The geosphere includes the crust, mantle, and core, encompassing all rocks, landforms, and the processes that shape them. An earthquake is the shaking of the Earth’s surface resulting from a sudden release of stored energy in the lithosphere, which generates seismic waves. This energy release occurs when accumulated stress along a geologic fault overcomes frictional resistance, causing a rapid slip. Earthquakes impose changes ranging from visible surface ruptures to deep, subterranean stress and fluid shifts.
Direct Surface Deformation
The permanent deformation of the Earth’s surface along the fault line is the most visible manifestation of a major earthquake. This ground rupture occurs when the crust breaks and displaces as the two sides of the fault slip past each other. The resulting movement is categorized into vertical or horizontal displacement, depending on the type of fault involved.
In dip-slip faults, the land surface experiences significant changes in elevation, either through uplift or subsidence. Uplift creates a fault scarp, a cliff-like feature where one side of the land is permanently raised relative to the other, sometimes reaching several meters in height. Conversely, subsidence is the sinking of the land surface, often seen on the downthrown side of a normal fault or on the side of a thrust fault.
Strike-slip faults, like the San Andreas Fault, involve horizontal motion, where the land on one side shifts laterally relative to the other. This shearing motion can offset linear features such as roads, fences, and stream channels by distances ranging from a few centimeters to many meters in a single seismic event. Cumulative displacements from multiple large earthquakes over geologic time contribute to the formation of dramatic landforms, such as mountain ranges and rift valleys.
Secondary Ground Instability
Away from the immediate fault rupture zone, the intense vibrational energy from seismic waves destabilizes slopes and loose, water-saturated sediments, leading to significant mass movement and ground failure. These secondary effects are induced by the traveling energy of the earthquake, not the primary fault slip itself.
Liquefaction occurs in loose, sandy or silty soils that are saturated with groundwater. The rapid back-and-forth motion of the seismic waves causes the water pressure between the soil grains to increase, temporarily transforming the solid ground into a fluid-like state. This loss of strength allows rigid structures, like buildings and pipelines, to tilt, sink, or be damaged by lateral spreading of the liquefied ground.
Seismic shaking is a major trigger for landslides, rockfalls, and slumping. The vibrations reduce the shear strength of the soil and rock on slopes, causing the material to be dislodged and mobilized downslope. These coseismic landslides can be devastating, altering the landscape by damming rivers or creating new landslide-prone areas.
Subsurface Stress Redistribution and Fluid Shifts
The subterranean effects of an earthquake involve changes in the forces and fluids within the crust. The sudden release of accumulated strain energy on one fault segment is often transferred to adjacent segments. This process, known as stress redistribution, increases the likelihood of failure on nearby faults.
This transferred stress can trigger aftershocks on nearby, smaller faults or increase the strain on entirely separate, major fault systems. This mechanical interaction suggests that seismic events are often linked in space and time. The changes in deep pressure and strain also impact the hydrogeology of the region.
Earthquakes cause significant shifts in subsurface fluids by altering the porosity and permeability of the rocks. Changes in pore fluid pressure (the pressure of water trapped within the rock’s pores) can lead to sudden changes in groundwater levels in wells, with some experiencing an abrupt rise or fall. The seismic deformation can also create new fractures or open existing pathways, which can change the flow rate of springs and hot springs, or lead to the sudden emergence of new springs. The shaking can facilitate the release of deep-seated gases, such as methane, from pockets within the ground into the atmosphere.