The Earth’s crust is composed of massive, moving segments known as tectonic plates. Their slow, grinding motion and constant interaction are the primary cause of nearly all earthquakes. An earthquake is the sudden slip on a geological fault, releasing stored-up energy as seismic waves. This occurs when stress built up from plate movement overcomes the friction holding the rocks together. The way these plates interface determines the earthquake’s characteristics, such as its depth and magnitude.
Divergent Boundaries Pulling Apart
Divergent plate boundaries are zones where two tectonic plates are actively moving away from each other, placing the crust under tensional stress. As the plates separate, the lithosphere is stretched and thinned, which leads to the formation of new crust from upwelling magma. This process is most clearly seen along mid-ocean ridges, such as the Mid-Atlantic Ridge, where seafloor spreading occurs.
The earthquakes generated here are characteristically shallow, typically occurring at depths less than 30 kilometers. This shallow depth occurs because the rock below this point is too hot and weak to fracture or store the necessary elastic energy for a large seismic event. Consequently, earthquakes at divergent boundaries are generally smaller in magnitude and pose a low risk, especially since most occur beneath the ocean. On continents, rifting zones like the East African Rift also exhibit shallow earthquake activity.
Transform Boundaries Sliding Past
Transform boundaries are defined by two plates sliding horizontally past one another, neither creating new crust nor destroying old crust. This grinding motion subjects the rocks to significant shear stress. The San Andreas Fault in California is the most widely recognized example of this boundary type, separating the Pacific and North American plates.
Earthquakes along transform faults are shallow, often occurring within the upper 25 kilometers of the crust. The movement is confined to a vertical fault plane, but the potential for high-magnitude events is significant. These faults can lock up for long periods, allowing massive strain to accumulate before a sudden, large-scale release. Such events are often destructive because they occur near densely populated continental areas, sometimes reaching magnitudes up to M8.
Convergent Boundaries Pushing Together
Convergent boundaries are the most geologically complex and seismically active zones on Earth, formed where plates move toward each other. This collision subjects the crust to immense compressional stress and produces the greatest variety of earthquake characteristics, including the world’s largest events. These boundaries are subdivided based on the type of crust involved in the collision.
When a denser oceanic plate sinks beneath a continental or younger oceanic plate, the process is known as subduction. This creates an inclined plane of seismic activity called the Wadati-Benioff zone, which extends deep into the mantle. Earthquakes in the shallowest part of this zone, where the two plates are locked together, can produce megathrust earthquakes, which are the largest recorded seismic events globally, sometimes exceeding magnitude M9. The sudden slip along this shallow fault can also displace the seafloor, generating destructive tsunamis.
As the subducting slab descends, it continues to generate earthquakes that become progressively deeper further inland from the plate boundary. Convergent boundaries produce the full spectrum of earthquake depths, detected down to a maximum depth of about 700 kilometers. These deep earthquakes occur because the cold, dense subducting slab remains brittle enough to fracture even under the extreme pressures and temperatures of the deep mantle.
In a continent-continent collision, such as the one that formed the Himalayas, neither buoyant plate fully subducts. Instead, the crust buckles, folds, and thickens, leading to extensive mountain building. The resulting earthquakes are strong and distributed over a wide swath of the thickened crust. These events are related to movement along major thrust faults and can occur down to depths of about 70 kilometers.
The Spectrum of Earthquake Activity
The mechanism of plate interaction directly dictates the earthquake profile observed at each boundary type, creating a distinct spectrum of seismic activity. Divergent boundaries, characterized by tensional stress, produce small-magnitude events concentrated at shallow depths, generally less than 30 kilometers, due to the thin, hot crust present at spreading centers. Transform boundaries, under shear stress, also produce shallow earthquakes but are capable of generating large-magnitude events up to M8, particularly where the faults are long and located on continents. In contrast, convergent boundaries, driven by compression and subduction, are responsible for the widest range of depths and the largest possible magnitudes. Convergent zones are the only setting where earthquakes occur from the surface down to approximately 700 kilometers deep, and the largest earthquakes, the megathrust events, are exclusively found at these boundaries.