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Earthquakes Table of Contents Forces in Earth’s Crust Earthquakes and Seismic Waves Monitoring Earthquakes Earthquake Safety Earthquakes - Forces in Earth’s Crust Forces in the Earth’s Crust • The movement of Earth’s plates creates enormous forces that squeeze or pull the rock in the crust. – Stress – force that acts on rock to change its shape or volume • Three different kinds of stress can occur in the crust: tension, compression, and shearing. – work over millions of years to change the shape and volume of rock Earthquakes - Forces in Earth’s Crust Types of Stress • Tension – pulls on the crust, stretching rock so that it becomes thinner in the middle. Earthquakes - Forces in Earth’s Crust Types of Stress • Compression – stress force that squeezes rock until it folds or breaks. Earthquakes - Forces in Earth’s Crust Types of Stress • Shearing – stress that pushes a mass of rock in two opposite directions Earthquakes - Forces in Earth’s Crust Kinds of Faults • Most faults occur along plate boundaries, where the forces of plate motion push or pull the crust so much that the crust breaks. – Fault – a break in the rock of the crust • There are three main types of faults: normal faults, reverse faults, and strike-slip faults. Earthquakes - Forces in Earth’s Crust Kinds of Faults • Tension in Earth’s crust pulls rock apart, causing normal faults. • Normal fault – hanging wall falls down below foot wall. Ex: Rio Grande rift valley, New Mexico 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. • Reverse fault – hanging wall move up above foot wall Ex: Northern Rocky Mountains 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. • Strike-slip fault – two sides slide past each other – Ex: San Andreas Fault 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 Folding Earth’s Crust • Anticline – upward fold in rock formed by compression of Earth’s crust. • Syncline – downward fold in rock formed by compression in Earth’s crust. • Folding produced some of the world’s largest mountain ranges – Ex: Himalayas in Asia and Alps in Europe Earthquakes - Forces in Earth’s Crust Stretching Earth’s Crust • Fault-Block Mountains – form from two normal faults, running parallel to each other – Two hanging walls fall leaving a raised area in between. Earthquakes - Forces in Earth’s Crust Uplifting Earth’s Crust • Plateau – large area of flat land elevated high above sea level. – Form when forces in Earth’s crust push up a large, flat block of rock – Ex. Colorado Plateau Earthquakes - Forces in Earth’s Crust Links on Faults • Click the SciLinks button for links on faults. Earthquakes - Earthquakes and Seismic Waves Earthquakes and Seismic Activity • Earthquake – shaking that results from the movement of rock beneath Earth’s surface. – Occur all the time, usually too small to notice – Most begin in the lithosphere (100 km below surface) • Focus – point beneath Earth’s surface where rock breaks under stress and causes an earthquake. • Epicenter – point on Earth’s surface directly above an earthquake’s focus. Earthquakes - Earthquakes and Seismic Waves Types of Seismic Waves • Seismic waves – carry energy from an earthquake away from the focus, throughout the Earth Earthquakes - Earthquakes and Seismic Waves Types of Seismic Waves • There are 3 main categories of seismic waves: P waves, S waves, and surface waves. • P waves - seismic waves that compress and expand the ground like an accordion. Earthquakes - Earthquakes and Seismic Waves Types of Seismic Waves • S waves – seismic waves that vibrate from side to side as well as up and down. Earthquakes - Earthquakes and Seismic Waves 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 • Click the Active Art button to open a browser window and access Active Art about seismic waves. Earthquakes - Earthquakes and Seismic Waves Measuring Earthquakes • There are three commonly used scales of measuring earthquakes: – Mercalli scale, Richter scale, and the moment magnitude scale. Earthquakes - Earthquakes and Seismic Waves Measuring Earthquakes • Mercalli scale – developed to rate earthquakes according to the amount of damage at a given place. Earthquakes - Earthquakes and Seismic Waves Measuring Earthquakes • Richter scale – rates an earthquake’s magnitude based on the size of its seismic waves. • Magnitude – measure of an earthquake’s strength based on seismic waves and movement along faults. • Seismograph – device that records ground movements caused by seismic waves Earthquakes - Earthquakes and Seismic Waves Measuring Earthquakes • Moment magnitude scale – rates earthquakes by estimating the total energy released by an earthquake. – Determined by studying data from seismographs • Each one-point increase in magnitude represents the release of roughly 32 times more energy. Earthquakes - Earthquakes and Seismic Waves Measuring Earthquakes • Magnitude below 3 = Scarcely noticed • Between 3-5 = little damage • Between 5-6 = Moderate damage • Above 6 = Great damage • Largest Earthquakes ever to be recorded had a magnitude measuring above 9 – Chile 1960 – Alaska 1964 Earthquakes It’s Your Turn!! Create an Earthquake!! 