About 90% of the world’s earthquakes occur along the edges of tectonic plates, the massive slabs of rock that make up Earth’s outer shell. These plates are constantly moving, and the zones where they grind past, pull apart from, or crash into each other produce the overwhelming majority of seismic activity on the planet. But earthquakes also strike in some surprising places far from any plate boundary.
The Ring of Fire
The single most earthquake-prone region on Earth is the Ring of Fire, a horseshoe-shaped belt that traces the edges of the Pacific Ocean. It stretches from New Zealand up through Southeast Asia and Japan, across to Alaska, and down the western coasts of North and South America. Roughly 90% of all earthquakes happen here, and it’s also home to about 75% of the world’s active volcanoes.
What makes this zone so active is that the Pacific Plate is being pushed beneath the surrounding plates in a process called subduction. When one plate dives under another, it generates enormous stress. The contact zone between the two plates produces very large, shallow earthquakes, typically within the top 60 kilometers of Earth’s surface. But the sinking plate itself remains brittle enough to crack as it descends, generating earthquakes as deep as 700 kilometers. The 2011 magnitude 9.0 earthquake in Japan and the 2004 magnitude 9.1 earthquake off Sumatra both originated at subduction zones in the Ring of Fire.
The Alpide Belt
The second major earthquake zone runs from Indonesia through the Himalayas, across the Mediterranean, and into the Atlantic. This is the Alpide belt, and it accounts for about 17% of the world’s largest earthquakes. The collisions here are largely between continental plates, which crumple and fold rather than one sliding cleanly beneath the other. The result is broad zones of faulting spread across thousands of kilometers.
Some of the most destructive earthquakes in recent history have struck along this belt. The 2005 magnitude 7.6 earthquake in Pakistan killed over 80,000 people. The 2004 magnitude 9.1 Indonesian earthquake, at the belt’s eastern end where it overlaps with the Ring of Fire, generated a tsunami that killed more than 230,000.
Mid-Ocean Ridges
Earthquakes also occur where plates are pulling apart, primarily along the underwater mountain chains called mid-ocean ridges. The Mid-Atlantic Ridge, for instance, runs down the center of the Atlantic Ocean where the Americas are slowly separating from Europe and Africa. These spreading centers produce frequent but generally smaller earthquakes. Studies of 50 large mid-ocean ridge earthquakes found that the faulting occurs at very shallow depths, just 1 to 10 kilometers beneath the seafloor. Because these quakes happen deep underwater and far from population centers, they rarely cause damage.
Transform Faults Like the San Andreas
Where two plates slide horizontally past each other, transform faults develop. The San Andreas Fault in California is the most famous example, marking the boundary where the Pacific Plate moves northwestward relative to the North American Plate. Earthquakes along continental transform faults like the San Andreas are confined to the shallow crust, typically no deeper than about 20 kilometers.
Some segments of the San Andreas creep steadily, producing frequent small to moderate earthquakes that cause little damage. Other segments lock up for decades or centuries, building enormous stored energy before releasing it all at once. Palaeoseismic studies at the Wrightwood site in Southern California identified thirteen intervals between great earthquakes, ranging from as short as 31 years to as long as 224 years. That variability makes predicting the next major rupture extremely difficult, but the long-term pattern confirms that large earthquakes on locked fault segments are inevitable.
How Deep Earthquakes Go
Not all earthquakes happen at the same depth, and depth matters for the kind of shaking you feel at the surface. Seismologists group earthquakes into three categories: shallow (0 to 70 kilometers deep), intermediate (70 to 300 kilometers), and deep (300 to 700 kilometers). Shallow earthquakes tend to cause the most damage because the energy has less distance to travel before reaching the surface.
The deepest earthquakes, near 700 kilometers, only occur in subduction zones where cold oceanic plates plunge into the hot mantle. At those depths, the surrounding rock is too hot and soft to fracture, but the subducting slab stays cool enough relative to its surroundings to remain brittle. Below about 700 kilometers, even the slab warms up enough that earthquakes stop entirely. On continents and along transform faults, earthquakes are restricted to the uppermost 20 kilometers or so of crust.
Earthquakes Away From Plate Boundaries
Earthquakes inside tectonic plates are far less common but not unheard of. The most notable example in the United States is the New Madrid Seismic Zone, which sits in the middle of the continent near the borders of Missouri, Arkansas, Tennessee, and Kentucky. In 1811 and 1812, this zone produced a series of earthquakes estimated at magnitude 7 or greater, among the largest ever recorded in North America east of the Rockies.
These intraplate earthquakes are harder to explain than their plate boundary counterparts. Research from the USGS suggests they result from ancient weaknesses in the crust, scars left behind by long-dead tectonic events like mountain-building or continental rifting that occurred hundreds of millions of years ago. Stress from the slow movement of the entire plate gets concentrated at these inherited weak points. The crustal stress map of the United States shows that the highest-hazard zones in the continental interior are places where local stress patterns differ from the broader stress driven by distant plate boundaries.
Human Activity and Induced Earthquakes
In recent years, some regions that historically experienced very few earthquakes have seen dramatic increases in seismic activity tied to human operations. The central United States, particularly Oklahoma, saw a sharp rise in earthquakes linked to the injection of wastewater from oil and gas production deep underground. The pressure from injected fluids can reactivate dormant faults, triggering earthquakes that would not have occurred naturally.
The increase was significant enough that the USGS released a special one-year seismic hazard forecast for 2016 that included contributions from both induced and natural earthquakes. Hydraulic fracturing (fracking) has also directly triggered earthquakes larger than magnitude 2.0 at sites in Oklahoma, Ohio, and England, though most induced earthquakes from fracking are smaller than those caused by wastewater disposal. Following earthquake swarms in Arkansas and Texas in 2011, federal regulators began developing new recommendations for managing injection wells to reduce seismic risk.
Why Some Regions Stay Quiet
The interior of most tectonic plates, particularly stable continental regions like central Canada, much of Australia, and large parts of Africa and South America, experience very few earthquakes. These areas sit far from active plate boundaries and lack the inherited crustal weaknesses that concentrate stress in zones like New Madrid. That doesn’t mean they’re completely immune, but the recurrence intervals for significant earthquakes in these regions can be thousands or tens of thousands of years, making the risk functionally negligible for most planning purposes.
The global pattern is clear: if you’re near a plate boundary, especially a subduction zone or transform fault, earthquakes are a regular feature of life. If you’re in the middle of a stable plate, they’re rare. And in a handful of places, ancient geology or modern industry has created exceptions that don’t fit neatly into either category.