An earthquake is a sudden shaking of the Earth’s surface. This occurs when stored energy within the Earth’s rigid outer layer, the lithosphere, is abruptly released. This energy travels outwards in the form of seismic waves, causing the ground to move and deform. Understanding how these powerful forces interact with the Earth’s surface helps explain the various ways ground cracking can occur.
Primary Cause: Surface Rupture
Earthquakes can directly cause cracks in the ground through surface rupture. This occurs when movement along a geological fault—a fracture in the Earth’s crust where rock masses slide past each other—extends to the surface. While not all earthquakes cause visible surface rupture, when they do, the ground breaks along the fault line. The ground on either side of the fault can then be displaced horizontally or vertically, creating a distinct crack or step in the landscape.
The extent of surface rupture can vary significantly, ranging from a few centimeters to many kilometers in length. For instance, the 1906 San Francisco earthquake caused the San Andreas Fault to slip approximately 430 kilometers, with up to 6 meters of horizontal ground displacement in some areas. These ruptures are the most direct and definitive way an earthquake creates large, visible cracks, clearly reflecting the underlying movement of the Earth’s crust.
Secondary Ground Deformations
Beyond direct fault rupture, intense seismic shaking can induce various secondary ground deformations, leading to cracks and other ground changes. Liquefaction is one such phenomenon, where strong shaking causes saturated, loose granular soils to temporarily lose their strength and behave like a liquid. This can lead to the ground settling, flowing laterally, or developing fissures as water and sediment are expelled to the surface. Lateral spreading, a type of ground failure associated with liquefaction, involves the horizontal movement of soil blocks over a liquefied layer, often resulting in large, irregular cracks and ground displacement.
Earthquake shaking can also trigger landslides and rockfalls, especially in unstable slopes or mountainous regions. These mass movements can create extensive cracks and scarps as large masses of soil and rock detach and move downhill. Additionally, the shaking itself can cause widespread cracking in unconsolidated sediments or weak soils, even without direct fault rupture or liquefaction. These cracks often appear as a complex network, reflecting the differential movement of the ground during the seismic event.
Factors Influencing Ground Cracking
Several factors determine whether an earthquake will cause visible ground cracking and the extent of that cracking. Earthquake magnitude plays a significant role; larger magnitude earthquakes are more likely to generate surface rupture and widespread secondary ground deformations compared to smaller ones. The depth of the earthquake’s origin, known as the hypocenter, also influences surface effects; shallower earthquakes tend to produce more pronounced ground cracking at the surface because the energy has less distance to dissipate before reaching the ground. The strength of ground shaking diminishes with increasing distance from the earthquake’s source.
Proximity to the fault line is another important consideration, as ground cracking is typically most severe closest to the active fault and the earthquake’s epicenter. Local geological conditions are also highly influential. Unconsolidated sediments, loose soils, or areas with a high water table are more susceptible to cracking, liquefaction, and landslides than solid bedrock. The presence of pre-existing weaknesses in the ground, such as older fault lines or areas of previous instability, can also predispose an area to more extensive ground cracking during an earthquake.
Identifying Earthquake-Related Cracks
Distinguishing earthquake-induced cracks from those caused by other environmental factors requires careful observation of their characteristics. Cracks resulting from surface rupture are typically linear, extending for considerable distances, and often show clear horizontal or vertical displacement of the ground on either side. These cracks directly align with known or newly formed fault lines. Cracks caused by liquefaction and lateral spreading, conversely, might appear more irregular, forming large blocky patterns or parallel fissures where the ground has spread or settled.
In contrast, cracks from non-earthquake phenomena, such as prolonged drought, expansive soils, or frost heave, usually lack the directional linearity or distinct displacement associated with fault movement. Drought-induced cracks often form polygonal patterns in clay-rich soils, while expansive soils can cause random cracking patterns due to swelling and shrinking. Subsidence due to groundwater withdrawal or natural compaction can create broad depressions and associated cracks, but these typically develop slowly over time, unlike the sudden appearance of earthquake cracks. Observing the context of the cracking, including the presence of other seismic effects like sand boils from liquefaction, can provide further clues to their origin.