Causes of Earthquakes - Almost all earthquakes occur at plate boundaries. All three boundary types divergent, convergent and transform are prone to earthquake activity. Plate tectonics causes the lithospheric plates to move. As you might imagine, having giant slabs of lithosphere moving about on a spherical shape is not smooth. When stresses build, they first cause the rocks to bend elastically. If the stresses persist, energy continues to build in the rocks.
When the stresses are greater than the internal strength of the rocks, the rocks snap. Although they return to their original shape, the stresses cause the rocks to move to a new position. This movement releases the energy that was stored in the rocks, which creates an earthquake. During an earthquake the rocks usually move several centimeters or maybe as much as a few meters. This description of how earthquakes occur is called elastic rebound theory.
The point where the rock ruptures is usually below the Earth’s surface. The point of rupture is called the earthquake’s focus. The focus of an earthquake can be shallow - less than 70 kilometers (45 miles), intermediate - 70 to 300 kilometers (45 to 200 miles), or deep - greater than 300 kilometers (200 miles). About 75% of earthquakes have a focus in the top 10 to 15 kilometers (6 to 9 miles) of the crust.
Shallow earthquakes cause the most damage because the focus is near the Earth’s surface where people live. Just above the focus on the land surface, is the earthquake’s epicenter. It is the epicenter of an earthquake that is reported by scientists and the media. The epicenter of the 1906 San Francisco earthquake, for example, was offshore, 1.5-3 kilometers (1-2 miles) west of Golden Gate Park.
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