A Canadian-German research team have documented a new type of earthquake in an injection environment in British Columbia. Unlike conventional earthquakes of the same magnitude, they are slower and last longer.
A study published in the journal Nature Communications stated that these events are a new type of induced earthquake that have been triggered by hydraulic fracturing, a method used in western Canada for oil and gas extraction.
Hydraulic fracturing process
Hydraulic fracturing, known as fracking, is a process used by the oil and gas industry that involves pumping pressurized liquids into a drilled well to create small fractures in subsurface rocks.
With a network of 8 seismic monitoring stations located within a few kilometers of an active gas well in British Columbia, Canada, researchers from the Geological Survey of Canada, Ruhr-Universität Bochum in Germany and McGill University in Montreal have recorded the Canada seismic data for approximately 350 earthquakes.
According to the December 6 McGill University press release, about 10% of existing earthquakes show unique features that indicate that seismic faults associated with them are slowly forming similar to what was previously observed in mainly volcanic regions.
Fluid-injection-induced earthquakes characterized by hybrid-frequency waveforms manifest the transition from aseismic to seismic slip
Not all earthquakes propagate at the same rate
According to a report on the study, published on the Science Alert website, the research team defines these recently discovered slow earthquakes as an intermediate form of conventional earthquakes and seismic slip, and therefore can be considered indirect evidence that seismic slip can occur also near the wells.
“We’d assumed that [fracking] induced earthquakes behave like most other earthquakes and have roughly the same rupture speed of two to three kilometers per second,” explains seismologist Rebecca Harrington of Ruhr-Universität Bochum, Germany.
But that doesn’t always seem to be the case. While the shaking from a conventional earthquake of magnitude 1.5 in the researchers’ data set had died down after about 7 seconds, an EHW earthquake of the same magnitude continued to shake for more than 10 seconds.
Despite this relatively weak intensity “so far”, researchers fear that hydraulic fracturing may cause more earthquakes, as the largest earthquake caused by fracking occurred in China in 2018 and its magnitude was 5.7, and thus it has the same intensity as the earthquake that naturally occurred in Pakistan in 2021 And caused the death of 20 people. So although these manmade earthquakes are rare, they have the potential to cause serious harm.
Theories about the origins of earthquakes
Many previous studies have documented the occurrence of earthquakes as a result of the hydraulic fracturing process according to two mechanisms.
The first says that the fluid pumped into the rock generates a pressure increase substantial enough to generate a new network of fractures in the subsurface rocks near the well. As a result, the pressure increase can be large enough to unclamp existing faults and trigger an earthquake.
The second mechanism states that the fluid pressure increase from injection in the subsurface also exerts elastic stress changes on the surrounding rocks that can be transmitted over longer distances. If the stress changes occur in rocks where faults exist, it can also lead to changes that cause the fault to slip and cause an earthquake.
Recently, numerical models and lab analyses have predicted a process on faults near injection wells that has been observed elsewhere on tectonic faults. The process, termed aseismic slip, starts out as slow slip that does not release any seismic energy. The slow slip can also cause a stress change on nearby faults that causes them slip rapidly and lead to an earthquake.
Although some scientists are convinced that a better understanding of earthquakes caused by fracking will help manage and mitigate the risks associated with them, others who are interested in the future of the Earth question the necessity of fracking from the ground up and the possibility of providing effective alternatives to help us get rid of fossil fuels that inevitably lead us to a catastrophic end.
Sources:
https://www.nature.com/articles/s411-x
https://www.mcgill.ca/research/channels/news/new-type-earthquake-discovered-335376