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3d - Students know why and how earthquakes occur and the scales used to measure their intensity and magnitude. • We have learned that Earth is constantly changing. • The surface of Earth changes everyday due to dynamic processes, such as weather and plate tectonics. • Remember…Plate Tectonics states, “Earth’s crust is broken into plates that move in different directions and at different rates”. • The surface of Earth is in constant motion because of forces inside the planet (heat – convection) • These forces cause sections (plates) of Earth’s surface to move • This movement puts stress on the rocks near the plate edges •To relieve this stress – rocks tend to bend, compress, or stretch • Earthquake – vibrations produced when rock snaps and breaks under force Objects can only withstand so much force: - A stick will snap when bent too far. - A rubber band will break if pulled too tight • Elastic limit – maximum force an object can withstand before breaking • Rocks have an elastic limit. • Rocks will actually bend until they reach their elastic limit. • When rocks bend too far, they will break. • Fault - break or crack in the rock of Earth’s crust. • Faults form when Earth’s plates move and place stress on one another….. Eventually forcing the crust to snap. • Earthquakes occur when rocks move past each other along a fault • Rocks have jagged edges • The jagged edges along a fault get stuck • Stress builds up in the stuck rock as the plates try to move • The rocks bend until they reach their elastic limit and break • The rocks snap back to their original position (elastic rebound) • The breaking of the rock creates an earthquake Stress and Strain • Stress is the forces per unit area acting on a Material. 3 kinds of stress (forces) that act on Earth’s rocks: 1) • 2) • 3) • Compressional – pressing together Decreases volume of material Tensional – pulls apart Extension of crust Shear – sliding past Causes rock to twist •Fractures form when stress exceeds the strength of the rocks involved. •The surface along which the movement takes places is called the fault plane. Tensional Strain Shear Strain • The three types of stress create the 3 main types of faults. •Foot wall – block of rock below the fault line (imagine being able to walk on it) •Hanging wall – block of rock above the fault line (imagine being able to hang from it) 1) • • • Reverse faults caused by compression Hanging wall moves up shortens the crust 2) • • • Normal faults caused by tension Hanging wall moves down lengthens the crust 3) Strike-slip • caused by shear • horizontal offset Reverse Fault If hanging wall is above the footwall it is a reverse fault! Normal Fault The red line marks equivalent layers on opposite side of the fault. Since the hanging wall dropped relative to the footwall, this is clearly a normal fault. Imperial Valley, California October 15, 1979. Furrows in a plowed field were offset about 11 inches by a magnitude 6.5 earthquake. Offset river system on the San Andreas!