Scientists discover a new type of earthquake caused by injecting fluids

In contrast to conventional earthquakes of the same size, the newly identified wave-frequency hybrid earthquakes are slower and last longer.

Study area and Hybrid Frequency Wave Seismicity (EHWs): (a) Hydraulic fracturing and seismic activity in northeastern British Columbia, Canada; The inset upper right shows the geographical location of the study area (red rectangle); Blue Diamond: hydraulic injection wells fractured between January 1, 2014 and December 31, 2016, according to a British Columbia Oil and Gas Commission report; Yellow dots: earthquakes during the same period; Stars: 10 M 4-5 earthquakes since 2008; black rectangle: the study area in which the dangerous alarms occurred; (b,c) Comparison of representative examples of a typical induced event (black trace) and EHW (purple trace); Both waveforms from the vertical component of the MG03 station have a similar distance to the source station (1.32 km vs 1.65 km) and magnitudes (MW1.5); The selected P/S access is manually selected; Comparison of the P/S-pulse shape shows the relatively wider pulses of EHWs; (d, e) the same as (b) and (c) but for the waveforms recorded at the MG02 station; Both events show longer periods with slightly larger center distances; Note that although both types of events have extended coda periods, EHW has a relatively greater proportion of lower-frequency energy in the coda. Image credit: Yu et al. , doi: 10.1038/s41467-021-26961-x.

“The injection of industrial fluids associated with unconventional oil and gas production causes earthquakes,” said Dr.

“Arguably the most common perception is that M4+ events are generally caused by large volumes of fluids associated with wastewater disposal, particularly in the central and eastern United States.”

“However, induced earthquakes associated with hydraulic fracturing injections have recently challenged conventional wisdom by the successive generation of larger and larger earthquakes.

“Using a high-intensity seismic array surrounding an active hydraulic fracturing well in the Montney Shale Formation, British Columbia, Canada, we report a new type of seismic signal that may represent a slow rupture related to hydraulic fracturing injection.”

With a network of eight seismic monitoring stations surrounding the injection well, the researchers recorded seismic data for 350 earthquakes.

About 10% of existing earthquakes have been shown to exhibit unique features indicating that they rupture more slowly, similar to what was observed previously mainly in volcanic regions.

“Recently, numerical models and laboratory analyzes predicted an operation on faults near injection wells observed elsewhere on tectonic faults,” the scientists said.

“The process, called seismic slip, begins as a slow slip that releases no seismic energy.”

“Slow slip can also cause a change in pressure on nearby faults causing them to slide quickly and result in an earthquake.”

“The lack of seismic energy from seismic slip and the magnitude of the faults involved make it difficult to monitor in nature. Therefore researchers have not yet been able to document large-scale seismic slip with any association with induced earthquakes.”

“Our work provides indirect evidence for seismic loading, and the transition from seismic slip to seismic slip.”

The authors interpret slow earthquakes – termed hybrid frequency wave (EHW) earthquakes – as an intermediate form of conventional seismicity and seismic slip – and thus are indirect evidence that seismic slip can also occur near wells.

“If we understand at what point subsurface fracturing is interacting with movements that do not result in an earthquake, and therefore cause no damage to the surface, it is best that we use this information to adjust the injection procedure accordingly,” said Dr. Harrington.

“We hypothesized that induced earthquakes behave like most other earthquakes and have roughly the same rupture velocity of 2-3 km/sec.”

But this does not always appear to be the case. Whereas the vibration from a conventional 1.5-magnitude earthquake in our data set wore off after about seven seconds, the EHW earthquake with the same intensity continued to shake for more than ten seconds.”

The team’s findings appear in the journal Nature Communications.

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H. Yu et al. 2021. Fluid-injection-induced earthquakes featuring hybrid-frequency waveforms show the transition from seismic slip to seismic slip. Nat Common 12, 6862; doi: 10.1038/s41467-021-26961-x

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