A team of researchers from Canada and Germany discovered and documented a new type of earthquake in an injection environment in British Columbia, Canada. The recently discovered seismic events are slower than conventional earthquakes. Their existence supports a scientific theory that until now had not been sufficiently substantiated by measurements. The researchers named the new type of earthquake as hybrid-frequency waveform earthquake (EHW).
Unlike conventional earthquakes of the same magnitude, these newly-discovered earthquakes are slower and last longer.
They are a new type of induced earthquake 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 approximately 350 earthquakes.
Around 10% of the located earthquakes turned out to exhibit unique features suggesting that they rupture more slowly, similar to what has previously been observed mainly in volcanic areas.
To date, researchers have explained the occurrence of earthquakes in the hydraulic-fracturing process with two processes.
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.
In 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 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 have been observed elsewhere on tectonic faults.
The process, termed aseismic slip, starts out as a slow slip event 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.
The lack of seismic energy from aseismic slip and the size of the faults involved make it 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.
The team interprets the recently discovered slow earthquakes as an intermediate form of a conventional earthquake and aseismic slip – and thus as indirect evidence that aseismic slip can also occur in the vicinity of wells. They dubbed the events as hybrid-frequency waveform earthquakes (EHW).
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 continued to shake for more than ten seconds.
- New type of earthquake discovered – RUB
- "Fluid-injection-induced earthquakes characterized by hybrid-frequency waveforms manifest the transition from aseismic to seismic slip" – Hongyu Yu, Rebecca M. Harrington, Honn Kao, Yajing Liu, Bei Wang – Nature Communications, 2021, – DOI: 10.1038/s41467-021-26961-x – OPEN ACCESS
Featured image credit: Authors, NatureCommunications
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