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Earthquakes Earthquakes - The movement of the ground, caused by waves of energy released as rocks move along faults Fault – a large fracture in rocks, from several meters to many kilometers long, where rocks not only crack but also move along either side of the break Types of Stresses; tension, compression, & shear Types of Faults; Normal, Reverse, Strike-Slip Earthquake Mechanics (cont.) Epicenter – the point on Earth’s surface directly above the earthquake’s focus Focus – the point in Earth’s interior or where movement releases energy to cause the earthquake Seismic waves: Waves of vibration caused by the energy released during an earthquake. • Types of waves produced: Surface and body waves • Seismograph: is the instrument used to detect, measure, and record seismic waves produced by earthquakes. • Magnitude - A measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. • Seismogram: the recording of an earthquake made by a seismograph Body Waves P waves – Primary Wave: compression & expansion - push-pull seismic waves • sound waves (too low to be heard) • fastest (~8 to 9 km/sec) depending on material • first to be recorded at a seismograph • causes rock particles to vibrate in the same direction the wave is traveling • denser the material, the faster the P-wave travels http://www.youtube.com/watch?v=2rYjlVPU9U4 Body Waves S waves – Secondary Wave: • shearing (lateral; side to side) seismic waves • travels through solid only (3 to 5 km/sec) - NOT liquid or gasses • second to reach and be recorded at a seismograph • causes the rock particles to vibrate at right angles to the direction of travel • http://www.youtube.com/watch?v=gl4FvHKzAlU Surface Waves Surface Waves – waves of energy, released during an earthquake, that reach Earth’s surface and travel outward from the epicenter in all directions ON THE SURFACE ONLY Travels only through solid Surface Waves L wave – Love wave: • shearing (lateral; side to side) seismic waves • speed is 2 to 6 km/sec • has the greatest height of the ‘waves’ recorded on a seismogram • has a horizontal (side to side) motion that is perpendicular to the direction of travel • causes the most destruction • (named after A.E.H. Love, the English mathematician who discovered it.) Surface Waves Rayleigh wave (Surface wave): • Slowest of the waves 1 to 5 km/sec • Most complex of earthquake waves; elliptical motion at the Earth's surface. • They are usually felt as a rolling or rocking motion • In the case of major earthquakes, can be seen as they approach • Often the largest and most destructive, of the wave types caused by an earthquake. • Named after Lord Rayleigh, the English physicist who predicted its existence. Surface Waves • Love waves – L-waves • Rayleigh waves Earthquake Machines Seismograph – instrument that detects & records earthquake seismic waves Seismogram – the recording data sheet of an earthquake made by a seismograph http://www.leo.lehigh.edu/projects/seismic/graphics/012601_SIndia.jpg Earthquake Intensity • Modified Mercalli Scale – intensity scale (modified for North American conditions); composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction. http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html • Seismic moment - measures the earthquake’s strength, or the energy released based the amount of displacement along a fault. Earthquake Intensity • Richter Scale – measures the strength of an earthquake – increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram – or approximately a 32-fold increase in the energy released. Therefore a magnitude 6.7 earthquake releases over 900 times (32 times 32) the energy of a 4.7 earthquake. Creation of a Tsunami Essential Questions -- Earthquakes • Name and describe the 3 different seismic waves • Explain how the structure of the earth’s interior affects seismic waves • Describe how seismographs are used to determine the epicenter of an earthquake • List the different scales used to measure the magnitude and intensity of an earthquake • Discuss the relationship between earthquakes and tsunamis • Identify the three types of stress and the resultant features (faults/folds) associated with each.