Fracking-related seismic activity refer to earthquakes of varying magnitudes that occur as a result of the hydraulic fracturing process. The general cause of these earthquakes is the weakening of pre-existing below ground faults by injecting high pressure fluids in the area surrounding these faults. In oil wells where hydraulic fracturing is used, water with chemical additives known as fracturing fluid is injected into a drilled well under high pressure to fracture the rock, allowing for oil and natural gas to flow into the well more easily. Additionally, proppant - sand or small ceramic spheres - are used to prop open the fractures. Sometimes, areas of an oil and gas reservoir that are already faulted will be targeted for fracturing as they already contain fractures. It is this process that can induce earthquakes.
Additionally, it is important to note that induced seismic activity can be produced by a number of different sources, not just the hydraulic fracturing process. Induced seismic activity has also been connected to enhanced geothermal systems, geologic CO2 sequestration - the process of storing carbon dioxide below ground, and with waste water disposal in wells.
It is the fluids that play an important role in inducing seismicity, in both natural and man-make scenarios. In nature, there are occasionally large earthquakes accompanied by changes in the level of the water table. Generally, fluids lubricate surfaces of fault lines that slip during earthquakes. By artificially injecting fluids - whether it is waste water or fracturing fluid - creates new fractures and injects highly pressurized fluids into fault zones, causing slips that wouldn't occur naturally.
The creation of these earthquakes during the fracturing process can be connected to the increased fluid pressure from the injected fracturing fluid. During the hydraulic fracturing process, when high-pressure fracturing fluid is injected into the ground, there may be an increase in pressure along a fault plane. This fault plane is a flat surface of rock which can slip during an earthquake. If there is enough stress upon this fault plane, it can be "reactivated" and slip, resulting in the possibility of earthquakes. This slipping as a result of pressure occurs when the stress on the fault plane is enough that it overcomes the static friction keeping the fault still.
Currently, there is a large amount of dispute over how large the earthquakes related to hydraulic fracturing activities can actually be. As a result of this, there is a number of different reports of whether or not these induced earthquakes pose any real threat to people. In general, there are two main arguments - one being that these induced earthquakes are so small they pose no threat, the other being that these earthquakes can reach a significant magnitude to cause property damage and pose a risk to humans. This is a topic that is still developing, and research is being conducted to clarify the topic further.
One point of view argues that earthquakes from the injection process in hydraulic fracturing are small - with magnitudes on the Richter scale, a logarithmic scale, below two - known as microearthquakes. Those who argue that these earthquakes are insignificant point to the fact that recently, hundreds of thousands of wells have been involved in the fracturing process. Additionally, these claims put forward that the largest recent fracking-related earthquake had a reported magnitude of 3.6, which can sometimes be detected by people in the upper levels of a building but create vibrations similar to that of a passing truck and pose no serious danger. These reports claim that the majority of the earthquakes that are created are so small that they cannot be detected and thus pose no danger, while some can create detectable earthquakes.
Although some reports still claim that the severity of these earthquakes are limited, recent research is connecting more larger-magnitude earthquakes to hydraulic fracturing activities. In 2013, a sequence of earthquakes - including a 5.7 magnitude quake - that occurred in 2011 near Prague, Oklahoma was reportedly connected to injection processes connected with hydraulic fracturing. This magnitude of quake is significantly larger than the microearthquakes that others argue are the only seismic result of hydraulic fracturing, with the Oklahoma quake damaging 14 homes and other structures.
It has been found that certain wells tend to be more problematic than others, particularly ones that inject a large quantity of fluid. As well, although most earthquakes are generally small, full reactivation of a fault could result in larger magnitude earthquakes. Although the severity of the earthquakes created by fracturing is widely disputed, other forms of induced earthquakes can pose a greater threat. Waste water disposal into deep wells can produce larger earthquakes, with the largest produced magnitude of 5.6. This magnitude of earthquake can move large items of furniture, chimneys and smokestacks can fall, and poorly built structures can crumble.