What is an Earthquake?

Types of Faults

What is an Earthquake?

An earthquake is the shaking of the ground caused by the sudden release of energy in the Earth’s crust. This energy is usually released along a fault, which is a crack where two blocks of rock can move past each other.

Earthquakes happen because of elastic rebound. As tectonic plates move, rocks along a fault bend and store energy, much like a stretched rubber band. When the stress becomes too great, the rocks snap back to their original shape, releasing energy. That energy travels outward in the form of seismic waves, which cause the ground to shake.

The point inside the Earth where the earthquake begins is called the focus or hypocenter. Directly above it on the surface is the epicenter, the location that usually feels the strongest shaking because it is closest to the source of the seismic waves.

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Earthquakes often come in a sequence. Small tremors called foreshocks sometimes occur before the largest quake, known as the main shock. Afterward, smaller quakes called aftershocks can continue for days, weeks, or even months as the crust adjusts to the new position.

In short, earthquakes are the result of stored energy being suddenly released as seismic waves, reminding us of the powerful forces at work beneath our feet.

Types of Faults

An earthquake fault is a crack in the Earth’s crust where blocks of rock move relative to one another. These movements are caused by the stress from shifting tectonic plates. There are three main types of faults, each linked to different kinds of plate motion:

Normal Fault

Normal faults occur when the crust is being pulled apart. One block of rock slips downward relative to the other. These faults are common at divergent boundaries, such as in rift valleys.

Reverse (Thrust) Fault

Reverse or thrust faults happen when the crust is being compressed or pushed together. One block is forced upward over the other. These are common in mountain-building zones and subduction zones at convergent boundaries. These types of faults often result in the most powerful and destructive earthquakes.

Strike-Slip Fault

In strike-slip faults, the blocks of rock slide past each other horizontally with little vertical movement. The San Andreas Fault in California is a famous example. These faults form at transform boundaries and are the type of fault that produce the most earthquakes.