Induced seismicity refers to earthquakes that are triggered by human activities associated with oil and gas operations. While seismic events have been observed near hydraulic fracturing sites, this topic is complex and often misunderstood. An earthquake is defined as the sudden slip along a fault, which releases energy and generates ground shaking. Most of these events are extremely small and occur below the surface detection limit, although larger events can sometimes be felt and, in rare cases, may cause damage.
Seismic activity can occur during hydraulic fracturing; however, the events generated are typically very small. The tensile failure associated with fracture creation produces minimal seismic energy, and these microseismic events are commonly used to monitor fracture growth. In contrast, most cases of induced seismicity are linked to long-term wastewater injection rather than the fracturing process itself. The injection of large volumes of fluid increases pore pressure in the subsurface, thereby reducing effective stress and potentially reactivating pre-existing faults.
Field observations indicate that overall earthquake frequency has changed very little over time, although localized increases have been recorded in areas with intensive injection activity. The majority of induced seismic events are very small and not felt at the surface. However, larger seismic events have been associated with high-volume wastewater disposal wells. In many cases, the total volume of water injected for disposal significantly exceeds the volume used during hydraulic fracturing operations.
A well-documented example is Oklahoma, where thousands of injection wells are used for wastewater disposal. Increased seismic activity in this region has been correlated with high injection rates. Many of these wells inject fluids into deep formations, such as the Arbuckle formation, which can hydraulically connect to underlying basement rocks containing pre-existing faults. This connection allows pressure changes to be transmitted to fault zones, increasing the likelihood of fault reactivation.
Seismic events are measured using the Richter magnitude scale, in which each unit increase corresponds to a tenfold increase in released energy. Most microseismic events associated with oil and gas operations are extremely small and difficult to detect at the surface. At these magnitudes, they generally pose no risk to life or property.
To reduce the risk of induced seismicity, operators implement several mitigation strategies. These include careful site selection to avoid known faults, control of injection rates and volumes, and continuous seismic monitoring. In addition, traffic light systems are often used to adjust or stop operations if seismic activity increases. Recycling produced water can also help reduce the need for wastewater disposal and lower the likelihood of inducing seismic events.
