A Canadian and German led research team have discovered and documented a new type of earthquake in an injection environment in British Columbia, Canada.
Unlike conventional earthquakes of the same magnitude, this newly discovered earthquake is slower and lasts longer. These documented events are a new type of induced earthquake that have been sparked by hydraulic fracturing, a method used in western Canada for oil and gas extraction.
With a network of eight seismic stations surrounding an injection well at distances of a few kilometres, researchers from the Geological Survey of Canada, Ruhr-Universität Bochum (RUB), and McGill University have recorded seismic data of approximately 350 earthquakes. Around 10% of the located earthquakes resulted in exhibiting unique features suggesting that they rupture more gradually, similar to what has previously been observed in volcanic areas.
The group, headed by Hongyu Yu, at the Canadian Geological Survey of Canada, and RUB Professor, Rebecca Harrington, illustrates the results in the journal Nature Communications, published online on 25 November 2021.
Earthquake origin theories
Currently, researchers have explained the occurrence of earthquakes in the hydraulic-fracturing activity with two processes. The first suggests that the fluid pumped into the rock generates a pressure increase that is substantial enough to generate a new network of fractures in the subsurface rocks near the well. This results in a pressure increase that can be large enough to unclamp existing faults, thus triggering an earthquake.
According to the second process, 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 produce the fault to slip, causing a quake.
Recently, researcher’s numerical models and lab analyses have predicted a process of faults near injection wells that have been observed elsewhere on tectonic faults. This process, labelled an aseismic slip, begins as a slow slip that does not release seismic energy. The slow slip can also produce a stress change on nearby faults that causes them slip rapidly, resulting in an earthquake. The lack of seismic energy from aseismic slip and the size of the faults involved make observation difficult to observe in nature. Researchers have therefore not yet been able to document aseismic slip broadly with any association to induced earthquakes. The work of the current study provides indirect evidence of aseismic loading, and a transition from aseismic to seismic slip.
Modifying the hydraulic fracturing process
The German-Canadian research team has interpreted the recently discovered slow earthquakes as an intermediate form of conventional earthquake and aseismic slip, and thus as indirect evidence that aseismic slip can also occur in the vicinity of wells. The researchers therefore dubbed the events hybrid-frequency waveform earthquakes (EHW).
“If we understand at which point the subsurface reacts to the hydraulic-fracturing process with movements that don’t result in an earthquake and, consequently, cause no damage to the surface, ideally we could use that information to adjust the injection procedure accordingly,” said Rebecca Harrington, Head of the Hydrogeomechanics Group at RUB.
Earthquakes spread at differing rates
“We’d assumed that induced earthquakes behave like most other earthquakes and have roughly the same rupture speed of two to three kilometres per second,” added Harrington. However, that does not 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 seven seconds, an EHW earthquake of the same magnitude is recorded to continue to shake for more than ten seconds.