A research team has documented a new type of earthquake in an injection environment in British Columbia, Canada.
The group is headed by Hongyu Yu – first at RUB, later at the Canadian Geological Survey of Canada – and RUB Professor Rebecca Harrington describes the results in the journal Nature Communications, published online on 25 November 2021.
This new type of earthquake featured as slower and long-lasting. The hydraulic fracturing triggered a new type of induced earthquake.
Induced earthquakes associated with hydraulic fracturing (HF) injection have recently challenged conventional wisdom by successively generating more prominent and more significant maximum magnitude earthquakes.
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 ten percent of the located earthquakes exhibited unique features, suggesting that they rupture more slowly, similar to previously observed 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 substantial pressure increase to generate a new network of fractures in the subsurface rocks near the well. As a result, the pressure increase can be significant enough to unclamp existing faults and trigger an earthquake.
The second process, 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 observed elsewhere on tectonic faults. The process, termed aseismic slip, starts as a slow slip that does not release any seismic energy. The slow slip can also cause a stress change on nearby faults that causes them to slip rapidly and lead to an earthquake.
The lack of seismic energy from aseismic slip and the faults’ size makes it challenging to observe in nature. Therefore, researchers have 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 German-Canadian research team interprets the recently discovered slow earthquakes as an intermediate form of a conventional earthquake and aseismic slip. It is indirect evidence that aseismic slip can also occur in the vicinity of wells. The researchers, therefore, dubbed the event hybrid-frequency waveform earthquakes (EHW).
The source characteristics of EHWs are identical to those of low-frequency earthquakes widely documented in plate boundary fault transition zones. The distribution of EHWs further suggests a possible role of aseismic slip in fault loading. EHWs could thus represent the manifestation of slow rupture transitioning from aseismic to seismic slip.
“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,” as Rebecca Harrington, Head of the Hydrogeomechanics Group at RUB, describes one implication of the study.
“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,” explains Rebecca Harrington. 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 seven seconds, an EHW earthquake of the same magnitude continued to shake for more than ten seconds.
- Hongyu Yu, Rebecca M. Harrington, Honn Kao, Yajing Liu, Bei Wang. Fluid-injection-induced earthquakes characterized by hybrid-frequency waveforms manifest the transition from aseismic to seismic slip. Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-26961-x