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What Are Earthquakes? Key Concept: Sudden motions along breaks in Earth’s crust can release energy in the form of seismic waves. Where Earthquakes Happen? • The majority of earthquakes happen near the boundaries of tectonic plates. • Large earthquakes have occurred in the interior of tectonic plates. • Earthquakes happen along faults. Faults at Tectonic Plate Boundaries Divergent Boundaries • Plates pull away from each other. • Tension causes plates to break up into fault blocks. • Some of the fault blocks drop down relative to the others. • Earthquakes happen as the fault block moves. Faults at Tectonic Plate Boundaries • An example of a divergent boundary is a mid-ocean ridge. • There the lithosphere is thin. • Earthquakes that happen at the mid-ocean ridge are shallow. • At divergent boundaries, earthquakes occur at depths less than 20 km below the ocean floor. Earthquakes at convergent Boundaries • When two plates collide with each other. • Two things may happen, 1) Two plates crumple up and make a mountain. 2) One plate subducts under the other. • Compression occurs as the two plates collide. Earthquakes at convergent Boundaries • Compression causes the lithosphere to break up into fault blocks. • These blocks thrust over one another as the plates move. • Two types of earthquakes may occur. • Between the two plates & • Inside the down going plate. Earthquakes at Transform Boundaries • As two plates move past each other, stress causes rocks to shear. • Earthquakes happen along these strike-slip faults as the plates move. • Most transform boundaries occur between oceanic lithosphere. • Earthquakes occur at shallow depths. Fault Zones • An area along a plate boundary, where there are many connected faults is called a fault zone. • At a fault zone can have different depths, length and cut through the lithosphere in different directions. Why Earthquakes Happen • • • 1) As plates move, stress is placed on the edges. In response to stress, rocks deform. Rocks deform in two ways: Plastic deformation: like a piece of clay being molded. 2) Elastic deformation: stretching like a rubber band. • Like a rubber band, plates will break if stretched too far. • The break releases energy as the rocks return to their original form. Elastic Rebound • The sudden return of deformed rock to its original shape is called elastic rebound • Elastic rebound occurs when the stress on a rock becomes so great, that it breaks. Elastic Rebound • This causes rocks on both sides of the fault to slide past each other. • In this sudden motion, stress is released in the form of energy waves. • Energy waves traveling through rock are called seismic waves. • The strength of an earthquake is determined by amount of energy released during elastic rebound. Earthquake Waves • Earthquakes are the result of energy movement as seismic waves. • There are 3 types of seismic waves: 1) P wave 2) S wave 3) Surface wave • Each wave travels thru Earth differently. P Waves • Also called pressure waves. • Are the fastest waves • Also called primary waves, because they are the first to be detected. • P waves can move thru liquids, solids and gasses. S Waves • • • • • Also called shear waves. Second fastest wave. Move side to side. Can not travel through liquids. Called secondary waves because always arrives after P waves. Surface waves • Only move along the Earth’s crust. • Produce motions along the top of the crust. • Since these waves focus on the surface, they cause the most damage. • Two type of surface waves: 1) Move up and down. 2) Move back and forth.