The Earth’s surface is fragmented into a mosaic of rigid tectonic plates. These massive pieces of the lithosphere are constantly in slow motion, driven by the planet’s internal heat. Most geological activity, including volcanism and seismic energy release, is concentrated along the narrow zones where these plates interact. Earthquakes occur when the stress built up by plate movement exceeds the strength of the rock, causing a sudden slip along a fault and radiating energy outward.
Earthquakes at Spreading Boundaries
Where tectonic plates move away from each other (divergence), the crust is pulled apart under tensional stress. This activity is primarily associated with mid-ocean ridges, where new oceanic crust is continuously formed by rising magma. Earthquakes in these spreading zones are typically caused by normal faults, where one block of crust slides down relative to the other.
These seismic events are characteristically shallow, generally occurring at depths less than 30 kilometers. The rock below that point is too hot and weak to store significant elastic energy. Earthquakes along the main spreading segments are frequent but tend to be low to moderate in magnitude. Larger earthquakes often occur along the short, perpendicular transform faults that offset these boundaries.
Earthquakes at Colliding Boundaries
Colliding (convergent) plate boundaries produce the most complex and powerful earthquakes globally due to intense compressional stress. These boundaries are divided into subduction zones and continental collision zones. In a subduction zone, a denser oceanic plate slides beneath a less dense plate, sinking into the mantle. This interaction generates massive megathrust earthquakes, which can reach magnitudes of 9.0 or higher as the plates lock together and suddenly release accumulated strain.
The seismic activity in subduction settings extends from the shallow trench to great depths, tracing the path of the descending slab. Shallow earthquakes involve reverse or thrust faulting due to the immense compression in the overriding plate and the upper part of the subducting slab. As the slab continues to sink, it remains brittle enough to fracture, creating deep-focus earthquakes down to 700 kilometers in the Wadati-Benioff zone. Subduction zones are the only boundaries capable of generating both the planet’s deepest and largest magnitude quakes.
When two continental plates collide, neither plate is easily subducted because of their similar, low density. Instead, the crust crumples and thickens, forming extensive mountain ranges like the Himalayas. This collision results in a broad zone of intense, scattered seismicity across hundreds of kilometers. The earthquakes here are dominated by thrust faulting, concentrated in the upper crust, making these events potentially very damaging.
Earthquakes at Sliding Boundaries
At sliding, or transform, boundaries, plates move past one another horizontally, neither creating nor destroying crust. This motion subjects the crust to immense shear stress. The resulting seismic events occur along strike-slip faults, where the movement is predominantly side-to-side, such as the San Andreas Fault in California.
The grinding action occurs in a broad zone of shearing, which can produce moderate to large earthquakes, sometimes reaching magnitudes up to 8.0. Like those at spreading boundaries, these earthquakes are almost exclusively shallow, typically occurring at depths less than 30 kilometers. Since the movement is purely horizontal, transform boundaries do not create the deep-focus quakes seen in subduction zones.
Earthquakes Within Tectonic Plates
While most seismic activity occurs at plate edges, a small percentage of earthquakes, termed intraplate quakes, happen far from the defined boundaries. These events occur within the stable interior of a plate, where the crust is generally considered undeformed. Intraplate earthquakes are often attributed to the reactivation of ancient, buried fault systems formed during past tectonic events.
The stress that triggers these slips is believed to be transferred into the plate’s interior from distant boundary movements. Examples include the 1811–1812 New Madrid earthquakes and the 1886 Charleston earthquake. Although they are less frequent and generally smaller than the largest interplate quakes, their shallow depth and lack of seismic preparedness can lead to considerable damage. The precise mechanisms for the timing and location of these interior quakes remain a subject of ongoing scientific investigation.