New research links a deadly earthquake sequence that jolted southwest China’s Sichuan Province in February 2019 to the region’s booming shale gas industry, underscoring the critical need to address threats fracking firms pose to local communities in the energy-hungry nation.
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The study, by researchers at the Earth System Science Programme of The Chinese University of Hong Kong, analyses the magnitude 4.9 quake that struck Rongxian County on 25 February last year. It finds compelling evidence that extractive activities nearby triggered two of the foreshocks, and possibly the main event.
As China’s demand for shale gas—natural gas trapped within sedimentary rock underground—grows amid ambitions to cut carbon dioxide emissions, industry players, together with policymakers and academia, must thoroughly evaluate seismic perils involved in the fuel’s extraction, reads the research article, published in Seismological Research Letters.
“Induced earthquakes are common, and especially in shale gas fields,” said Professor Hong Feng Yang, a researcher at the Earth System Science Programme and one of the main authors behind the study. “The industry needs to formulate a strategy to mitigate those risks, and it needs a better understanding of how these shocks occur.”
To extract shale gas, companies drill to depths of several kilometres and fire a mixture of water, sand, and chemicals into rock under high pressure to rupture it.
The method—called hydraulic fracturing, or fracking—has been extensively used around the world, and a substantial number of tremors triggered in the process have been recorded in several countries, including in Canada, the United States, and China. In the United Kingdom, shale gas production was halted due to earthquake concerns last year.
According to the Human-Induced Quake (HiQuake) Database, which tracks human-caused earthquakes, there have been 1,196 projects with reported induced seismicity as of October 2020, of which 33 per cent have been attributed to fracking.
Professor Richard Davies, from Newcastle University in the United Kingdom, said: “There is no doubt that earthquakes can be caused by fracking. We have known this for quite a while, and the evidence is undeniable.”
He said a mechanism many scientists had investigated was the invasion of fluid into a geological fault—a thin zone of crushed rock which separates vast blocks of the earth’s crust.
When rock on one side of a fault suddenly slips due to tectonic stress, waves of energy are released, causing the shaking felt during an earthquake. If fluid is pushed in, it can lubricate the fault, reducing the amount of friction that holds it in place. In most cases, however, it is difficult to confirm with absolute certainty that shocks are man-made, Davies explained.
In Sichuan, a hilly area prone to quakes due to its proximity to fault lines, shale gas production has been expanding rapidly since 2012. The Province—one of three basins home to the majority of estimated national shale gas reserves—accounts for about a third of China’s total shale gas production.
Several tremors in the region have been attributed to the shale gas industry, including the 2018 Xingwen quake with a local magnitude of 5.7.
In the wake of last year’s disaster, thousands of residents held protests at local government headquarters, with many claiming the earthquake was the result of fracking. Companies immediately halted all operations. Officials said the suspension was a safety measure.
Beyond quake concerns, critics of fracking have pointed to a host of potential environmental pollution problems it brings, such as the contamination of drinking water and the leakage of methane—a powerful greenhouse gas—calling the role of natural gas in energy transition plans into question. The process is also extremely water-intensive, requiring millions of litres of injection fluid.
Unusually destructive
Although deemed relatively weak, last year’s quake left a considerable trail of devastation in its wake, killing two people and damaging more than 10,000 buildings. Economic losses were estimated to be more than 14 million RMB (US$2.1 million).
Yang, of the Earth System Science Programme, found this puzzling. The earthquake scientist suspected something rare had happened: If mild shocks aren’t normally this destructive, could the quake have taken place at an extremely shallow depth?
Yang and his team of researchers began to investigate. Using satellite images, they discovered the ground in the area had sagged in some places and lifted in others. Such deformation—uncommon for a quake of such magnitude—confirmed Yang’s initial hypothesis.
Seismic reflection data helped the geologists locate the fault responsible for the shock, at an approximate depth of one kilometre. “Before we started our work, we had not expected a depth this shallow. This was very unusual,” Yang told Eco-Business.
But how exactly the event had come about was still unknown. Earthquakes generally happen at greater depths, where tectonic plates generate more stress as they crunch together, grind side by side, or descend under one another.
Eventually, the team found signs that shale gas firms could be behind the disaster. Fracking happened nearby at the time, with one injection well located precisely at the hypocentre—the point within the earth where an earthquake originates—of one of two major foreshocks, both boasting magnitudes larger than 4. Moreover, both these foreshocks occurred at a depth of around 2.7 kilometres, coinciding with the injection depth.
The researchers identified several mechanisms that might have triggered the foreshocks. They believe when fracking fluid was pumped into the well, it lubricated the fault and caused a dramatic increase in pore pressure within the rock, forcing its grains apart and facilitating its slipping.
The main quake, however, happened well above the shale formation where the foreshocks occurred. And while there was a shale gas well in the ruptured zone, it had an injection depth of about 2.7 kilometres, at least 1.7 kilometres deeper than the focal depth of the mainshock.
This means fracking fluid could not have migrated up into the fault, although stress created within the rock might have. But due to the lack of accurate injection data, the team has not been able to verify this theory.
“We don’t have concrete evidence that the mainshock was induced by fracking. But then, a fault at very shallow depth should not generate an earthquake under normal conditions. We are still conducting further investigations,” said Yang.
Can frackers ever rule out quake risks?
As China’s shale gas production expands, Yang said fracking companies should be obliged to inspect an area’s subsurface prior to launching operations and stay away from all known fault lines, although the distance sufficient to minimise threats might be difficult to determine.
The greater problem, however, is whether there is adequate data available in the first place. “A lot of quakes triggered by anthropogenic activities have occurred on unmapped faults. Some fault lines are very small and not well recorded. This makes it challenging to address these risks,” observed Yang.
Newcastle University’s Davies said depending on their orientation, fault lines could fly under the radar, and even 3D imaging techniques had in some cases failed to pick them up. “Fracking is causing earthquakes, but we can’t predict where they’ll occur if they happen along faults we can’t see.”
“There’s a massive question here: What is enough certainty? What is enough data to be confident the geology isn’t conspiring against you?” he added.
