What Causes Earthquakes? A Simple Explanation

The Basics: Earth's Moving Puzzle

Imagine Earth's outer layer as a cracked eggshell. These cracks divide the surface into large pieces called tectonic plates. Unlike an eggshell, though, these plates aren't stationary. They're constantly moving, sliding over a layer of partially molten rock deep below the surface.

These plates move incredibly slowly, typically just a few centimeters per year. That's about as fast as your fingernails grow. But over millions of years, this slow movement has created mountains, ocean trenches, and shaped the continents as we know them today.

Why the Ground Suddenly Shakes

As tectonic plates move, they don't glide smoothly past each other. Instead, they often get stuck at their edges due to friction. Meanwhile, the rest of the plate keeps moving, building up enormous pressure along these stuck zones, called fault lines.

Think of it like bending a stick. As you apply more force, the stick stores energy. Eventually, when the stress becomes too great, the stick suddenly snaps, releasing all that stored energy at once. The same thing happens with tectonic plates.

When the pressure finally overcomes the friction holding the plates together, they suddenly slip and release years or even centuries of built-up energy in seconds. This rapid movement sends shock waves, called seismic waves, radiating outward in all directions through the Earth's crust. When these waves reach the surface, we feel them as an earthquake.

The Three Main Types of Plate Boundaries

Earthquakes occur most frequently where tectonic plates meet. These meeting points, called plate boundaries, come in three main types, each producing earthquakes in different ways.

Convergent Boundaries: Where Plates Collide

At convergent boundaries, plates move toward each other. When they collide, one plate is often forced beneath the other in a process called subduction. This creates some of the most powerful earthquakes on Earth, including those that can trigger tsunamis. The 2011 Japan earthquake, which reached magnitude 9.1, occurred at this type of boundary.

Divergent Boundaries: Where Plates Pull Apart

At divergent boundaries, plates move away from each other. As they separate, magma rises from below to fill the gap, creating new crust. These boundaries produce frequent but generally smaller earthquakes. The Mid-Atlantic Ridge, where new ocean floor is constantly being created, is a prime example.

Transform Boundaries: Where Plates Slide Past Each Other

At transform boundaries, plates slide horizontally past one another. California's San Andreas Fault is the most famous example of this type. When the plates stick and then suddenly slip, they can produce major earthquakes. The 1906 San Francisco earthquake, estimated at magnitude 7.9, occurred along this fault system.

Other Causes of Earthquakes

While most earthquakes result from tectonic plate movement, other factors can also cause the ground to shake.

Volcanic Activity

As magma moves beneath a volcano, it can fracture rock and cause earthquakes. These volcanic earthquakes are typically smaller than tectonic ones but can signal an impending eruption. Scientists monitor these earthquake swarms to help predict volcanic activity.

Human Activities

Human actions can occasionally trigger smaller earthquakes. Activities like injecting wastewater deep underground, mining, or filling large reservoirs behind dams can increase stress on existing faults. While these induced earthquakes are generally minor, they've become more common in areas with extensive oil and gas operations.

Why Some Places Experience More Earthquakes

Earthquakes aren't distributed evenly across the globe. About 90% of the world's earthquakes occur in the "Ring of Fire," a horseshoe-shaped zone around the Pacific Ocean where several tectonic plates meet.

Countries like Japan, Indonesia, Chile, and the western United States experience frequent seismic activity because they sit near active plate boundaries. In contrast, regions in the middle of tectonic plates, like the eastern United States or central Africa, experience far fewer earthquakes.

However, no place on Earth is completely immune to earthquakes. Even areas far from plate boundaries can experience seismic activity from ancient fault lines or rare intraplate earthquakes.

The Bottom Line

Earthquakes are a natural consequence of our planet's dynamic nature. The same forces that cause earthquakes also created the mountains we climb, the ocean basins we explore, and the diverse landscapes we inhabit.

Understanding what causes earthquakes helps us prepare for them, design safer buildings, and ultimately save lives in earthquake-prone regions. While we can't prevent earthquakes, knowing why and where they occur allows us to be ready when they strike.