Earthquakes generally originate along the edges of tectonic plates, the massive slabs of rock that make up Earth’s outer shell. Nearly 95% of all earthquakes occur at these plate boundaries, where plates collide, pull apart, or grind past each other. The remaining 5% strike within plate interiors, sometimes thousands of kilometers from the nearest boundary.
The Point Where an Earthquake Begins
Every earthquake starts at a specific point underground called the hypocenter (or focus), where rock first fractures and begins to slip. The epicenter is simply the spot on the surface directly above the hypocenter. When you see a location reported for an earthquake, that’s the epicenter, but the actual origin is always below ground.
Earthquakes are classified by how deep their hypocenter sits. Shallow earthquakes occur between 0 and 70 km deep. Intermediate earthquakes range from 70 to 300 km. Deep earthquakes reach 300 to 700 km beneath the surface. The vast majority of damaging earthquakes are shallow, because the energy released has less rock to travel through before reaching the surface.
Convergent Boundaries: Where Most Earthquakes Happen
About 80% of earthquakes occur where plates collide, known as convergent boundaries. At these boundaries, one plate is forced beneath the other in a process called subduction. This creates a sloping zone of earthquake activity that can extend from the surface all the way down to about 700 km deep.
The earthquakes at different depths along a subduction zone happen for different reasons. Near the surface (down to about 25 km), the plate bends as it starts its descent, stretching the upper surface and causing it to crack. Deeper, between the two plates, enormous friction generates powerful thrust earthquakes as one plate grinds beneath the other. These are the earthquakes responsible for the largest recorded events, including the kinds of megathrust quakes that trigger tsunamis.
Between 70 and 300 km deep, something different happens. Water-bearing minerals in the sinking plate undergo rapid chemical changes, releasing water and triggering faulting. Below 300 km, the mechanism shifts again: minerals in the rock suddenly transform from one crystal structure to another under extreme pressure, causing the rock to fracture internally. This mineral transformation is essentially complete by 700 km depth, which is why no earthquakes are detected below that point.
The Ring of Fire
The most earthquake-prone region on Earth is the Ring of Fire, a horseshoe-shaped belt of subduction zones and volcanic arcs that encircles the Pacific Ocean. According to the U.S. Geological Survey, roughly 90% of the world’s earthquakes occur there. This belt runs from New Zealand up through Indonesia, Japan, and the Aleutian Islands of Alaska, then down the western coasts of North and South America.
The concentration is so extreme because the Pacific Plate is being subducted beneath several surrounding plates simultaneously. Japan, Chile, Indonesia, and the Pacific Northwest of the United States all sit along this belt, which is why these regions experience frequent and sometimes catastrophic seismic activity.
Transform Boundaries: Shallow and Sideways
At transform boundaries, two plates slide horizontally past each other. The San Andreas Fault in California is the most famous example. These faults produce exclusively shallow earthquakes because the plates are grinding side by side rather than diving beneath one another. The result is broad zones of crustal deformation, with masses of rock displaced tens to hundreds of miles over geologic time.
Transform boundary earthquakes can still be highly destructive. Because they’re shallow, the shaking reaches the surface with full force. The 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake both originated along the San Andreas system.
Divergent Boundaries: Small but Constant
Where plates pull apart, at mid-ocean ridges and continental rifts, earthquakes are frequent but relatively small. These spreading centers generate new crust as magma rises to fill the gap between separating plates. The earthquakes tend to be shallow and low in magnitude because the crust at these boundaries is thin and hot, making it less capable of storing the stress needed to produce a large quake.
The Mid-Atlantic Ridge, which runs down the center of the Atlantic Ocean, is the longest divergent boundary on Earth. Iceland sits directly on top of it, which is why the island experiences regular small earthquakes alongside its volcanic activity.
Earthquakes Far From Plate Edges
About 5% of earthquakes occur within plate interiors, far from any boundary. These intraplate earthquakes are less common but can be genuinely surprising, both to the people who feel them and to scientists trying to explain them.
The traditional explanation was straightforward: forces applied at distant plate edges transmit stress across the entire plate, and that stress occasionally releases at weak points in the interior. More recent research has complicated this picture. A 2019 study of the southeastern United States, a region thousands of kilometers from any plate boundary, found that variations in the thickness of Earth’s crust can largely explain the stress patterns and earthquake locations observed there. The 2011 Virginia earthquake, which was felt as far away as Washington, D.C., is a notable example of this type of event.
Intraplate earthquakes tend to be felt over much larger areas than boundary earthquakes of the same magnitude. The rock in plate interiors is older, colder, and more rigid, so seismic waves travel farther before losing energy. The New Madrid Seismic Zone in the central United States, responsible for a series of massive earthquakes in 1811 and 1812, sits squarely in the middle of the North American Plate and remains active today.