Mining can cause earthquakes, triggering seismic events known as induced seismicity. Induced seismicity refers to tremors caused or significantly influenced by human industrial activities that alter the stresses and strains within the Earth’s crust. This is distinct from natural tectonic earthquakes, which are caused solely by the movement of the Earth’s plates. The connection involves two primary mechanisms: removing solid material and altering fluid pressures.
Seismicity Caused by Material Removal
The physical act of extracting large volumes of rock, coal, or ore in deep underground operations directly modifies the geological stress field that has been stable for millennia. When material is removed, the enormous weight of the overlying rock is redistributed to the surrounding pillars and walls of the excavation. This process creates highly localized areas of increased stress concentration and decreased stress, disrupting the pre-existing equilibrium.
This stress change can trigger failures in the rock mass, a phenomenon often referred to as a rock burst in metal mines or a bump in coal mines. These seismic events represent the rapid release of stored energy when the rock’s strength is exceeded. Deep mines, particularly those extending below 1,000 meters, are vulnerable because they operate under naturally high stress conditions, making even small disturbances more likely to initiate a significant seismic response.
The process can also activate pre-existing geological features. Mining-induced stress changes cause movement or slip along these discontinuities. The extraction provides the mechanical trigger needed to overcome the fault’s frictional resistance, leading to an earthquake-like event. Historically, deep coal mining operations have been linked to a significant portion of recorded seismic events in certain regions.
Seismic Events Linked to Subsurface Fluid Alterations
Another major mechanism for induced seismicity involves the manipulation of subterranean fluids, a process distinct from the physical removal of rock. This mechanism operates by changing the pore pressure, which is the pressure of fluids within the small spaces and cracks of underground rock formations. Increasing fluid pressure can effectively “lubricate” or destabilize existing faults by reducing the normal stress clamping the fault faces together.
Large-scale industrial activities like the deep injection of wastewater, a common byproduct of oil and gas production, are frequently associated with this type of induced seismicity. The injected fluid migrates into the porous rock and can travel substantial distances to reach a previously locked fault. When the fluid pressure increases enough to overcome the fault’s shear strength, it triggers a slip event and an earthquake.
Examples include the significant increase in earthquakes in parts of the central United States, such as Oklahoma and Kansas, strongly linked to the disposal of oilfield wastewater into deep wells. Other fluid-related activities, such as geothermal energy production and hydraulic fracturing (fracking), can also induce seismic events through similar pore pressure changes.
Assessing the Magnitude and Reach of Induced Earthquakes
Induced earthquakes typically occur at low magnitudes, often below what is felt by the public, but they can occasionally reach magnitudes that cause concern and damage. The largest known human-induced event was a magnitude 6.3 caused by the filling of a large water reservoir. Mining-related events are generally smaller, often in the Magnitude 4 to 5 range, with the largest recorded mining-induced earthquake being magnitude 5.4.
Induced seismicity often occurs in regions that have historically been seismically quiet, introducing earthquake risk to areas where infrastructure is not built to withstand shaking. The events are generally shallow and highly localized, meaning ground shaking is concentrated over a smaller area compared to a major natural tectonic earthquake. While the overall energy release is lower, this localized nature still poses a risk to nearby communities and local infrastructure.
Induced earthquakes have been observed to cause less intense ground shaking than natural earthquakes of the same magnitude, meaning they are felt less severely further away. However, the destructive power of induced quakes near the epicenter can still be substantial. Continuous monitoring is necessary to manage the risk, as the frequency of induced events correlates with the volume and rate of industrial operations.