A divergent plate boundary is a location where two tectonic plates are actively moving away from each other. This motion creates new crust as magma rises from the mantle to fill the gap, and it generates seismic activity. Earthquakes do occur at divergent boundaries, but their characteristics are fundamentally different from the large, deep events associated with plates colliding or sliding past one another. The seismicity along these spreading zones is a direct result of the Earth’s lithosphere being pulled apart.
The Mechanism of Tensional Stress and Normal Faulting
The entire process of plates separating is driven by tensional stress, or extensional stress. This stress acts to stretch and pull the brittle outer layer of the Earth, the lithosphere, in opposite directions. As the crust stretches, it experiences strain that eventually exceeds the rock’s strength, causing it to fracture and move.
The fracturing that occurs under tensional stress results in a specific type of break known as a normal fault. In a normal fault, the block of crust situated above the fault plane (the hanging wall) slides downward relative to the block below it (the footwall). This downward motion accommodates the lengthening and thinning of the crust as the plates pull away from the spreading center.
The sudden slip along these normal faults releases the stored energy in the form of an earthquake. This contrasts with the compressional stress at convergent boundaries that creates reverse faults. The rigid, colder upper crust can only stretch so far before it breaks instantly. This brittle failure and subsequent rapid movement along the fault plane is the mechanism for generating seismic waves at divergent zones.
Seismic Properties of Divergent Zone Earthquakes
Earthquakes at divergent boundaries are characterized by a consistently shallow depth and low-to-moderate magnitudes. These seismic events occur at depths less than 30 kilometers, often closer to 10 kilometers beneath the surface. This shallow focus is directly related to the high heat flow and thinness of the lithosphere at spreading centers.
Below a certain depth, usually around 20 to 30 kilometers, the temperature of the rock becomes so high that it behaves in a ductile, or plastic, manner. In this hot, weak region, the rock deforms slowly without fracturing, preventing the buildup of elastic strain necessary to produce a large earthquake. Therefore, all seismic activity is confined to the cooler, more rigid upper crust.
The magnitudes of these earthquakes are smaller than those at subduction zones, generally falling below a moment magnitude of 6.0. While some events may reach up to magnitude 7.0, the limited brittle zone prevents the massive ruptures that generate the largest, most destructive earthquakes globally. The energy is released in more frequent, smaller bursts rather than accumulating over centuries for a single catastrophic event.
Key Locations Where Divergent Earthquakes Occur
Divergent boundaries manifest in two primary geological settings: oceanic ridges and continental rift zones, both of which exhibit distinct patterns of seismicity. The Mid-Atlantic Ridge (MAR) is the most prominent example of an oceanic spreading center, a vast underwater mountain range where the North American and Eurasian plates, and the South American and African plates, separate.
Seismic activity along the MAR is frequent but low-level, with most events occurring along the numerous transform faults that perpendicularly offset the main spreading axis. These transform faults accommodate the differential spreading rates along the jagged ridge. The side-by-side shearing motion on these segments creates a higher concentration of moderate earthquakes, typically in the magnitude 4.5 to 5.4 range, with a focal depth of 10 kilometers.
The East African Rift Valley (EAR) provides the best example of a continental rift, where the African plate is slowly pulling apart. Since continental crust is thicker and more complex than oceanic crust, the seismicity here can be more scattered and sometimes reach higher magnitudes than ocean ridge events. Earthquakes in the EAR system have been recorded with moment magnitudes up to magnitude 7.1, though the average focal depth remains shallow, clustering around 10 kilometers.