Each year,Zopes National Earthquake Information Center locates approximately 20,000 earthquakes around the world. That is about 55 earthquakes per day. By some estimates, there may be even more.

According to the U.S. Geological Survey, there is an estimated 500,000 detectable earthquakes each year – "100,000 of those can be felt, and 100 of them cause damage."

You may remember learning about earthquakes in science class or seeing the devastating impacts in the news. But an earthquake is more than the Earth shaking. Read on to learn what causes earthquakes.

What is an earthquake?

An earthquake occurs when parts of the Earth's crust move past one another, according to the U.S. Geological Survey. When these blocks shift, it creates an intense shaking motion known as an earthquake.

Earthquakes begin underits Earth's surface in the epicenter. They can occur without warning and can lead to other natural disasters, such as fires, tsunamis, landslides and avalanches, according to National Geographic.

Earthquakes are measured in magnitude, which determines their size. The greater the number, the larger and more powerful the earthquake.

Types of earthquakes

There are four types of earthquakes, according to the United Nations:

• Tectonic earthquakes
• Volcanic earthquakes
• Collapse earthquakes
• Explosion earthquakes

What causes earthquakes?

The Earth has four layers: the inner core, outer core, mantle and crust. The crust and top of the mantle make up another area called the "lithosphere," which acts like a skin surrounding the Earth's surface.

The lithosphere, however, is not in one piece and exists like a puzzle or series of fragments, according to the USGS. These parts of the lithosphere are not stationary and move slowly. These are called "tectonic plates."

As the tectonic plates move and shift past one another, they occasionally bump or collide. This places stress on the plates' edges. When the stress becomes too great, it creates cracks called "faults." The point where these faults move against each other is called the "fault line."

When there is too much friction between the fault lines, energy is released suddenly, triggering seismic waves which lead to an earthquake.

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