Tectonic plates are constantly in motion, and this movement is the ultimate source of nearly all seismic activity. These rigid plates float atop the warmer, more fluid mantle, leading to continuous interactions along their boundaries. Earthquakes are concentrated along the narrow zones where these plates meet. Understanding the specific nature of the plate boundary—whether the plates are pulling apart, sliding past each other, or colliding—explains why certain regions experience frequent, small tremors while others face the threat of massive, deep-seated shakers.
The Fundamental Cause: Stress, Strain, and Faults
The generation of an earthquake begins with stress, the force applied to a rock unit, and the resulting strain, or deformation, that the rock undergoes. Plate movement continuously applies one of three primary types of stress—tensional, compressional, or shear—to the crustal rocks near the plate edges. Rocks can absorb this strain by deforming elastically, much like a stretched rubber band, temporarily storing the energy.
As the stress continues, the rock eventually reaches its breaking point, known as the limit of elastic deformation. When this limit is exceeded, the rock fractures, and the two sides abruptly slip past each other along a fault plane. This sudden rupture, described by the elastic rebound theory, releases the stored energy in the form of seismic waves, causing the ground shaking. The extent of the rupture area and the amount of slip determine the magnitude of the resulting earthquake.
Divergent Boundaries: Tensional Stress
Divergent boundaries are areas where tectonic plates move away from one another, generating tensional stress that stretches the crust, typically forming normal faults. Geologically, these boundaries are characterized by rifting in continental crust or by mid-ocean ridges in oceanic crust, like the Mid-Atlantic Ridge.
Earthquakes at divergent boundaries are generally frequent but tend to be shallow and of low-to-moderate magnitude. The crust here is relatively thin and warm due to the upwelling of molten material from the mantle. This makes the rock weaker and less capable of storing vast amounts of elastic energy. Brittle failure that generates earthquakes is restricted to the upper part of the crust, typically less than 30 kilometers deep. While most seismic activity occurs along transform faults that offset the ridge segments, earthquakes also happen when the crust stretches and breaks along the spreading axis.
Transform Boundaries: Shear Stress
Transform boundaries occur where plates slide horizontally past one another, subjecting the crust to intense shear stress. This sideways motion results in a strike-slip fault. These boundaries can be found in oceanic crust, connecting segments of mid-ocean ridges, or in continental crust, creating long, linear fault systems.
Friction causes the plates to lock up, preventing smooth movement. Over long periods, immense elastic strain builds up along the fault zone until the frictional resistance is overcome. The subsequent rupture releases this stored energy in a powerful, shallow earthquake, often within 20 kilometers of the surface. Transform boundaries, such as the famous San Andreas Fault in California, can produce large magnitude earthquakes, with the most mature fault systems capable of generating events up to a magnitude 8.
Convergent Boundaries: Compressional Stress
Convergent boundaries are the sites of plate collision, where compressional stress squeezes the rock and generates the largest and deepest earthquakes. This boundary includes subduction zones and continental collision zones.
Subduction Zones
Subduction zones form where an oceanic plate slides beneath a continental or another oceanic plate. Friction along this gently sloping boundary, the megathrust fault, can lock the plates together, accumulating strain. When the locked zone ruptures, it produces megathrust earthquakes, which are the most powerful seismic events on Earth, capable of reaching magnitude 9 and higher. Earthquakes also occur within the overriding plate and throughout the descending slab. This activity defines the Wadati-Benioff zone, which can extend to depths of up to 700 kilometers as the cold slab sinks into the mantle.
Continental Collision Zones
Continental collision zones, such as the Himalayas, involve two continental plates colliding. Neither plate easily subducts, resulting in massive crustal thickening and uplift. This process causes broad zones of shallow-to-intermediate earthquakes.