6.8: Induced Seismicity

Induced Seismicity

In some intraplate areas of the country, like Oklahoma, earthquake numbers in a single year can exceed even what we experience here in California. The majority of these earthquakes are a result of human activity and are known as induced seismicity Links to an external site.. Humans have induced earthquakes in the past, but the increasing frequency of episodes of induced seismicity has led to more dedicated research into the problem. Evidence points to several contributing factors, all related to types of fluid injection used by the oil industry. Hydraulic fracturing Links to an external site., also referred to as fracking, has been used for decades by oil and gas companies to improve well production. Fluids, usually water, are injected at high pressure into low-permeability rocks to fracture the rock. As more fractures open within the rock, fluid flow is enhanced and more distant fluids can be accessed, increasing the production of a well. In the past, this practice was utilized in vertical wells. However, with the recent advent of horizontal drilling technology, the fracking industry has really taken off. Drillers can now access thin horizontal oil and gas reservoirs over long distances, greatly increasing well production in rocks that formerly were not exploited, driving a boom in US natural gas and oil production.

While there have been many reports in the media that blame the process of hydraulic fracturing for all the increased seismicity rates, this is not the full story. Fracking mainly produces very minor earthquakes, less than magnitude 3 (although it has been shown to produce significant earthquakes on occasion). Instead, the majority of induced earthquakes are actually caused by the injection of wastewater deep underground. Wastewater injection is a byproduct of fracking, so ultimately the industry is to blame. As wells are developed (by fracking or other processes), large amounts of waste fluid, which typically contain potentially hazardous proprietary chemicals, are created. When the fluids cannot be recycled or stored in retention ponds above ground, they are injected deep underground, theoretically deep enough to not encounter oil reservoirs or water supplies. These wastewater wells are quite common and are considered a safe option for wastewater disposal. By injecting this water in areas that contain faults, however, the stress conditions on the faults change as friction is reduced, which can result in movement along faults and earthquakes.

In 2005, the White Wolf fault Links to an external site. located southeast of Bakersfield (Figure 6.20), experienced an earthquake swarm. Peak activity occurred on September 22, 2005 with multiple quakes ranging from M4.3-4.6. Nearby is the Tejon Oil field, which between 2001 and 2010 increased the rate of wastewater injection by over 26.4 million gallons of water each month. Ultimately, this put pressure on the White Wolf fault, which then ruptured, producing the swarm. Want to learn more about fracking? Visit the USGS Myths and Misconceptions About Induced Earthquakes Links to an external site..