Where Do Earthquakes Occur?
Earthquakes shake the ground and rattle buildings around the world every day. But while quakes can happen anywhere, the largest and most devastating ones occur in a large area known as the Ring of Fire—a horseshoe-shaped belt of volcanoes and tectonic plates surrounding the Pacific Ocean.
The tectonic plates that make up the Earth’s outer skin are always moving—bumping into each other or sliding past each other at about the same speed as your fingernails grow. When they suddenly slip, it creates energy and pressure that we feel as earthquakes.
Plate boundaries are places where Earth’s continental and oceanic plates meet. They contain systems of deep cracks called faults, and most earthquakes occur along these fault lines. Friction keeps the rock masses on either side of the fault from moving. But when the friction is finally overcome, sudden movement occurs and releases energy in the form of an earthquake.
There are three types of plate boundary: those that subduct into or rise above other plates (Convergent); those that separate from other plates (Divergent); and those that slide past each other horizontally (Transform). Earthquakes are most common at convergent and divergent boundaries.
In contrast, only a few earthquakes happen at mid-plate boundaries and the cause of those isn’t well understood. This makes it difficult to predict when or where an earthquake will happen. But we do know that there are several prominent seismic belts where earthquakes regularly occur, including the Pacific Ring of Fire where about 80% of the world’s largest earthquakes occur. This belt stretches from Chile up through Central America and Mexico to the West Coast of North America and beyond Alaska to Japan, the Philippines and New Guinea.
Over 80% of the world’s largest earthquakes occur along the edge of the Pacific Ocean in an area known as the Ring of Fire. These earthquakes are caused by movement on faults.
Friction between plates slows the edges of the fault down, building up stress over time. Eventually the pressure becomes too great and sections of rock suddenly break or slide past each other, releasing the built-up tension in seismic waves that shake the Earth. The point on Earth’s surface directly above the focus of an earthquake is called the epicenter.
The motion of the rocks on a fault is usually described as either a strike-slip or a reverse fault. Strike-slip faults are found at transform boundaries and often cause earthquakes. Reverse faults are associated with collision zones and tend to produce large earthquakes. Both types of fault motion result in sudden movements characterized by “spasms” that can last a few seconds. These sudden movements are what give earthquakes their energy and make them so dangerous.
When an earthquake happens, the shifting rock causes vibrations called seismic waves that can be detected by seismographs. These waves can also transmit the energy of an earthquake to other parts of Earth, causing damage and sometimes generating tsunamis.
Earthquakes occur mainly at the boundaries between the Earth’s plates, where rocks are pushing against each other or scraping past each other. These movements build pressure until the friction between the plates becomes so great that one or more of them slips suddenly. The sudden movement releases the built-up pressure in a burst of energy that makes the ground shake.
Most plate boundary zones experience many small earthquakes a year, while the very large “great” quakes that level cities and spawn tsunamis happen only about once a decade on average. However, some earthquakes happen far from the edges of the plates, such as those in New Madrid, Missouri (1811-1812) and Charleston, South Carolina (1986). These quakes are called intraplate tremors.
The point on Earth’s surface directly above the earthquake focus is called the epicenter. The epicenter is where the strongest vibrations of the seismic waves occur, but the amplitude of those waves decreases with distance from the center.
The plates that make up the Earth’s crust don’t move continuously but rather in ‘fits and starts’. Sometimes they snag on each other causing friction that builds up over time. Eventually that pressure becomes too great and the rock slips, releasing the energy that we feel as earthquakes.
Most earthquakes happen at tectonic plate boundaries. Some are also caused by magma erupting from volcanoes and at continental rift zones.
Most of the world’s earthquakes happen along the so-called Pacific Ring of Fire, an area that stretches in a horseshoe shape 40,000 km around the Pacific Ocean. This makes the region the most earthquake-prone on Earth. However, earthquakes can happen anywhere. They can be shallow or deep, and can vary in magnitude from minor to very large (magnitude 7.0 and higher). The type of geology in the area also affects the severity of an earthquake: rocks that are solid do less damage than sands and clays which can liquefy during an earthquake and cause buildings to collapse.