Tectonics and Landforms

Plate Boundaries and Earthquakes

An earthquake is the shaking of the Earth as a result of crustal movement. Most earthquakes occur at the boundaries where plates meet. Submarine earthquakes create seismic ocean waves that travel long distances across the ocean. They often strike land with devastating consequences. Earthquakes emanate from faults, or fractures in the Earth where movement occurs. [For an animation of how earthquakes are propagated see "The Savage Earth" - The Restless Planet: Earthquakes animation , NOVA, PBS].The location of earthquakes and the ruptures associated with them are what scientists use to define plate edges. The association between plate boundaires and earthquakes is shown in Figure TL.17.

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Figure TL.17 Cumulative global earthquakes 1960 -1995
Courtesy NASA (Source)

There are several different kinds of faults classified according to the type of movement that occurs.Where plates move horizontally past one another, transform faults occur. Such is the case of the San Andreas Fault in California. Where the crust is spreading apart, like the mid-ocean ridge, shallow earthquakes (within 30 kilometers of the surface) are common.  Shallow to deep earthquakes occur along subduction zones where one plate overrides another, like that which occurs along the northwest coast of the United States, western Canada and southern Alaska. [ Watch "Angry Earth" from National Geographic]

Figure TL.18 San Andreas Fault, California, is where numerous earthquakes occur due to slippage along two plate boundaries. (Source: USGS Used with permission)

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