A study team in British Columbia, Canada, has discovered a new sort of earthquake in an injection setting. Seismic occurrences occur at a slower rate than regular earthquakes. Their existence validates a scientific notion that had hitherto gone unsubstantiated by measurements.
In British Columbia, Canada, a Canadian-German research team discovered a novel sort of earthquake in an injection setting. They are slower and endure longer than conventional earthquakes of the same magnitude. Earthquakes are a novel type of induced earthquake caused by hydraulic fracturing, a process employed for oil and gas extraction in western Canada. Researchers from the Geological Survey of Canada, Ruhr-Universität Bochum, and McGill University recorded seismic data from around 350 earthquakes using a network of eight seismic stations located a few kilometers around an injection well.
Around 10% of the earthquakes discovered had distinctive characteristics indicating that they ruptured more slowly, similar to what had previously been observed mostly in volcanic zones. The findings were published in the journal Nature Communications by a group led by Hongyu Yu – initially at RUB, then at the Canadian Geological Survey of Canada – and RUB Professor Rebecca Harrington.
If we understand when the subsurface reacts to the hydraulic-fracturing process with movements that do not result in an earthquake and, as a result, cause no damage to the surface, ideally we could use that information to adjust the injection procedure accordingly.
Rebecca Harrington
Several theories on the origins of earthquakes
To present, two methods have been proposed by researchers to explain the occurrence of earthquakes during the hydraulic-fracturing process. According to the first, the fluid injected into the rock causes a significant pressure increase, resulting in the formation of a new network of fractures in the subsurface rocks around the well. As a result, the pressure rise may be significant enough to unclamp existing faults and cause an earthquake. According to the second mechanism, the increased fluid pressure from subsurface injection causes elastic stress changes in the surrounding rocks that can be communicated over longer distances. If stress changes occur in rocks with faults, it can result in changes that cause the fault to slip and trigger an earthquake.
Recently, computational models and laboratory analyses anticipated behavior on faults near injection wells that has been observed on tectonic faults elsewhere. The process, known as aseismic slip, begins slowly and does not produce any seismic energy. Slow slide can also induce a stress change on adjoining faults, causing them to slip quickly and cause an earthquake. It is difficult to witness in nature due to the absence of seismic energy from aseismic slide and the size of the faults involved. As a result, researchers have failed to document aseismic slip in general with any association to produced earthquakes. The current study’s findings show indirect evidence of aseismic loading and a transition from aseismic to seismic slip.
Modifying the hydraulic fracturing process
The recently detected slow earthquakes are seen by the German-Canadian research team as an intermediate kind of conventional earthquake and aseismic slip – and hence as indirect evidence that aseismic slip can also occur in the vicinity of wells. As a result, the researchers called the occurrences hybrid-frequency waveform earthquakes (EHW).
“If we understand when the subsurface reacts to the hydraulic-fracturing process with movements that do not result in an earthquake and, as a result, cause no damage to the surface,” says Rebecca Harrington, Head of the Hydrogeomechanics Group at RUB, “ideally we could use that information to adjust the injection procedure accordingly.”
Social media, online news, and television news all provide rapid access to information. While rapid knowledge has numerous benefits, it can also lead to cognitive biases like clustering illusion. When we see earthquakes in the news on a regular basis, we believe there are more of them.
Not all quakes spread at the same rate
“We assumed that generated earthquakes behaved like most other earthquakes, with the same rupture speed of two to three kilometers per second,” Rebecca Harrington explains. However, this does not always appear to be the case. While the shaking from a conventional earthquake of magnitude 1.5 faded down after around seven seconds in the researchers’ data set, an EHW earthquake of the same magnitude continued to shake for more than ten seconds.
