The Earth’s surface is covered by rigid slabs known as tectonic plates, which are composed of the lithosphere (the crust and upper mantle). These plates are constantly moving at rates of up to 10 centimeters a year, driven by forces within the planet. The locations where two or more plates meet are called plate boundaries. The friction and stress generated by their motion are the primary cause of most global earthquakes. Plates often lock up, allowing immense strain energy to accumulate, which is suddenly released as seismic waves when the rock finally breaks.
Earthquakes at Divergent Boundaries
Divergent boundaries are areas where tectonic plates move away from each other, placing the crust under tensional stress. This motion results in the formation of normal faults, where the land stretches and one block of crust drops down. New crust is created as magma rises from the mantle to fill the gap, a process known as seafloor spreading in oceanic environments. Earthquakes along these boundaries, such as the Mid-Atlantic Ridge, are typically shallow, occurring at depths less than 30 kilometers. Since the rock in these zones is relatively hot and ductile, the resulting seismic events are generally of a lower magnitude and are less destructive than those at other boundary types.
Earthquakes at Transform Boundaries
At transform boundaries, plates slide horizontally past one another along a strike-slip fault, generating powerful shear stress. This movement neither creates nor destroys rock. As the plates grind past, their irregular edges catch and lock, causing elastic energy to build up. When the stress overcomes the friction, the plates suddenly slip, releasing the stored energy as a shallow, intense earthquake. The San Andreas Fault in California, where the Pacific Plate slides past the North American Plate, is the most famous example capable of generating large-magnitude quakes.
Earthquakes at Convergent Boundaries
Convergent boundaries are where two plates move toward each other, resulting in compressional stress. These are the most seismically active regions on Earth, accounting for approximately 80% of all global earthquakes and responsible for the largest and deepest events.
Oceanic Subduction
When an oceanic plate collides with a continental plate or another oceanic plate, the denser plate sinks beneath the other into the mantle in a process called subduction. This area is known as a subduction zone, and it produces megathrust earthquakes, which are the most powerful seismic events recorded, capable of magnitudes greater than 9.0. Earthquakes in these zones can be shallow near the trench, intermediate, or very deep, tracing the path of the descending slab up to depths of 670 kilometers. The intense friction between the two plates generates massive quakes, such as the 2011 M9.0 TÅhoku earthquake off the coast of Japan. As the subducting plate plunges deeper, the pressure and temperature cause it to release fluids that trigger melting in the overlying mantle, leading to volcanic arcs.
Continental Collision
In contrast, when two continental plates converge, neither plate is dense enough to fully subduct, leading to a continental collision. This immense compression causes the crust to crumple, fold, and thicken, resulting in the formation of huge mountain ranges. The Himalayas, formed by the collision of the Indian and Eurasian plates, are a prime example of this mechanism. While these collisions create significant seismic activity, the earthquakes are generally shallower than those found deep within subduction zones, as the deformation is concentrated in the thickened crust.
Earthquakes Not Caused by Plate Edges
Although the vast majority of seismic activity occurs at plate boundaries, earthquakes can happen within the interior of a plate, a phenomenon called intraplate seismicity. These events are caused by stresses transmitted from distant boundary forces or by the reactivation of ancient, buried faults that represent zones of weakness in the crust. The New Madrid seismic zone in the central United States is an example where ancient rift structures are stressed by the ongoing movement of the North American Plate. Intraplate earthquakes are less frequent and generally lower in magnitude than plate-boundary quakes, but they still pose a significant threat. Since regions away from boundaries are often unprepared for seismic shaking, these quakes, like the 1886 Charleston earthquake, can inflict heavy damage.