Thrust Faults

Introduction
Thrust faults occur when a section of land slips over another at a low angle. A reverse fault forms when two landmasses are being compressed together. The hanging wall on one side of the fault moves upward.

Thrust fault, Picture by Myrna Martin

Diagram showing how one section of land slips
over another along a fault line.
Myrna Martin

Coast ranges in subduction zones
Subduction zones often have multiple faults where land on the ocean floor is scraped off the oceanic plate.The faults pile up on the continent. The coast range Oregon, Washington and parts of California formed in this manner.

Formation thrust fault
Thrust faults form when the angle is less than 45 degrees. The angle allows one section of the land to go over the top of the other block of land.

Crust thickens
This type of fault movement thickens and shortens the crust. A normal fault develops where land is pulling apart and one block of land drops down. Blocks of land that move up or down in normal and reverse faults have steeply inclined planes.

A blind fault occurs in areas a shallow-dipping reverse fault terminates before it reaches the Earth's surface. Tension in the crust will cause the rocks to fracture but the fractures does not have any surface features. Many unknown thrust faults are suspected in California. Some faults are not discovered until a major earthquake occurs.

1994 Northridge earthquake
The 1994 Northridge earthquake was caused by a blind fault that scientists did not know about until it occurred. The earthquake killed 61 people and approximately $40 billion in damage. The fault line is between 12 miles at its southern edge and 3 miles below the Earth's surface at its northern edge. About 10 to 12 miles of land broke along the fault plane during the earthquake.

Break along fault line
The actual break only took about 8 seconds but the shaking in the area due to sediments lasted between 20 and 30 seconds. The movement along the fault was approximately 13 feet.

No foreshocks
Everyone in the region included seismologists were taken by surprise. There were no foreshocks and no strain was seen in the rocks at any time before the earthquake occurred.

More Plate Tectonic Links

Divergent Boundary These boundaries form the global mid-ocean ridge system which is the largest mountain chain on Earth.

Crustal Plates There are large and small plates that cover the Earth. Subduction zones form where these plates are recycled.

Mid Ocean Ridge The global mid-ocean-ridge system is the longest chain of mountains on Earth and was not discovered until the 20th Century.

Earthquake Epicenter The epicenter of an earthquake is a point on the Earth's surface, not where the earthquake originates.


Caribbean Plate This plate is small but very complex. It has a subduction zone, transform fault, and triple junction.

Thrust Faults These faults occur where large blocks of land can be forced over the top of another block of land at a low angle.

Plate Tectonics Find out lots of fascinating facts and interesting trivia on plate tectonics.

Kids Fun Science The links on our home page include information about volcanoes, science activities, plate tectonics, the rock cycle and much more.