New type of earthquake discovered

A Canadian-German research team has documented a new type of earthquake in an injection environment in British Columbia, Canada. Unlike conventional earthquakes of the same magnitude, they are slower and last longer. The events are a new type of induced earthquake that was triggered 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 kilometers, researchers from the Geological Survey of Canada, Ruhr-Universität Bochum and McGill University recorded seismic data of around 350 earthquake. About ten percent of localized earthquakes were found to have unique characteristics suggesting that they rupture more slowly, similar to what has been observed previously mainly in volcanic areas.

The group led by Hongyu Yu – first at RUB, then at the Geological Survey of Canada of Canada – and RUB professor Rebecca Harrington describe the findings in the journal Nature Communications, published online November 25, 2021.

Several theories on the origins of earthquakes

To date, researchers have explained the occurrence of earthquakes in the process of hydraulic fracturing with two processes. The first says that the fluid pumped into the rock generates a pressure increase large enough to generate a new network of fractures in the underground rocks near the well. As a result, the increase in pressure may be large enough to unblock existing faults and trigger an earthquake. According to the second process, increasing the pressure of the fluid from the injection into the subsoil also exerts changes in elastic stresses on the surrounding rocks which 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 laboratory analyzes have predicted a fault process near injection wells that has been observed elsewhere on tectonic faults. The process, called earthquake slip, begins with a slow slip that does not release any seismic energy. Slow sliding can also cause a stress change on neighboring faults, causing them to slide quickly and lead to an earthquake. The lack of seismic energy from the seismic slip and the size of the faults involved make it difficult to observe in nature. Researchers have therefore not yet been able to widely document the seismic slip with any association with induced earthquakes. The work of this study provides indirect evidence of the seismic load and of a transition from a seismic slip to a seismic slip.

Modification of the hydraulic fracturing process

The German-Canadian research team interprets the recently discovered slow earthquakes as an intermediate form of conventional earthquake and seismic landslide – and therefore as indirect evidence that seismic landslide can also occur near wells. The researchers therefore dubbed the events the hybrid frequency waveform (EHW) earthquakes.

“If we understand how the subsoil reacts to the process of hydraulic fracturing with movements that do not cause an earthquake and, therefore, do not cause any damage to the surface, we could ideally use this information to adjust the injection procedure accordingly, ”as Rebecca Harrington, head of the hydrogeomechanics group at RUB, describes an implication of the study.

Not all earthquakes propagate at the same rate

“We had assumed that the induced earthquakes behaved like most other earthquakes and had about the same breaking speed of two to three kilometers per second,” says Rebecca Harrington. But that doesn’t always seem to be the case. While the tremors of a conventional 1.5-magnitude earthquake in the researchers’ data set subsided after about seven seconds, an EHW earthquake of the same magnitude continued to tremble for more than ten seconds.

Yu H, Harrington RM, Kao H, Liu Y, Wang B. Fluid injection induced earthquakes characterized by hybrid frequency waveforms manifest the transition from seismic slip to seismic slip. Nat Common. 2021; 12 (1): 6862. do I:10.1038 / s41467-021-26961-x

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