Faults: When Earth Literally Rocks


In geology, faults do not mean mistakes; faults are cracks in the Earth’s crust, but not all cracks are considered faults. The Earth’s surface is made up of tectonic plates, which are very close to each other; they move very slowly and when they hit, earthquakes and volcanoes take place. Faults exist between tectonic plate boundaries; for a crack to be considered a fault, at least one of the two sides must be moving.

There are three causes to faults: tensional stress, compressional stress, and shear stress. Tensional stress happens when rocks are pulled away from each other; compressional stress, on the other hand, happens when rocks are pushed towards each other. Shear stress, however, happens when rocks slide across each other in opposite direction; the sliding rocks in this case do not hit each other.

Types of faults depend on the type of movement that created them. Normal faults happen when rocks pull away creating space and one of the two sides moves downward. Reverse faults happen when rocks are pushed together; this way the rock on one side moves on top of the other. Strike slip faults move horizontally unlike other fault types; the two blocks move in opposite direction.

However, some faults are not that simple; one fault might be a combination of a normal fault, reverse fault, and a strike slip fault. For all faults, one side of the fault is called the “hanging tree” and the other one is called “footwall”. The footwall is the side that is sliding, and therefore, people can literally stand on it. On the other hand, the hanging tree has that name because it protrudes and people can hang things on it.

Faults can also be classified as active faults, inactive faults, and reactivated faults. Active faults are the faults that are expected to move again at any time in the future and cause earthquakes. It is important to note that faults are considered active if they have moved once, or more than once, any time during the past 10,000 years. Inactive faults have not moved for millions of years; as for reactivated faults, they are the result of a movement happening in the opposite direction of the fault’s original movement.

The study of faults helps scientists discover where earthquakes are likely to occur. The areas with large faults are where most deadly earthquakes take place. Some of the biggest faults in the world include San Andreas Fault System, Gulf of California Rift Zone, Alpine Fault, Main Uralian Fault, and Central African Shear Zone. The biggest fault line is Sunda Megathrust, which extends approximately from Mynamar (Burma) in the north, running along the southwestern side of Sumatra, to the south of Java and Bali before terminating near Australia; this fault line was responsible for the 2004 India earthquake that led to the deadly tsunami, which killed at least 230,000 people. Thus, faults are what make Earth literally rock below your feet.


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