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Cornell Notes Earthquakes Poseidon Earthquakes Worldwide Should we care? Loss of Life and Economic Costs Northridge, California 1994 E:\HHS_2010\METROPOLI TAN.WMV Earthquakes Since 1900 Montana Earthquakes What is an Earthquake? Seismic Waves http://wwwrohan.sdsu.edu/~rmellors/lab8/l8awa http://www.geo.mtu.edu/UPSeis/wa v2.htm ves.html What happens to seismic wave in Earth’s interior? Shadow Zones Locating & Measuring Earthquakes Seismograph Seismograms Seismographs Seismographs Seismograph station at Cornell University A modern seismograph station usually has three seismographs. The first one records up-and-down motions, the second one records side-to-side motions in a north/south direction, and the third one records side-to-side motions in an east/west direction. Measuring quakes. Locating an Earthquake Epicenter Step 1- Analyzing 3 Seismograms Step 2- Read Time-Travel Graph Step 3 – Triangulate to find Epicenter. Measuring Earthquakes – Magnitude is a quantitative measurement that estimates the energy released. • Richter Scale • Moment Magnitude – Intensity is a qualitative measurement that evaluates the degree of earthquake shaking perceived by individuals • Modified Mercalli Scale Richter Scale (Magnitude) • Developed by Charles F. Richter in 1935. • Measurement of ground movements. • Each increase of one whole number in Richter magnitude is a 10 x increase in shaking (32 x increase in energy release). For example, a magnitude 6 earthquake is about 10 times more powerful than a magnitude 5 earthquake. A magnitude 7 earthquake is more than 100 times (10x 10) more powerful than a magnitude 5 earthquake. Magnitude Magnitude is a measure of the amount of energy released in an earthquake. Reading a Nomogram Richter Scale (Magnitude) Richter Scale Description Less than 3.5 Not generally felt by humans 5.5 - 6.0 Slight damage to structures 7.0 – 7.9 Serious damage 8.0 and greater Total destruction of nearby communities Moment Magnitude (Magnitude) Moment Magnitude is the current method used for measuring earthquakes (especially large ones) and uses three measurements: 1. Average area of slip along the fault; 2. Distance traveled; and 3. Considers the strength of the rock. Example Magnitudes Location Date Richter Moment Magnitude Magnitude San Francisco 1906 8.3 7.9 Alaska 1964 8.5 9.2 Alaska released at least twice as much energy because it involved greater movements along a much larger fault plane. Mercalli Scale (Intensity) • Mercalli Scale (Giuseppi Mercalli 1902) measures the intensity of a quake by using observed affects of ground movement. Earthquake Risk Where in the United States is the risk for earthquakes the greatest? Alaska and California are high risks since they are found near plate boundaries. The middle of the country by Missouri is also at a higher risk, because many faults are buried deep beneath sediments deposited by the Mississippi, Missouri, and Ohio rivers. Damage from Earthquakes • • • • • Ground shaking Foundation failure Aftershocks Fire Tsunamis Ground Shaking Ground shaking is produced by the waves set in motion by an earthquake’s sudden release of energy. Some of the ground vibrations are up-and-down, but the largest are side-to-side motions. Most buildings can withstand fairly violent up-and-down shaking; however, few buildings can survive violent side-to-side shaking, and as a result many buildings collapse. Foundation Failure As a result of severe ground shaking, soils under buildings may settle or even liquefy. Liquefaction occurs when loose soil temporarily takes on some of the properties of a liquid. A building located on soil that settles is no longer safely supported and may collapse. For example, buildings located on bog muds or soil landfill can suffer severe damage or can collapse because of foundation failure. Foundation Failure Liquefaction caused the building in the front to tip nearly 22°. The arrow points to a building that is tipped almost 70°. Aftershocks A large earthquake may be followed by a smaller earthquakes. Aftershocks are a series of smaller earthquakes originating close to the focus of the large earthquake. Aftershocks can be as great as 1000 per day, though the frequency usually diminishes quickly over time. Fire One of the most damaging effects of an earthquake may be the fires caused by the rupturing of gas lines by ground movement. Tsunami Underwater earthquakes and landslides sometimes cause huge ocean waves called tsunami. Tsunami can travel very quickly across large expanses of water. Its speed depends on the depth of the water. Tsunami When a tsunami reaches shallower water near a shoreline, it will slow down and increase dramatically in height. A tsunami warning system is now in place for regions in the Pacific Ocean. Scientists monitor wave activity at various stations throughout the Pacific. Tsunami Animation Elastic Rebound Theory Stress is released and crust is occassionally cracked called the elastic-rebound theory normally occuring at faults. Stresses: • Plate Tectonics • Thermal expansion and contraction. • Gravitational attraction • Mineral phases Tensional stress pulls rock away Normal faults occur when rock from adjacent rock thinning along one side of the fault slide crust. down relative to the other. Divergent Boundary Compressional stress Reverse faults occur when pushes rock together causing. rocks are pushed together and one side of the fault moves up. Convergent Boundary Rocky Mountain Front • Shearing stress pushes Strike-slip faults occur when rock in two different directions blocks of rock on each side of the fault move in opposite adjacent to one another. directions. TRANSFORM PLATE BOUNDARY