The ground beneath our feet can shake for reasons beyond the natural movement of tectonic plates. While most earthquakes result from massive geological forces, a distinct category of seismic events is directly linked to human activity. These events demonstrate that industrial and engineering practices can alter the deep subsurface environment enough to trigger ground shaking. Understanding this phenomenon is critical for assessing seismic hazard in modern society.
Defining Induced Seismicity
The specific term for earthquakes caused by human actions is induced seismicity, sometimes referred to as anthropogenic earthquakes. These events differ from natural, or tectonic, earthquakes because the trigger is external and artificial, not geological strain accumulation. Induced events occur when human activities alter the existing stress balance on faults that are already primed for movement. The resulting ground shaking is physically identical to a natural earthquake, involving sudden slip along a fault plane. The key distinction lies solely in the source of the stress change that initiates the slip. Unlike tectonic quakes that primarily occur along plate boundaries, induced seismicity can happen in areas previously considered seismically quiet, such as the central and eastern United States.
Key Human Activities That Trigger Earthquakes
A variety of industrial activities cause induced seismicity by disrupting the natural subsurface equilibrium. The most common cause, particularly in the United States, is the disposal of wastewater via deep injection wells. This wastewater is a byproduct of oil and gas production and is injected deep underground into porous rock formations for permanent storage.
While hydraulic fracturing, or “fracking,” is often cited, the majority of larger, felt earthquakes are linked to these disposal wells, which inject larger volumes of fluid over longer periods. Hydraulic fracturing itself uses high-pressure fluid to fracture rock and release hydrocarbons, and is associated with numerous, smaller micro-earthquakes rarely felt at the surface.
Beyond fluid injection, other large-scale human projects also trigger seismic events:
- The impoundment of large reservoirs by constructing massive dams adds immense weight to the Earth’s crust, increasing stress on underlying faults.
- Mining operations, especially deep-level extraction, can trigger quakes by removing significant amounts of rock and destabilizing the ground.
- Enhanced geothermal systems (EGS) use fluid injection to create or enlarge subsurface fractures, allowing water to circulate and extract heat.
How Human Activities Alter Earth Stress
The scientific mechanism by which fluid injection triggers earthquakes revolves around effective stress and the role of pore pressure. Earthquakes occur when the shear stress overcomes the frictional resistance holding a fault locked. This resistance is controlled by the normal stress, the perpendicular pressure exerted by the surrounding rock mass.
The presence of fluid in the pores and fractures of rock exerts pressure, known as pore pressure. Injecting large volumes of fluid deep underground raises the pore pressure within the rock formation. This increase in fluid pressure effectively pushes the rock grains apart, which reduces the effective normal stress clamping the fault together.
Reducing the effective normal stress lowers the frictional strength of the fault, making it easier for the existing tectonic shear stress to cause a slip. Even a small increase in pore pressure can be enough to trigger an earthquake on a fault that is already close to failure, or “critically stressed.” This pressure change can diffuse outward from the injection well, sometimes triggering earthquakes many kilometers away and months or years after injection began.
Other human activities alter the Earth’s stress field in different ways. The sheer weight of water in a large reservoir created by a dam directly increases the stress on the crust beneath it, a process called reservoir loading. Conversely, the removal of material through mining or the extraction of large volumes of oil, gas, or groundwater can lead to a decrease in pressure and stress, which can also destabilize faults. This change in the stress field can push a pre-existing fault past its breaking point.
Managing and Predicting Induced Earthquakes
Induced earthquakes are potentially manageable because they are tied to controllable human actions. The primary strategy for mitigation involves extensive seismic monitoring networks, which use sensitive instruments to detect small tremors in real time. This monitoring provides data that helps scientists and regulators understand the relationship between operational parameters, such as injection volume and pressure, and the resulting seismic activity.
Regulatory bodies often implement protocols to manage the risk, especially in the oil and gas industry. These protocols include setting prescriptive limits on the maximum pressure or volume of fluid injected into a well, or mandating safe distances from known faults. A common approach is the “traffic light system,” where operations are slowed down or halted entirely if seismic activity reaches a predefined magnitude or frequency threshold.
Predicting the exact timing and magnitude of an induced earthquake remains challenging due to the complexity of subsurface geology and the difficulty of mapping all pre-existing faults. However, detailed site characterization and the use of machine learning algorithms are improving the ability to forecast which areas are most susceptible to induced seismicity. The goal of these management efforts is to prevent damaging earthquakes by modifying or stopping operations before a significant event can occur.