Economic Observer reporter Zhang Ling

According to the official measurement of China Earthquake Networks Center, at 23:00 on August 4, a 3.7 magnitude earthquake occurred in Gaoxian County, Yibin City, Sichuan Province, with a focal depth of 5 kilometers. Less than two hours later, a 3.0-magnitude earthquake occurred in Gao County, with a depth of 7 kilometers.

According to data from the Sichuan Provincial Seismological Bureau, since the 3.2-magnitude earthquake occurred in Jiang'an County, Yibin City on June 21, there have been 32 earthquakes of magnitude 3 or above in Yibin within a month and a half, of which 30 occurred in Gao County. Four earthquakes with strong tremors also occurred in Gao County: the 5.5-magnitude earthquake on June 29th caused 15 injuries; On July 8th, two 5.0-magnitude earthquakes occurred in Gao County within one day; On August 3rd, a 4.8-magnitude earthquake occurred in Gao County. The focal depths of these four earthquakes were all 6 kilometers.

Earthquakes occur frequently in the short term, causing concern among many local residents. Many people suspect that this is caused by large-scale shale gas extraction in the local area.

The Economic Observer noted that on July 9th, on the "Sichuan People's Voice" platform, a netizen asked: "Every time there are people mentioning shale gas during the Yibin earthquake, the authorities come out to refute the rumors and claim that it was not caused by shale gas mining. Is this really not related at all

On July 16, Yibin Earthquake Monitoring Center replied to the netizen that Yibin is located at the junction of Sichuan and Yunnan, at the eastern edge of the central southern section of the north-south seismic belt in Chinese Mainland, and in the central southern section of the Huaying Mountain basement fault zone in the Sichuan seismogeological structure. The geological structure and faults in the territory are relatively complex, providing a geological structure environment for moderate and strong earthquakes. Furthermore, there is no clear evidence to prove the correlation between shale gas development and earthquakes.

On July 10th, on the "Wenzheng Sichuan" platform, a netizen suggested "stopping shale gas mining in Changning, Gaoxian, and Gongxian". He wrote, "From July 8th to 10th, earthquakes occurred frequently in Gaoxian, Yibin. The epicenter depth was shallow, with 15 earthquakes in 3 days. Yibin is located in an earthquake zone, is it really okay to mine in the earthquake zone

Three days later, the Natural Resources and Planning Bureau of Gong County, Yibin City, replied to the netizen, stating that the local shale gas extraction process strictly follows relevant regulatory requirements, and the China Earthquake Administration and Sichuan Provincial Seismological Bureau have not made any qualitative conclusions related to earthquakes and shale gas extraction.

Is the Yibin earthquake related to shale gas? Why frequent earthquakes? Is it possible for a larger earthquake to occur in the future? In response to public concerns, the Economic Observer interviewed Jia Qunlin, an emergency rescue expert from the China Earthquake Administration, and Meng Yuanku, a professor at the School of Earth Science and Engineering, Shandong University of Science and Technology.

Why is it unrelated to shale gas

Based on multiple indicators such as the depth of the earthquake source, the range of influence, and energy release, it can be concluded that the multiple earthquakes in Yibin over the past month are natural tectonic earthquakes and not caused by shale gas mining. Meng Yuanku said that the depth of shale gas mining in Yibin is 2.5-3.5 kilometers, while the recent earthquake source depth in Yibin area is 5-8 kilometers, much deeper than the depth of shale gas mining.

The Chuannan shale gas field of PetroChina is the largest shale gas production base in China, and Yibin is one of the core areas of the base. China Petroleum's first shale gas field with a capacity of billions of cubic meters, the Changning shale gas field, is mainly located in Yibin. The shale gas field spans multiple districts and counties including Xingwen County, Changning County, Gong County, Gao County, Yunlian County, and Xuyong County in Luzhou City.

Meng Yuanku introduced that earthquakes can be divided into natural tectonic earthquakes and artificial earthquakes according to their causes. If measures are not taken properly, shale gas, oil and gas, and coal mining may all lead to earthquakes, but generally only in the mining area. Hydroelectric power station water storage, tunnel excavation, underground passage construction, and subway construction may also cause earthquakes. Overall, artificial earthquakes are mostly microseismic or small earthquakes, and there are unlikely to be medium or large earthquakes.

Why shale gas extraction may trigger earthquakes? Meng Yuanku explained that when extracting shale gas, the "hydraulic fracturing" process is required. During the operation, high-pressure fracturing fluid is injected into the underground shale layer through drilling, forming a large number of artificial fractures inside the rock layer, connecting the natural gas in the shale micropores, and allowing it to be collected and extracted to the surface. Injecting high-pressure fluid can alter the pressure state of underground formations, causing slippage between existing fractures and weak geological structures, thereby posing a risk of inducing earthquakes.

Meng Yuanku explained that if an earthquake is induced by shale gas mining, its impact is mainly concentrated in the core mining area, and there is a risk of damage to surface buildings in the mining area. However, the impact of vibration on the periphery of the mining area is limited. The impact range of natural earthquakes is usually larger and can also exhibit a linear distribution. In addition, natural earthquakes can occur multiple times in a short period of time in some areas, and even trigger hundreds or thousands of aftershocks in multiple locations; The energy released by shale gas mining induced earthquakes is limited and will not produce large-scale aftershocks. Even if aftershocks occur, they are limited to the core mining area.

Jia Qunlin explained that there is a significant difference between artificial earthquakes and natural tectonic earthquakes:

One is the difference in magnitude.

