Induced or natural? New insights into an unusual series of earthquakes in Canada: GFZ

summary

Between November 2022 and March 2023, an unusual series of earthquakes struck the area around the Peace River in the northern Canadian province of Alberta. Since there is also industrial activity in the geological subsurface of the region, the question has arisen as to whether the earthquakes are caused by action or of natural origin. A study by researchers led by Hannes Vaciura-Bathke and Torsten Damm of the German Research Center for Geosciences in Potsdam, together with colleagues from Canada, confirms suspicions that the quakes were caused by industrial wastewater disposal that began 10 years ago. The results are based on a detailed analysis of earthquakes using seismic measurements and satellite-based radar (InSAR), which were analyzed using modern statistical methods. The study was published in the journal Nature Communications Earth & Environment.

Series of earthquakes near the Peace River: natural or man-made?

In November 2022, a series of earthquakes struck a relatively seismically quiet site near the town of Peace River in northern Alberta, Canada. The M5.2 main earthquake on November 30 was preceded by four foreshocks with magnitudes higher than 4.0. This was followed by a longer series of aftershocks, including two with magnitudes of 4.5 and 4.6 Mw on March 16, 2023.

There is extensive industrial activity in the area, including extraction of heavy oil by steam injection and disposal of wastewater in large quantities at various depths in the rocks. This has led to debate about whether unusually strong earthquakes are of natural origin or caused by human activity.

Earthquakes caused by human activity usually occur in weak areas in rock. They are similar to natural earthquakes, but they begin at shallower depths. Although the area is fairly quiet seismically, there have been natural earthquakes and earthquakes caused by industrial activity within 500 kilometers of the Peace River in the past.

In 2023, local authorities initially assumed that the earthquakes were natural, as there had been no hydraulic fracturing in the vicinity during the period in question, injection rates for saltwater disposal had not changed in the previous year, and a preliminary estimate of the magnitude of the earthquakes was made. The focal depth of six kilometers was below expectations for induced seismicity at this location.

However, initial scientific investigations led to a different assessment: Schultz et al1 interpreted the earthquakes primarily as a result of wastewater injection into a deep disposal well about 3.5 km east of the main shock epicenter. It extends into the aquifer system within the so-called Leduc Formation.

Research project on the origin of earthquakes

A team led by Hannes Vaciura-Bathke, scientist in the GFZ 2.1 section “Physics of Earthquakes and Volcanoes,” and Torsten Dahm, head of the same department and professor at the University of Potsdam, has now been able to contribute to further clarification. From seismic processes. In collaboration with scientists from Canada – from the University of Calgary, the Canadian Hazard Information Service and the Canadian Center for Mapping and Earth Observation – they investigated the series of earthquakes in the Peace River in detail. Their study, published in the journal Nature Communications Earth and Environment, confirms colleagues' assumptions that the earthquakes were influenced by wastewater injection.

“We wanted to analyze exactly how, where and when the earthquakes occurred so that we could draw conclusions about their origin,” says Hannes Vaciura-Bathke, explaining the aim of the study. The researchers therefore analyzed the main earthquake and more than 130 aftershocks that occurred between November 2022 and March 2023. They used different methods to understand the characteristics of the main shock, current weak zones, and the potential for future earthquakes.

In particular, they relied on both seismic data and a special type of remote sensing data called interferoscopic radar (InSAR), which can measure deformations of the Earth's surface. To analyze the data, they used a statistical method known as Bayesian inference. This is a concept that allows hypotheses resulting from modeling to be tested using successively integrated data. “Combining InSAR and seismology helps significantly reduce the uncertainty in our analyses,” says Vaciura-Bathke.

Anatomy of a fault system

The researchers were able to reconstruct the source of 132 earthquakes and the exact location and rupture characteristics of eight major events. The distribution of earthquakes shows an anatomy of a rupture regime with four nearly parallel zones of weakness extending to the northwest, which are favored by reactivation in the current stress regime. However, these zones are intersected by an occasional fault zone that is difficult to reactivate. It appears that some areas of the weak zones have not been ruptured, indicating the continued risk of earthquakes.

The main shock originated at a depth of about five kilometers, propagated upward along the weak zone toward the injection source and terminated at a depth of about two kilometers.

Earthquakes caused by industrial salt water disposal

“Our results support the conclusion that wastewater drainage in the Leduc Formation may have played a major role in stimulating this sequence,” Hans Vaciura-Bathke summarizes. “If the assessment is correct, the main earthquake in this series is the strongest earthquake to occur in Western Canada to date.”

In general, proximity to crystalline rock is an important geological risk factor for earthquakes caused by human activity. The Leduc Formation, into which wastewater was injected, lies directly above the Precambrian basement. This direct connection increases the likelihood of a hydrological connection between the Leduc Formation and deeper weak areas. Such a correlation supports the argument that earthquakes were influenced by continuous injection.

Diagnosis with consequences: Even ten years after the start of injection, earthquakes can occur for the first time

“The unusual time period of eleven years between the start of the wastewater injection and the activation of the fault is particularly remarkable. This reflects the long time it apparently took for the rock to reach the critical state,” says Hannes Vaciura-Bathke.

“Our observations show that weak zones in the rock can be activated by continuous injection of wastewater, even ten years after disposal activities begin. So it is possible that more disposal could lead to more earthquakes,” says co-author Torsten Dahm. “The results from Canada show how long it can take for earthquakes to trigger and how important it is to monitor areas with underground industrial activities in the long term.”

Original post

Faciura-Bathke, H., Detmer, J., Biegel, K. et al. Bayesian inference explains the anatomy of a fault system and renewed earthquakes caused by continued saltwater disposal. Common Earth Environment 4, 407 (2023). DOI: 10.1038/s43247-023-01064-1https://doi.org/10.1038/s43247-023-01064-1

1 Schultz, R. et al. Disposal of bitumen recovery at the site triggered the ML 5.6 Peace River earthquake. Geophys. Precision. Lett. 50, e2023GL102940 (2023).

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