The Earth’s surface is broken into numerous large pieces called tectonic plates, which are constantly moving. An earthquake is the sudden, violent shaking of the ground that occurs when two blocks of the Earth slip past one another along a fault. The majority of seismic activity, including almost all powerful earthquakes, occurs along the edges of these moving plates, known as plate boundaries. This interaction drives the accumulation and rapid release of seismic energy.
The Underlying Mechanics of Plate Movement and Stress
The continuous, slow motion of tectonic plates causes immense friction where their edges meet. These opposing forces lead to the buildup of stress and strain within the Earth’s crust. Because fault surfaces along the boundaries are rough and irregular, the plates often become temporarily locked together, resisting the motion driven by deeper forces.
As the plates continue their slow movement, the rocks near the boundary deform, storing accumulated energy like a stretched rubber band. This process is described by the elastic rebound theory, which explains how rocks accumulate strain energy over time. The rocks sustain this deformation until the stored stress exceeds the rock’s internal strength and the frictional resistance along the fault.
Once the rock’s limit is reached, the fault ruptures suddenly, causing the locked sections to slip and “snap back” toward their original shape. This abrupt movement releases the stored energy in the form of seismic waves, which travel through the Earth and cause the ground shaking felt during an earthquake. The magnitude of the resulting earthquake is directly related to the extent of the rupture area and the total displacement along the fault.
Earthquakes at Convergent Boundaries
Convergent boundaries, where plates push toward each other, produce the largest and deepest earthquakes globally, accounting for approximately 80% of seismic activity. This boundary type is divided into subduction zones and continental collision zones, each generating distinct seismic patterns. Subduction occurs when a denser oceanic plate sinks beneath a less dense plate, which can be continental or oceanic.
The interface between the subducting and overriding plates, known as the megathrust fault, generates the planet’s most powerful earthquakes, reaching magnitudes of 9 or greater. As the oceanic slab descends, it remains cold and brittle, leading to deep-focus earthquakes extending down to 700 kilometers. The sudden upward movement of the overriding plate during a megathrust event can vertically displace the seafloor, generating destructive tsunamis.
When two continental plates converge, neither is dense enough to fully subduct, leading instead to a continental collision. This process results in the crumpling and thickening of the crust, forming vast mountain ranges like the Himalayas. Unlike the narrow seismic band of a subduction zone, collision-zone deformation and associated earthquake activity are scattered across a broader area. These earthquakes are shallow to moderate in depth, and though they can be large, they do not reach the extreme magnitudes seen in megathrust events.
Earthquakes at Divergent and Transform Boundaries
At divergent boundaries, plates move away from each other, creating tensional stress that pulls the crust apart. This motion occurs primarily along mid-ocean ridges where magma rises to form new crust. Earthquakes in these spreading centers are frequent but consistently shallow, typically occurring at depths less than 30 kilometers. Due to the tensional stress and the high temperature of the rock, these earthquakes are small and infrequent along the spreading segments.
Transform boundaries involve two plates sliding horizontally past each other, generating significant shear stress. Since there is no subduction or spreading, crust is neither created nor destroyed at these boundaries. The resulting earthquakes are characterized by a shallow depth, as the rupture occurs near the surface along the fault plane.
These strike-slip faults, such as the San Andreas Fault system, can produce moderate to large earthquakes, sometimes reaching magnitudes near 8. Although they do not generate the megathrust magnitudes of convergent zones, the shallow rupture means substantial energy is released close to the surface. The high concentration of seismic activity is due to the constant friction and stick-slip motion as the plates move past one another.