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Earthquakes • An Earthquake is any vibrating, shaking or rapid motion of the Earth’s crust. • Most Earthquakes occur at zones of weakness or a break in bedrock known as a fault. • When the stress built up in the rock is greater than what the rock can resist, the crust shifts. • A sudden release of energy radiates in all directions as vibrations. • The focus is the place underground where the actual break occurs. • The epicenter is the surface directly above the focus. • After the earthquake waves move away from the epicenter. • When the waves reach a location the earth seems to shake. • The further a location is from the epicenter the weaker the waves are when they reach that location. San Andreas Fault Video(45MIN) https://www.youtube.com/watch?v=b5f09mh-gIM San Andreas Fault Video (4.5 min) https://www.youtube.com/watch?v=_YLjIvJXhpg P (primary waves)- travel through liquids and solids. P waves move faster than S waves. P waves cause particles to move in the direction of the wave. S (secondary waves)- travel only through solids. S waves are slower than P waves. S waves cause particles to move at right angles to the direction of the wave. L (Land) Waves are caused when P Waves and S Waves Combine to cause a circular motion of the rock particles. L waves cause the most destruction during an Earthquake • The characteristics waves have in different conditions allows geologists to infer properties of the Earth’s interior. • http://sunshine.chpc.utah.edu/labs/s eismic/index.htm Measuring Earthquakes There are two different ways to measure earthquakes. • Magnitude scales- based on seismometer readings. • Intensity scales- based on reports of damage. Magnitude Scales • Seismometer- an instrument designed to detect and measure seismic waves. A pen is attached to a heavy mass on a spring. When the ground shakes, the mass remains stationary and the paper moves under it. • Seismograph- a seismometer that makes a recording of seismic waves. • Each magnitude is 10 times greater than the previous number. • A magnitude 5 earthquake is 10 times stronger than a magnitude 4 earthquake. A magnitude 6 is 100 times stronger than a magnitude 4. Intensity scale• The Mercalli scale is based on the damage to structures and accounts of witnesses. • 1-2 - may not be felt • 5-6 - some dishes and windows break • 7-8 - hard to stand, collapse of some structures. • 10-11 - Visible waves on ground, total Since we know how fast P-waves and S-waves travel, we can use this information to calculate the distance to an epicenter Damage from Earthquakes • Shaking- buildings may be damaged or collapse. • Movement of crust- damages roads and railroads. • Tsunamis- earthquakes in oceans cause large waves which may swamp coastal areas thousands of miles away. Human Dangers • Fires- broken gas pipes often ignite. • Broken water mains make it hard to fight fires. • Landslides often occur in soft ground. Areas of high risk have stricter building codes to help keep structures from falling apart during quakes. Locations of Earthquakes and Volcanoes • Most all earthquakes and volcanoes occur in a belts of crustal activity. • The largest belt surrounds the Pacific ocean which is an old ocean plate and is subducting all around its borders. It is called the “Ring of Fire”. Damage from Volcanoes • Ash- may cover large areas, very difficult to deal with. Machines may not work, clogs air filters. • Lava- smaller areas covered but nearly unstoppable. • Toxic gasses- hot, fast-moving gasses may suffocate people.