces of plate movement can change a flat plain into landforms such as anticlines and synclines, folded mountains, fault-block mountains, and plateaus. Earthquakes - Forces in Earth’s Crust Building Vocabulary A definition states the meaning of a word or phrase. As you read, write a definition of each Key Term in your own words. Key Terms: stress hanging wall tension footwall strike-slip fault compression anticline shearing syncline normal fault plateau reverse fault Examples: Stress is aofforce on rocka to change itsisshape The block rock that that acts lies above normal fault or volume. called the hanging wall. The force on the crust, The stress rock that liescalled belowtension is calledpulls the footwall. stretching rock so that it becomes thinner in the In a strike-slip fault, the rocks on either side of the middle. fault slip past each other sideways, with little up or The stress force called compression squeezes rock down motion. until it folds or breaks. A fold inthat rock that bends upward into is an Stress pushes a mass of rock in an twoarch opposite anticline. directions is called shearing. A fold in in rock that bends downward to form a valley is Tension Earth’s crust pulls rock apart, causing a syncline. normal faults. A plateau a large of flatstructure land elevated high reverse is fault has area the same as a normal above seathe level. fault, but blocks move in the opposite direction. Earthquakes - Earthquakes and Seismic Waves Measuring Earthquakes The Mercalli scale was developed to rate earthquakes according to the amount of damage at a given place. Earthquakes Richter Scale The Richter Scale for measures the 'size' or 'strength' of an earthquake. This scale is quantitative and based on the amount of energy released by an earthquake. The energy of a quake is a function of both the amplitude and the duration of a single wave. The seismogram below shows waves with a wide range of amplitude and duration. This would be magnitude 8 quake! Earthquakes - Earthquakes and Seismic Waves Earthquakes - Earthquakes and Seismic Waves Seismic Wave Speeds Seismographs at five observation stations recorded the arrival times of the P and S waves produced by an earthquake. These data are shown in the graph. Earthquakes - Earthquakes and Seismic Waves Seismic Wave Speeds Reading Graphs: What variable is shown on the x-axis of the graph? The yaxis? X-axis––distance from the epicenter; y-axis––arrival time. Earthquakes - Earthquakes and Seismic Waves Seismic Wave Speeds Reading Graphs: How long did it take the S waves to travel 2,000 km? 7 minutes Earthquakes - Earthquakes and Seismic Waves Seismic Wave Speeds Estimating: How long did it take the P waves to travel 2,000 km? 4 minutes Earthquakes - Earthquakes and Seismic Waves Seismic Wave Speeds Calculating: What is the difference in the arrival times of the P waves and the S waves at 2,000 km? At 4,000 km? 2,000 = 3.5 minutes 4,000 = 4.5 minutes Earthquakes - Earthquakes and Seismic Waves Locating the Epicenter Geologists use seismic waves to locate an earthquake’s epicenter. Earthquakes - Monitoring Earthquakes The Modern Seismograph Seismic waves cause the seismograph’s drum to vibrate. But the suspended weight with the pen attached moves very little. Therefore, the pen stays in place and records the drum’s vibrations. Earthquakes - Monitoring Earthquakes Instruments That Monitor Faults In trying to predict earthquakes, geologists have developed instruments to measure changes in elevation, tilting of the land surface, and ground movements along faults. Earthquakes - Monitoring Earthquakes Using Seismographic Data The map shows the probability of a strong earthquake along the San Andreas fault. A high percent probability means that a quake is more likely to occur. Earthquakes - Earthquake Safety Earthquake Risk Geologists can determine earthquake risk by locating where faults are active and where past earthquakes have occurred. Earthquakes - Earthquake Safety How Earthquakes Cause Damage A tsunami spreads out from an earthquake's epicenter and speeds across the ocean. Earthquakes - Earthquake Safety Designing Safer Buildings To reduce earthquake damage, new buildings must be made stronger and more flexible. Earthquakes - Earthquake Safety Asking Questions Before you read, preview the red headings and ask a what, how, or where question for each heading. As you read, write answers to your questions. Question Answer Where is the quake risk highest? Earthquake risk is the highest along faults and where past earthquakes have occurred. How do earthquakes cause damage? Earthquake damage occurs as a result of shaking, liquefaction, aftershocks, and tsunamis. How can you stay safe during an earthquake? The best way to stay safe during an earthquake is to drop, cover, and hold. What makes buildings safe from earthquakes? Buildings can be made safer by being built stronger and with greater flexibility.