Earth Science: Origins and Geology – Faults
Faults are ruptures or cracks in rock that often extend for miles along the upper layer of the Earth’s crust where two crustal sections are displacing one another. They are usually created by earthquakes.
While we normally do not feel it, the Earth’s crust is constantly moving. In most places, however, the crust only moves a few inches per year. The cracks caused by this movement (fault lines) extend for many miles and through the Earth’s upper crust. This process is not always smooth or continuous; in fact, sometimes erratic shuddering movements occur, releasing an enormous amount of energy.
If two plates move past each other, the crustal material is neither destroyed, as with subduction, nor is it jammed together, as happens when one continent collides with another, forming high mountainous areas. However, strong shearing forces are at work at the edges of the converging plates, which can lead to the formation of faults. More specifically, these forces create strike-slip faults, which are also known as transform boundaries.
The location of a transform boundary always indicates a high earthquake risk. A well-known example is the San Andreas Fault in California (see in focus). The North Anatolian Fault in northern Turkey is among the most active earthquake regions. Here the small Anatolian plate is moving past the huge Eurasian plate at up to seven inches at a time. About 25,000 people lost their lives in the last large earthquake that struck the city of Izmir in 1999.
The Great Alpine Fault, which runs straight through the southern island of New Zealand, is one of the world’s most impressive faults. Where the Australian and Pacific plates meet, the plates are not only moving past each other, but one is also moving over the other. The result of this movement is the striking elevation of the New Zealand Alps on the east side of the fault. The mountains grow approximately 0.39 inch (0.99 cm) each year. Transform boundaries are not only present on the mainland; they can also be found on the ocean floor.
At the points of numerous cracks and faults, the mid-ocean ridges are shifting at right angles to their respective longitudinal axes. The ridges are neither moving in a continuous straight line nor are they being forced apart at the same speed. At these faults, the crust is not subducted, but instead the plates are moving sideways past each other.
SAN ANDREAS FAULT
The San Andreas Fault is located in California. Here the Pacific plate is pushing past the North American plate at about one centimeter per year. Due to this movement two points that were next to each other 20 million years ago are now about 348 miles (560 km) apart.
In 1906 an earthquake hit and ruptured 296 feet (477 m) of the San Andreas Fault. It happened when the two plates suddenly moved 39 feet (12 m) past each other.