Artificial earthquakes are generally small local earthquakes, with a maximum magnitude of around 5 or higher, but they do not produce large earthquakes of magnitude seven or eight, and the proportion of energy released is much lower than that of natural tectonic earthquakes. For example, a mining earthquake occurred in Jixi City, Heilongjiang Province in 2015, with a magnitude of 3.0 and a depth of 0 kilometers.

The second difference is the depth of the earthquake source.

At the same magnitude, shallow earthquakes have greater destructive power on the surface because seismic waves consume energy during their propagation towards the surface. Natural tectonic earthquakes generally have deeper focal depths, with shallow earthquakes typically occurring between 5-33 kilometers. The focal depth of artificial earthquakes is generally related to the depth of mining or construction operations, and is usually much shallower than natural tectonic earthquakes. Recently, three earthquakes of around magnitude 5 occurred in Yibin City, all of which were shallow earthquakes with a depth of 6 kilometers. When the depth of the earthquake source far exceeds the depth of the shale gas well, there is generally no correlation between the two.

In artificial earthquakes, the depth of the earthquake source is positively correlated with the depth of mining. For example, if the depth of a shale gas well is 1 kilometer, the depth of the epicenter is generally around 1 kilometer, and the epicenter is located at the collapse site. If there are special circumstances such as explosions, the impact may be even greater.

The third difference is the seismic wave.

The seismic waves formed by different types of earthquakes have certain differences, and there may be some changes in the propagation of transverse waves, longitudinal waves, and initial waves, which can be distinguished by instruments. The fracturing layer and fluid pressure are difficult to conduct vertically, while natural earthquakes have a vertical propagation process from the epicenter to the surface, so it is relatively easy to distinguish between the two.

Jia Qunlin introduced that the country attaches great importance to safety work. When shale gas is extracted, there are usually relevant earthquake monitoring points and equipment with high sensitivity and a second level warning. At present, most earthquakes caused by shale gas extraction captured by monitoring stations are of magnitude 1-2. If monitoring stations discover that a certain project frequently causes small earthquakes, they will take measures such as suspending work for rectification.

Will there be larger earthquakes happening again

In the context of frequent earthquakes in the short term, some people are concerned about whether there will be larger earthquakes in the Yibin area in the future.

Jia Qunlin believes that there is no need to worry too much. Based on the current monitoring situation, the geological conditions in Yibin City do not have the structural background for the occurrence of earthquakes of magnitude 7 or above.

He explained that Yibin is located in the Huaying Mountain basement fault zone, where there is periodic small earthquake activity and similar earthquake frequencies have occurred in history. Recently, the main reason for frequent earthquakes in Yibin is the compression between the Indian Plate and the Eurasian Plate, and the secondary reason is the stress changes and secondary adjustments caused by the Wenchuan earthquake. Due to the continuous subduction of the Indian Plate towards the Eurasian Plate and the obstruction of the stable block in the Chengdu Plain, stress is prone to accumulate in areas such as Yibin, leading to the sliding or rupture of seismic zones and the formation of small and medium-sized earthquakes. In addition, after the Wenchuan earthquake, it takes a long time to adjust the stress between the plates to reach equilibrium, which may be accompanied by some small and medium-sized earthquakes during the process.

According to the Yibin Earthquake Prevention and Disaster Reduction Bureau, the Yibin area is located in an earthquake zone and has experienced multiple moderate to strong earthquakes in history. It is a key earthquake monitoring and defense area designated by Sichuan Province. The history of earthquake records in Yibin region is quite long. According to the Book of Han, as early as 26 BC, Yibin had a record of a magnitude 5 earthquake, which is the earliest earthquake recorded in clear historical documents in Sichuan region. In 1610, a 5.5-magnitude earthquake occurred in Gaoxian, marking the first recorded earthquake activity exceeding 5.0 in Yibin City.

At present, it is scientifically impossible to accurately predict the time, location, and magnitude of earthquakes, but from a general perspective, the continuous occurrence of small and medium-sized earthquakes means that energy has been appropriately released, thereby reducing the risk of larger earthquakes, "said Jia Qunlin.

The region itself is distributed with natural faults, coupled with the continuous northward compression of the Indian Plate. The tectonic stress continuously accumulates on the southern edge of the Sichuan Basin and is eventually released through fault displacement, forming earthquakes. Meng Yuanku explained that Yibin is located at the junction of Sichuan and Yunnan, with complex geological structures and highly concentrated crustal stress. However, recent earthquakes in the region have all been induced by secondary fault activity, rather than the overall movement of large fault zones, so the magnitude is generally low, and there have been no earthquakes with a magnitude of 6.0 or above.

Jia Qunlin believes that based on the law of stress release, there may be earthquakes of magnitude 3 to 5 in the Yibin area in the future. This is a process of energy release, and once released to a certain extent, the magnitude will gradually weaken until it subsides.

In the face of earthquake risks, the public should remain vigilant and not overly anxious, "Jia Qunlin said. In recent years, new buildings in China have generally been designed and constructed in accordance with current seismic fortification requirements." Small earthquakes do not damage, medium earthquakes can be repaired, and large earthquakes do not collapse. "Formally constructed houses usually do not collapse on a large scale in earthquakes of around magnitude 6. The public should prepare emergency supplies in advance, pay attention to authoritative information releases in a timely manner, carry out orderly risk avoidance and self rescue and mutual aid according to local government arrangements, and minimize casualties and property losses as much as possible.

(Author Zhang Ling)