Why do Earthquakes Happen?

Many people associate earthquakes with destruction caused by falling buildings or by the creation of a tsunami. While earthquakes may be associated with destruction in the time frame of human activity, in the evolution of the Earth they signal the geological forces that build our mountains and create our oceans. In many ways, earthquakes are one of nature's reminders that we are living on the thin outer crust of a planet whose interior is still cooling. Earth's rigid outer surface is broken into what geologists call plates. In general, earthquakes occur when plates move under, over, or slide past each other. The plates are moving in millimeters per year--about as fast as your fingernails grow.


  • Earth’s rigid outer surface is broken into what geologists call plates
  • The locations of earthquakes provide evidence for the boundaries of these plates
  • In general, earthquakes occur when plates move under, over, or slide past each other


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An asperity is an area on a fault that is stuck or locked. Scientists study areas along long fault zones that have not had earthquakes in a long time in order to determine where the next earthquake may occur. As long faults move, all areas of it will, at some point, become "unstuck" causing an earthquake relative to the the size of the asperity that finally breaks.

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View looking into a fault zone with a single asperity. Regional right lateral strain puts stress on the fault zone. A single asperity resists movement of the green line which deforms before finally rupturing.

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This demonstration, squeezing uncooked spaghetti noodles in a wood template set in a vise, effectively shows how asperities (stuck patches) on a fault rupture at different times.

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Video lecture about elastic rebound and brittle material in the crust using a yardstick as a mechanical analog. This demonstrates elasticity, brittle fracture, and why it is difficult to predict earthquakes.

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