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Lessons 1-4 Lesson 1: What are earthquakes and where do they occur Where do Earthquakes Occur? • Earthquakes can occur near the Earth’s surface or far below the surface. • Most earthquakes occur at plate boundaries, but some happen at faults located in the middle of tectonic plates. http://www.dosecc.org/html/volcanic_monitoring.ht ml PLATE BOUNDARIES Earthquake Locations Around the World What Causes Earthquakes? • Earthquakes are caused by movement along faults. • When stress is placed on rocks it deforms, or changes. • This is called Elastic Deformation (remember faults and folds, tension and compression) • Rock is stretched and bent until it can no longer take the stress. • When enough stress builds up in the rock it, it slips and energy is released. • The rock then returns to its original shape. This is called Elastic Rebound. Think of a rubber band, you can only stretch it so far until it breaks and return to its original shape. • This energy is felt as an earthquake. Elastic ELASTIC REBOUND Earthquakes in Alabama?? • Do we live near a plate boundary? • Do we live near a fault? The New Madrid Fault • If there is no plate boundary in the middle of the United States, why did these earthquakes take place? • Geologists are beginning to understand the answer. The New Madrid Fault Zone is part of an ancient plate boundary. In this area, the North American Plate tried to form a divergent plate boundary about 500 million years ago. The splitting stopped before new plates could form. The faults in the New Madrid Zone are remnants of this old event. Earthquakes occur because the North American Plate is still "settling down". The faults in the New Madrid Zone do not reach the Earth’s surface. They are buried beneath thousands of feet of rock and sediment deposited by the Mississippi River. Geologists have located them by looking at the patterns of earthquakes in the zone. Several of the largest earthquakes ever recorded in the United States occurred in the Midwest, far from any plate boundary. These earthquakes took place in an area called the New Madrid Fault Zone, named after the town of New Madrid, Missouri. • Over a three-month period in the winter of 1811 to 1812, the New Madrid Fault Zone was struck by three huge earthquakes estimated to be greater than magnitude 8.0 • The New Madrid Fault is what causes Alabama to experience earthquakes. • The map shows the earthquakes recorded in Alabama since 1886. The last earthquake to occur in Alabama was on July 27, 2007. It measured a 2.6 on the Richter scale. This was not a very strong earthquake, but it was recorded by a seismograph, an instrument used to measure earthquakes.