Ground rupture is the physical breaking, tearing, or displacement of the Earth’s surface, a dramatic geological event often triggered suddenly. This phenomenon is distinct from simple surface cracking, as it involves measurable, permanent offset or movement of the ground over a significant distance. Ground rupture occurs when subsurface stresses or instabilities become so great that the overlying soil and rock layers fail.
Tectonic Faulting During Earthquakes
The most direct and forceful cause of ground rupture is the movement of tectonic plates along a fault line during an earthquake. This rupture happens when the subterranean movement on a fault plane extends all the way up through the crust to the surface layer. The sudden release of accumulated strain energy physically tears the ground along the fault trace.
The type of surface break observed depends directly on the style of fault movement. A normal fault occurs in areas where the crust is being pulled apart (extensional stress), causing the overlying block of rock to move downward. This movement creates a feature known as a fault scarp, a visible step or cliff face on the ground surface. Conversely, a reverse fault, common in areas where the crust is compressed, pushes the upper block up and over the lower block, also forming a scarp.
Strike-slip faults, such as the San Andreas Fault, involve blocks of crust sliding past each other horizontally with little vertical motion. This lateral movement creates fissures and linear features that run parallel to the fault trace, often displacing roads, fences, and stream channels sideways.
Ground Instability from Soil Liquefaction
Ground rupture can also be a secondary effect of strong ground shaking, particularly through the process of soil liquefaction. This occurs when saturated, loose, granular soils temporarily lose their strength and stiffness, causing them to behave like a dense liquid. The shaking from an earthquake increases the water pressure between the soil particles, reducing the effective stress that holds them together.
This fluid-like behavior can lead to the tearing of the ground surface by a mechanism called lateral spreading. On gentle slopes or near free faces like riverbanks, the liquefied soil layer begins to flow horizontally, carrying the solid, non-liquefied crust above it. The movement tears the overlying ground apart, creating extensive fissures and scarps that parallel the direction of the flow.
Another manifestation of liquefaction-induced rupture is the formation of sand boils, where pressurized water and sand erupt upward through cracks in the ground surface. The venting of the water and sediment creates localized, often circular, ruptures on the surface. These phenomena are common in areas with a shallow water table and deposits of unconsolidated sediment.
Gravity-Driven Mass Movements
Large-scale slope failures, broadly categorized as mass movements or landslides, represent another significant cause of ground rupture, frequently independent of tectonic activity. These events occur when the downward pulling force of gravity exceeds the internal shear strength of the soil or rock mass. Triggers often include heavy rainfall, which increases the weight of the slope material and reduces the frictional resistance, or erosion at the base of the slope.
The rupture associated with a landslide typically manifests in two primary ways: the head scarp and the lateral tears. The head scarp is the steep, newly exposed face of the undisturbed ground at the top of the failure, representing the initial break in the surface.
Lateral rupture occurs along the sides of the failing mass, defining its boundaries as it moves downslope. This tearing is a result of differential movement, where the central portion of the slide moves faster than the edges, which are constrained by the adjacent stable ground.
Fissure Formation from Subsurface Volume Loss
Ground rupture can also be caused by the loss of volume beneath the surface, a process that leads to ground subsidence and the formation of long, tensional fissures. This is frequently observed in areas where large quantities of fluids, such as groundwater or oil and gas, are extracted from underground reservoirs. The removal of fluid reduces the pressure that supports the overlying sediment layers, causing them to compact, a process known as hydro-compaction.
This compaction leads to differential settlement, where the ground subsides unevenly, often cracking the brittle surface layers under tension. These earth fissures tend to form at the boundaries of the subsiding basin, particularly near geological features like bedrock ridges that do not compact. The cracks can be several meters wide, running for miles, and are a direct result of the ground being stretched as it sinks.
A more localized and sudden form of rupture occurs in karst terrain, characterized by soluble rock like limestone. Water dissolves the rock, creating underground voids and caves. When the roof of one of these cavities can no longer support its own weight, it collapses abruptly, forming a cover-collapse sinkhole. This sudden, vertical collapse creates a deep, circular depression that can appear with little warning.