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What Causes Earthquakes?

It's no secret that an earthquake is one of the most destructive natural disasters on Earth. Not only can they swallow whole buildings and level entire cities, but they often trigger massive tsunamis capable of engulfing miles of coastland. How does something so powerful and destructive seem to appear out of nowhere? What even causes earthquakes?

The Structure of Earth

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To uncover what causes earthquakes, you must first understand the structure of the Earth. The Earth isn’t a solid ball. In the very center is its core. The core consists of two layers: the inner core and the outer core. The inner core is made of solid metal — mostly iron and nickel, while the outer core is made of hot molten iron and nickel. Next comes the mantle. The mantle is the thickest part of the Earth. It’s made of dense magnesium and iron-silicate rocks.

The crust sits on top of the mantle and is made of solid rock. The crust is more popularly known as the Earth’s surface or "home." At the bottom of the crust layer where it meets the mantle, friction between the rubbing rocks creates intense heat, which melts the rock and metal — creating a liquid layer that allows the crust to “float” and move.

All About the Crust

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When it comes to earthquakes, it’s all about the crust. The Earth's crust isn’t just one continuous sheet of rock; it’s made of several tectonic plates that fit together like pieces of a puzzle. The only difference is that these puzzle pieces aren’t fixed into place. They move, grind, and push against one another.

Tectonic movements are very slow and can sometimes take hundreds, if not thousands, of years to move only a few feet. Mountains are created when two plates push against one another and start to rise. Valleys and trenches form when two plates collide and push downward. Sometimes, a tectonic plate can slip underneath another tectonic plate or the two can move in opposite directions. This is when earthquakes occur.

Fault Zone

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Tectonic plates move at a constant slow and steady speed. When one plate rubs against another, rock-on-rock friction stops the movement. Eventually, the force of the moving plate becomes too much for the friction to hold and the rocks slip. The sudden movement creates a massive release of energy. The scraping and bumping of billions of tons of rock and metal is felt as an earthquake. The bigger the slip, the more powerful the earthquake. Earthquakes happen only at the edges of tectonic plates or faults. The regions where plates meet are called fault zones.

Subduction vs. Strike-Slip

Aerial view of a layer of the earth's crust exposed on a hillside in Ecuador
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There are two kinds of faults where earthquakes are most common:

  • Strike-slip faults: These faults are created when one tectonic plate slides horizontally past another. While earthquakes are very common in these areas, they’re usually not very powerful. The San Andreas Fault in California is an example of a strike-slip fault.
  • Subduction zones: These zones occur when one plate is pushed underneath another. Earthquakes in subduction zones are often much more powerful than quakes around strike-slip faults. The Ring of Fire around the Pacific Ocean is the strongest subduction zone on Earth. About 75% of the world’s volcanoes and 90% of the world’s earthquakes occur along these major fault lines.

Richter Scale

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The Richter scale was developed in 1935 to measure the power of an earthquake by calculating the size of the seismic waves produced. It uses a numeric scale to represent the strength of an earthquake.

  • 4 or below: A minor earthquake that probably won’t cause any damage.
  • 5-6: An earthquake that could cause some damage to weak structures.
  • 6-7: An earthquake that can move furniture and damage buildings.
  • 7-8: A serious earthquake that can knock over chimneys and walls.
  • 8+: An earthquake with severe destruction over large areas.

The strongest earthquake ever recorded occurred near Valdivia in southern Chile in 1960. It was classified as a 9.5 on the Richter scale. The earthquake was so powerful that the effects were felt across the Pacific Ocean in Japan and the Philippines. Over two million people were left homeless from the quake itself and the several tsunamis that followed.

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