An earthquake is the sudden release of stored energy in the Earth’s lithosphere, typically along a geological fault, which generates seismic waves and results in intense ground shaking. The consequences of this massive energy discharge create a complex chain of physical, geological, and secondary hazards. These effects range from direct ground deformation to wide-ranging environmental and infrastructural failures.
Immediate Physical Ground Effects
Ground shaking and vibration are the most recognizable and widespread effects of an earthquake, acting as the primary mechanism of destruction. This vibration is caused by the passage of seismic energy waves, including P-waves, S-waves, and the more damaging surface waves like Love and Rayleigh waves, which cause both horizontal and vertical motion. The intensity of this shaking is influenced by the earthquake’s magnitude, distance from the fault rupture, and local geological conditions, which can amplify wave energy in softer sediments. The duration of strong ground motion can vary significantly, with major events sometimes causing violent shaking for several minutes.
Structural damage and collapse result when ground shaking subjects buildings, bridges, and infrastructure to forces they may not be designed to withstand, particularly the lateral forces generated by seismic waves. Structures built on loose, unconsolidated sediment often experience greater damage due to the amplification of seismic waves compared to those built on solid bedrock. Failures commonly manifest as a “soft-story” collapse, where a less rigid ground floor gives way, or as the “pancaking” of upper floors when column and slab connections fail.
The most direct geological consequence is surface faulting and rupture, which occurs when the fault break extends all the way to the Earth’s surface. This effect is confined to a narrow zone directly above the fault line, but the resulting displacement is permanent and devastating. The ground can be offset laterally or vertically by several meters, creating a visible scar known as a fault scarp. Any structure built directly across this rupture zone, such as a pipeline, road, or building foundation, is inevitably torn apart by the differential movement of the ground on either side of the fault.
Geologic and Slope Instability
A severe secondary hazard triggered by ground shaking is liquefaction, a process where saturated, loose, granular soils temporarily lose their strength and behave like a viscous liquid. The cyclic stresses from seismic waves cause the water pressure within the soil pores to increase rapidly, reducing the effective stress between soil particles to near zero. This transformation undermines the foundation of structures, causing buildings to sink, tilt, or even float in the liquefied material. Evidence of this process often appears as “sand boils,” where a mixture of water and sand erupts onto the surface through fissures in the ground.
Earthquake-induced landslides and rockfalls represent the mass movement of earth and debris destabilized by seismic forces. Strong shaking reduces the internal friction and shear strength of soil and rock masses on slopes, initiating their rapid downward movement. These landslides can involve massive volumes of material, blocking transportation routes, damming rivers, and burying entire communities. The potential for this hazard is highest in mountainous regions or areas with steep coastal cliffs, where the natural stability of the terrain is already marginal.
A more subtle, regional consequence is ground subsidence and elevation change, which involves the permanent vertical deformation of the Earth’s crust over a large area. This effect is particularly noticeable in coastal zones where tectonic uplift or subsidence occurs along a fault boundary. A sudden drop in elevation can cause permanent inundation of coastal land, while uplift can expose new areas of the seafloor. Additionally, the compaction of soil, particularly following liquefaction, can cause widespread, permanent lowering of the ground surface.
Water-Mediated and Secondary Hazards
The most far-reaching water-related consequence is the tsunami, a series of immense ocean waves primarily generated by the sudden vertical displacement of the seafloor during a subduction zone earthquake. When one tectonic plate thrusts beneath another, the overlying plate can snap upward, displacing the entire column of water above it. In the deep ocean, these waves travel at speeds comparable to a jet airliner. However, as they approach a shallow coastline, their speed decreases and their height dramatically increases, resulting in devastating coastal flooding and erosion.
In urban areas, fires are a major secondary hazard that often causes more extensive damage than the initial ground shaking. Earthquakes frequently rupture natural gas lines, break electrical wiring, and overturn heating appliances, providing both a fuel source and an ignition point. The widespread failure of water mains, often compounded by liquefaction, severely hampers the ability of firefighters to suppress the resulting conflagrations. The 1906 San Francisco earthquake is a historic example where uncontrolled fire ultimately consumed the majority of the city.
A seiche is a phenomenon involving standing waves that oscillate in enclosed or semi-enclosed bodies of water, such as lakes, reservoirs, bays, or even swimming pools. The seismic waves cause the water body to rock back and forth, similar to water sloshing in a bathtub. While typically smaller than tsunamis, large seiches can cause localized flooding and damage to infrastructure like dams or harbor facilities. The 1964 Alaska earthquake, for instance, generated seiches that were recorded across water bodies throughout North America.
Seismic activity can cause noticeable changes in groundwater flow and well levels. The intense pressure changes and fracturing of rock layers can temporarily or permanently alter the permeability of underground aquifers. This can result in existing wells drying up due to the shifting of water tables or, conversely, the appearance of new springs and mud volcanoes as pressurized water is forced to the surface. Water quality may also be temporarily affected as shaking dislodges fine sediment, causing well water to become turbid.