Earthquakes occur primarily along the boundaries where tectonic plates meet. Earth’s outer shell is broken into roughly 15 major plates that constantly shift, and the zones where these plates grind against, pull apart from, or slide past each other produce the vast majority of seismic activity on the planet. There are three main boundary types, and each generates earthquakes in a distinct way.
Convergent Boundaries: Where Plates Collide
Convergent boundaries produce the most powerful earthquakes on Earth. At these zones, two plates move toward each other, and when one plate is oceanic crust, it gets forced beneath the other in a process called subduction. The friction and pressure between the descending slab and the overriding plate builds enormous stress that releases in massive quakes. The largest earthquake ever recorded, a magnitude 9.5 in Chile in 1960, occurred at a subduction zone. So did the magnitude 9.2 that struck Alaska in 1964.
Earthquakes at convergent boundaries span an exceptional range of depths. Near the surface, the plates lock and release in shallow quakes that cause intense shaking. But as one plate sinks deeper into the mantle, it can still generate earthquakes hundreds of kilometers below the surface. Deep-focus earthquakes occur between 300 and 700 kilometers down within these cold, sinking slabs. The deepest significant earthquake on record struck beneath Japan’s Bonin Islands in 2015 at a depth of about 680 kilometers, with a magnitude of 7.9.
The faulting at convergent boundaries is dominated by reverse faults, where one block of rock is thrust upward over another. This vertical motion is also why subduction zones are the primary source of tsunamis. When the leading edge of the overriding plate snaps upward, it displaces the ocean floor and the water above it. Most tsunamis in the global historical record, about 89%, were triggered by large earthquakes or earthquake-caused landslides, and the biggest ones almost always originate at subduction zones. An earthquake generally needs to exceed magnitude 8.0 to send a dangerous tsunami across an ocean basin.
The Pacific Ring of Fire is the most famous convergent boundary system, ringing the Pacific Plate where it collides with the North American, Eurasian, Philippine, and other plates. It hosts the majority of the world’s most powerful earthquakes and volcanic eruptions.
Divergent Boundaries: Where Plates Pull Apart
Divergent boundaries form where two plates move away from each other. As the plates separate, hot rock from the mantle rises to fill the gap, creating new crust. Most divergent boundaries run along the ocean floor as mid-ocean ridges, massive underwater mountain chains that stretch tens of thousands of kilometers across the globe.
Earthquakes here are frequent but typically smaller than those at convergent boundaries. Along ultraslow spreading ridges in the Arctic, for example, earthquakes above magnitude 5.5 do occur, but much of the local seismic activity involves small quakes below magnitude 2. These small events reflect the constant cracking and settling as new crust forms and the seafloor spreads apart. The faulting is predominantly normal faulting, where blocks of rock drop downward as the crust stretches and thins.
On land, divergent boundaries show up as rift valleys. East Africa’s Great Rift Valley is the most prominent example, where the African plate is slowly splitting in two. Earthquakes in continental rift zones can be more damaging than their oceanic counterparts simply because people live on top of them.
Transform Boundaries: Where Plates Slide Past Each Other
Transform boundaries form where two plates grind horizontally past one another. The dominant motion is strike-slip faulting, meaning the ground on either side of the fault moves sideways rather than up or down. The San Andreas Fault in California is the best-known example, marking the boundary where the Pacific Plate slides northwest past the North American Plate.
These boundaries produce frequent, sometimes destructive earthquakes, but they rarely reach the extreme magnitudes seen at subduction zones. The quakes tend to be relatively shallow, often originating within the upper 15 to 20 kilometers of the crust, which means even moderate-magnitude events can cause significant shaking at the surface. A shallower earthquake concentrates its energy closer to where people live and build, producing stronger ground motion than a deeper quake of the same magnitude.
While shearing motion dominates at transform faults (accounting for the large majority of strain release), some vertical movement also occurs. Studies of oceanic transform faults show that about 17% of seismic energy can be released through normal faulting rather than pure strike-slip motion, creating complex fault zones with both horizontal and vertical displacement.
Earthquakes Away From Plate Boundaries
Not every earthquake happens at a plate boundary. Intraplate earthquakes occur well within the interior of a tectonic plate, far from the nearest boundary. These are less common and generally smaller, but they can still pose serious hazards, partly because the regions where they strike are often less prepared.
The southeastern United States sits in the middle of the North American Plate, yet it hosts several active seismic zones. The Eastern Tennessee Seismic Zone produced a magnitude 4.4 earthquake in 2018 at a shallow depth near the Watts Bar Nuclear Power Plant. Charleston, South Carolina experienced a devastating earthquake in 1886. The New Madrid Seismic Zone in the central Mississippi Valley generated a series of massive quakes in 1811 and 1812.
The exact mechanisms driving intraplate earthquakes are still not fully understood. The same forces that push and pull plates at their boundaries transmit stress through plate interiors, and ancient faults buried deep in the crust can reactivate under that stress. What makes these zones tricky is that large earthquakes recur at very long intervals, making the hazard easy to underestimate. Identifying which buried faults are capable of producing significant quakes remains an ongoing challenge.
Why Boundary Type Matters
The type of plate boundary determines nearly everything about the earthquakes it produces: how deep they occur, how large they can get, what kind of ground motion they generate, and whether they can trigger tsunamis. Convergent boundaries hold the record for both the deepest and the most powerful earthquakes. Transform boundaries tend to produce shallower quakes with intense local shaking. Divergent boundaries generate the most frequent but generally smallest events.
If you’re trying to understand earthquake risk for a specific region, the first question is which boundary (or boundaries) influence that area, and how close the population centers are to the fault zones. Depth matters enormously: a magnitude 6.0 earthquake at 5 kilometers depth will feel far more violent than the same magnitude at 50 kilometers. And while most seismic energy on the planet is released at plate boundaries, the possibility of intraplate earthquakes means that distance from a boundary doesn’t guarantee safety.