What causes earthquakes and why are they more common in certain areas?

An Afghan man cries next to his house after the recent earthquake in Sahaq village, Engil district, Herat province, Afghanistan, on October 11, 2023. Ali Kara/Reuters

Earthquakes, large and small, Happens every day In zones that go around the world like the seams in a baseball. Most don’t bother anyone, so they don’t post news. But every now and then a catastrophic earthquake strikes people somewhere in the world with terrible destruction and great suffering.

On October 7, 2023, a magnitude 6.3 earthquake struck near the historic city of Herat, Afghanistan. More than 1,000 people According to estimates, died in the rubble. following that Another earthquake, just as powerfulOn October 11.

read more: After powerful earthquakes, Afghan communities dug out the dead and injured from the rubble

A few weeks ago, on September 8, a magnitude 6.8 earthquake struck Morocco’s Atlas Mountains. Divided ancient villages, nearly 3,000 people were killed. In February 2023, a large part Turkey and Syria It was destroyed by two major earthquakes in quick succession.

as A geologist, I study the forces that cause earthquakes. Why are some seismic zones very active while others can be quiet for generations before the stress develops into a catastrophic event?

Earth’s crust is colliding in on itself and breaking apart

Earthquakes are part of the Earth’s natural behavior. They occur with the movement of the tectonic plates that form them The outer layer of the planet.

You can think of the plates as a more or less solid outer shell to allow the Earth’s internal heat to escape.

A map of all earthquakes greater than magnitude 5 from 1960 to 2023 clearly shows the outlines of the tectonic plates. USGS/GMRT

These plates carry the continents and oceans, which are constantly contained within Slow motion crashes together with each other. Cold and dense oceanic plates sink under the continental plates in a process that descends back into the Earth’s crust. It is called subduction. When an oceanic plate sinks, it pulls everything behind it and opens a rift somewhere else, which fills with hot material from the surface, which then cools. These fissures are long chains of underwater volcanoes Mediterranean ridges.

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Earthquakes are accompanied by both subduction and faulting. In fact, that’s how plate boundaries were first discovered.

In the 1950s, when A A global seismological network was established To monitor nuclear tests, geophysicists observed that most earthquakes occur in relatively narrow bands that cut along the edges of ocean basins such as the Pacific, or in the middle of basins. Like the Atlantic Ocean.

They also observed that earthquakes in subduction zones are shallower in the ocean Deep under the continent. If you plot earthquakes in 3D, they define slab-like features where the plates are seen sinking into the mantle.

Tens of thousands of earthquake sites from 1980 to 2009 have detected the Pacific plate under northern Japan. The top image is a side view showing the depth of the earthquakes below the rectangle on the map. Jaime Toro, CC BY-ND

An Experiment: How an Earthquake Works

To understand what happens during an earthquake, put your palms together and press with some force. You are modeling a plate boundary fault. Each hand is a plate, the surface of your hands is wrong. Your muscles are a plate tectonic system.

Now, add some forward force to your right arm. As the forward force overcomes the friction between your palms you will eventually see it move forward. That sudden forward jerk is an earthquake.

Google Earth image of creeks offset by movement along the San Andreas Fault in southern California as the Pacific plate moves northwest relative to North America. Jaime Toro

Scientists explain this using so-called earthquakes Theory of regeneration.

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Fast plates move up to 8 inches (20 centimeters) per year, driven mostly by subducting oceanic layers. Over time, they stick to each other through plate boundary friction. The attempted motion elastically deforms the plate boundary zone, like a loaded spring. At some point, the accumulated elastic energy overcomes friction and pulls the plate forward, causing an earthquake.

But the Plate driving forces Don’t stop, so the plate boundary starts accumulating elastic energy again, causing another earthquake – maybe soon or maybe in the future.

In the oceans, plate boundaries are narrow and well-defined because the underlying rocks are very hard. But within continents, plate boundaries are often vast zones of deformed highlands crisscrossed by many faults. Even if a plate boundary fails, those faults can persist for eons. That’s why earthquakes sometimes occur across plate boundaries.

Earthquakes, fast and slow

The rotational behavior of faults allows seismologists to Statistically assess earthquake hazards. Plate boundaries with fast movements, such as those along the Pacific Rim, accumulate elastic energy quickly and are prone to frequent, large earthquakes.

Slow-moving plate boundary faults take longer to reach a critical point. With some faults, hundreds or even thousands of years can pass between major earthquakes. This allows time for cities to grow and for people to lose their ancestral memory of past earthquakes.

read more: How to help Morocco earthquake victims

An example is the earthquake in Morocco. Located on the border with Morocco between the African and Eurasian platesslowly bumping into each other.

This plate collision resulted in a huge belt of mountains stretching from the Atlas of North Africa to the Pyrenees, the Alps and most of the mountains across southern Europe and the Middle East. Yet because these plate movements are slow near Morocco, large earthquakes do not occur often.

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Afghanistan is More prone to earthquakes. It contains many faults created by India’s collision against Eurasia. The old and rigid Indian plate has been plowing along the southern edge of Eurasia for the past 40 million years. You can see the slow-moving evidence of this conflict in the way that mountain ranges – and earthquakes – circle India on both sides.

Getting ready for adulthood

An important fact about devastating earthquakes is that, in most cases, earthquakes don’t kill people—buildings fall.

Most Americans have heard of it California’s San Andreas Fault and seismic risk to San Francisco and Los Angeles. The last major earthquake was along the San Andreas Fault Hit and Loma Prieta, in the San Francisco Bay Area, in 1989. Its magnitude, 6.9, was comparable to the earthquake in Morocco, but 63 people died compared to thousands. This is because building codes in these earthquake-prone U.S. cities are now designed to stabilize structures when the ground shakes.

The Exceptions are tsunamis, large waves generated when an earthquake shifts the ocean floor, displacing the water above it. Tsunami hit Japan in 2011 Regardless of the quality of engineering in coastal cities, the consequences were dire.

Unfortunately, earthquake scientists It is impossible to predict exactly when An earthquake may occur; They can only assess risk.

This article, originally published on September 13, 2023, has been updated with the powerful earthquake in Afghanistan.

This article has been republished Conversation Under Creative Commons License. Read on Original article.

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