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 Locating the Epicenter • Seismic waves travel at different speeds. – P waves travel faster than S waves – To tell how far the epicenter is from the seismograph, scientists measure the difference between the arrival times of the P waves and S waves. – The longer the time difference, the further the epicenter is from the seismograph station. Earthquakes Locating the Epicenter • For scientists to figure out the location of the epicenter they need P and S wave information from at least three different locations – Once the distance is calculated for each station a circle is drawn on a map (with a radius = the distance calculated) around each of the three seismograph stations – The place where the three circles intercept should be the earthquake’s epicenter Earthquakes Earthquakes - Earthquakes and Seismic Waves Seismic Waves in the Earth • Click the Video button to watch a movie about seismic waves in the earth. 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 Reading a Seismogram • Seismogram – record of an earthquake’s seismic waves produced by a seismograph. – The height of the jagged lines are greater for a more severe earthquake. Earthquakes - Monitoring Earthquakes Instruments That Monitor Faults • Geologists have developed instruments to measure: – changes in elevation, tilting of the land surface, and ground movements along faults. Earthquakes Instruments That Monitor Faults • Tiltmeter – measures tilting or raising of the ground • Creep meter – uses a wire stretched across a fault to measure horizontal movement of the ground • Laser-ranging device – uses a laser beam to detect horizontal fault movements • GPS – (Global Positioning System) measure tiny movements of markers set up on the opposite sides of a fault. Earthquakes Using Seismograph Data • Seismographs and fault-monitoring devices provide data used to map faults and detect changes along faults. • Geologists are also trying to use these data to develop a method of predicting earthquakes. Earthquakes Monitoring Changes Along Faults • How rocks move along a fault depends on how much friction there is between the sides of the fault • Friction – force that opposes the motion of one surface as it moves across another surface. – Ex: San Andreas Fault, California Earthquakes - Monitoring Earthquakes Monitoring Changes Along Faults • 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 - Monitoring Earthquakes Links on Earthquake Measurement • Click the SciLinks button for links on earthquake measurement. Earthquakes - Earthquake Safety Earthquake Risk • Geologists can determine earthquake risk by locating where faults are active and where past earthquakes have occurred. Earthquakes How Earthquakes Cause Damage • Causes of earthquake damage include: – shaking, liquefaction, aftershocks, and tsunamis. • Shaking - triggers landslides and avalanches – Destroys buildings and bridges, topples utility poles, and fractures gas and water mains • Liquefaction – violent movements suddenly turn loose soil into liquid mud. Earthquakes How Earthquakes Cause Damage • Aftershock – earthquake that occurs after a larger earthquake in the same area. – Can sometimes cause the most damage • Tsunami – large wave produced by an earthquake on the ocean floor. – Ex: December 26, 2004 – earthquake on the seafloor of the Indian Ocean caused a tsunami to hit the shores of Indonesia killing 200,000 people Earthquakes - Earthquake Safety How Earthquakes Cause Damage • Tsunamis spread out from an earthquake's epicenter and speeds across the ocean. – The waves are amplified as they approach the shore. Earthquakes Steps to Earthquake Safety • The best way to protect yourself during an earthquake is to drop, cover, and hold. • Before an earthquake occurs be prepared – Keep a supply of canned food, bottled water, flashlights, batteries and a portable radio Earthquakes - Earthquake Safety Designing Safer Buildings • To reduce earthquake damage, new buildings must be made stronger and more flexible. Earthquakes Designing Safer Buildings • Base-isolated building – buildings mounted on bearings designed to absorb the energy of an earthquake. – rests on shock-absorbing rubber pads or springs – Flexible joints can be installed in gas and water lines to keep them from breaking. Earthquakes - Earthquake Safety Earthquake Damage • Click the Video button to watch a movie about earthquake damage. Earthquakes - Earthquake Safety More on Earthquake Risk • Click the PHSchool.com button for an activity about earthquake risk.