8.1 Earthquakes 101

Earthquakes, Faults, and Deformation

To know about earthquakes, we have to revisit deformation. Specifically, the type of deformation that will cause an earthquake is located near the Earth’s surface. This is called Brittle Deformation, which we covered in the section on Metamorphism. Let’s quickly review!

The story of most earthquakes begins at faults. These faults can be caused by stress brought on at a nearby plate boundary, which is why we witness so many earthquakes in California which has the San Andreas transform fault boundary. Other times, there are faults due to other causes such as the movement of the mantle below the tectonic plate or the extension or compression of a region due to ancient tectonic processes. Not every earthquake occurs at a tectonic boundary, but they are most often caused by movement along a fault plane.

Faults are simply planes of weakness in the rock. These rocks move along the fault plane in starts and stops, each time “stretching” out the rocks like a rubberband. Eventually, the tension on the rocks at the fault becomes too much, and they break or rupture on part of the fault plane, then snap back towards their original shape in what is called elastic rebound, but often leave a significant amount of displacement along the fault.

The place, often well beneath the Earth’s surface, where the fault rupture occurs is called the hypocenter, or focus. The spot directly above the hypocenter on the surface is called the epicenter. You will often hear people talk about finding the epicenter as the location where the earthquake occurred, and this definition is why it is so important.

When the fault ruptures, energy is released which travels through the earth as seismic waves. Let’s learn about these waves in the next section…