Lecture 11 - Chapter 11 Earthquakes What is an earthquake?...

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Chapter 11 Earthquakes What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy released radiates in all directions from its source, the focus Energy is in the form of waves Sensitive instruments ( seismographs ) around the world record the event Earthquakes and faults Movements that produce earthquakes are usually associated with large fractures in Earth’s crust called faults Most of the motion along faults can be explained by the plate tectonics theory Vast majority of faults are inactive and do not generates earthquakes at all ! What is a mechanism of an earthquake? Elastic rebound (fig.11.5 text-book) Mechanism for earthquakes was first explained by H.F. Reid Rocks on both sides of an existing fault are deformed by tectonic forces Rocks bend and store elastic energy Frictional resistance holding the rocks together is overcome and Slippage at the weakest point (the focus) occurs Vibrations (earthquakes) occur as the deformed rock “springs back” to its original shape ( elastic rebound ) Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome Earthquake – before and after Foreshocks and aftershocks Small earthquakes , called foreshocks , often precede a major earthquake by days or, in some cases, by as much as several years Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks Monitoring of the foreshocks has been used as a means of predicting forthcoming major earthquakes, with mixed success. San Andreas: An active earthquake zone San Andreas is the most studied fault system in the world Displacement occurs along discrete segments 100 to 200 kilometers long Each small segment behaves somewhat differently from the others. Some portions exhibit slow, gradual displacement known as fault creep Other segments regularly slip producing small earthquakes
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Displacements along the San Andreas fault The fault is exhibits alternating periods of locked behavior, followed by sudden slip-page and release the strain. The process which described as stick-slip motion Still other segments store elastic energy for hundreds of years before rupturing in great earthquakes Great earthquakes should occur about every 50 to 200 years along these sections
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Lecture 11 - Chapter 11 Earthquakes What is an earthquake?...

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