Unlike conventional earthquakes of the same magnitude, the newly-identified hybrid-frequency waveform earthquakes are slower and last longer.

Study area and hybrid-frequency waveform earthquakes (EHWs): (a) hydraulic fracturing activity and seismicity in northeast British Columbia, Canada; the top-right inset shows the geographic location of the study area (red rectangle); blue diamonds: hydraulic fracturing injection wells between January 1, 2014 and December 31, 2016, reported by the British Columbia Oil and Gas Commission; yellow dots: earthquakes during the same period; stars: 10 M 4-5 earthquakes since 2008; black rectangle: study area in which EHWs occurred; (b, c) comparison between representative examples of a typical induced event (black trace) and an EHW (purple trace); both waveforms are from the vertical component of station MG03 with comparable source-station distance (1.32 km vs. 1.65 km) and magnitudes (MW1.5); manually picked P/S-arrivals are marked; the comparison of P/S-pulse shape demonstrates the relatively wider pulses for EHWs; (d, e) the same as (b) and (c) but for waveforms recorded at station MG02; both events show longer coda durations with slightly larger epicentral distances; note that although both types of events have extended coda durations, the EHW contains a relatively larger proportion of lower-frequency energy in the coda. Image credit: Yu et al., doi: 10.1038/s41467-021-26961-x.

“Industrial fluid injection related to unconventional oil and gas production induces earthquakes,” said Dr. Rebecca Harrington, a researcher with the Institut für Geologie, Mineralogie und Geophysik at Ruhr-Universität Bochum, and colleagues.

“The most common perception is arguably that M4+ events generally result from large fluid volumes related to wastewater disposal, particularly in the Central and Eastern United States.”

“However, induced earthquakes associated with hydraulic fracturing injection have recently challenged the conventional wisdom by successively generating larger and larger maximum magnitude earthquakes.

“Using a high-density 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 slow rupture related to hydraulic fracturing injection.”

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

“Recently, numerical models and lab analyses have predicted a process on faults near injection wells that has been observed elsewhere on tectonic faults,” the scientists said.

“The process, termed aseismic slip, starts out as slow slip 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.”

“Our work provides indirect evidence of aseismic loading, and a transition from aseismic to seismic slip.”

The authors interpret the slow earthquakes — dubbed hybrid-frequency waveform earthquakes (EHW) — 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.

“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,” Dr. Harrington said.

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

“But that doesn’t always seem to be the case. While the shaking from a conventional earthquake of magnitude 1.5 in our dataset had died down after about seven seconds, an EHW earthquake of the same magnitude continued to shake for more than ten seconds.”

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

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