Bay Area Fault Planes Labeled

Faults are fractures in Earth’s crust where movement has occurred. Sometimes faults move when energy is released from a sudden slip of the rocks on either side. Most earthquakes occur along plate boundaries, but they can also happen in the middle of plates along intraplate fault zones. During the winter of 1811–1812, a series of earthquakes struck New Madrid, Missouri. More recently, the Sichuan region in China suffered a devastating intraplate earthquake in 2008.

Some faults are visible at the surface, but others lie deep within the crust. Just as there are various types of plate movements, there are also different types of faults. They’re based on the type of movement they exhibit.

  • Normal Faults: This is the most common type of fault. It forms when rock above an inclined fracture plane moves downward, sliding along the rock on the other side of the fracture. Normal faults are often found along divergent plate boundaries, such as under the ocean where new crust is forming. Long, deep valleys can also be the result of normal faulting.
  • Reverse or Thrust Faults: The opposite of a normal fault, a reverse fault forms when the rocks on the “uphill” side of an inclined fault plane rise above the rocks on the other side. Reverse faults often form along convergent plate boundaries.
  • Strike-slip Faults: Sometimes referred to as a lateral fault, this type forms when the blocks of rock on either side of a vertical (or nearly vertical) fracture move past each other. These faults are described as right lateral or left lateral, depending on which way the movement goes.

This post is part of Exploring Earthquakes, a rich collection of resources co-presented by the California Academy of Sciences and KQED. This material is also available as a free iBooks textbook and iTunes U course.

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