Evaluation of Various Forms of Geothermal Energy Release in the Beijing Region, China

Abstract

The energy inside the Earth can not only be released outward through earthquakes and volcanoes but also can be used by humans in the form of geothermal energy. Is there a correlation between different forms of energy release? In this contribution, we perform detailed seismic and geothermal research in the Beijing area. The results show that the geothermal resources in Beijing belong to typical medium-low temperature geothermal resources of the sedimentary basin, and some areas are controlled by deep fault activities (e.g., Xiji geothermal well (No. 17)). The heat sources are upper mantle heat, radioactive heat in granite, and residual heat from magma cooling. The high overlap of earthquakes and geothermal field locations and the positive correlation between the injection water and earthquakes indicate that the exploitation and injection water will promote the release of the earth’s energy. The energy releases are partitioned into multiple microearthquakes, avoiding damaging earthquakes (M_L ≥ 5) due to excessive energy accumulation. Therefore, the exploitation of geothermal resources may be one way to reduce destructive earthquakes. Furthermore, the use of geothermal resources can also reduce the burning of fossil energy, which is of great significance in dealing with global warming.

1. Introduction

The interior of the earth is filled with energy, which originates from the magma and the decay of radioactive materials. The energy can be released into the shallow surface or atmosphere in various ways. They can be fierce and destructive, like earthquakes and volcanoes, or relatively gentle, like hot springs. The difference is that earthquakes and volcanoes represent disasters, while hot springs are clean energy that can be used by humans. It is worth noting that they all originate from the release of energy inside the Earth. Is there a correlation between the different forms of energy release?

Unlike earthquakes and volcanoes, geothermal resources can be used by humans in a gentle way. Geothermal resources are considered one of the ways to combat global warming. The exploitation of geothermal resources has always been the focus of attention [1,2,3,4,5,6,7,8,9,10,11]. According to statistics, the geothermal energy reserves in the upper crust (3–10 km) are 1.3 × 10²⁷ J. In the World Energy Association’s “Energy and Sustainability Challenges” report published in 2000, geothermal energy ranked first among all renewable energy sources [12]. However, induced earthquakes have been observed at various production stages of geothermal energy extraction, including initial injection of geothermal working fluid during stimulation, withdrawal of working fluid from geothermal reservoirs, reinjection of working fluid after heat extraction, and post-well closure [13]. Along with the disturbance of the crustal stress state, some exploitation projects have induced sizable earthquakes, even causing significant disasters and social problems [14,15,16,17,18,19]. For example, Soultz-Sous-Forêts in France [20], Basel in Switzerland [21], and Pohang in South Korea [22]. Therefore, exploring the mechanism of earthquakes induced by geothermal energy mining has been a hot topic in the world.

Efforts have long been made to mitigate or even eliminate induced earthquakes, whether from geothermal or oil and gas extraction. Induced seismicity is often perceived as an unsolicited and uncontrollable side effect of geothermal development [16,23,24,25,26,27,28,29,30,31]. But in fact, in most cases of induced seismicity, many events usually have magnitudes smaller than M_L = 3 and hence without economic consequences [15,32]. Seismicity triggered by fluid injection-induced earthquakes are still natural earthquakes, and their energy still comes from the Earth itself. Therefore, we put forward a conjecture: the total amount of Earth’s energy is fixed, and earthquakes and geothermal are different forms of energy release. Is it possible to reduce the energy released by earthquakes by increasing the energy released by geothermal development?

To test the assumption, we choose the Beijing area for seismic and geothermal research. There are abundant geothermal resources in the Beijing area. Statistically, from 1971 to 2013, the total amount of geothermal resources exploitation quantity in Beijing is 2.87 × 10⁸ m³, and the injection water is 3.02 × 10⁷ m³ (Data from Beijing Geological Archive). In addition, there is a complete seismic network in the Beijing area, with detailed records of earthquakes (M_L ≥ 1) since 1970. Therefore, Beijing is a natural laboratory for studying the relationship between seismic activity and geothermal energy.

2. The Study Area

The North China Craton (NCC) is one of the ancient cratons in the world [33]. It is bounded by the Central Asian orogenic belt in the north and the Qinling—Dabbe orogenic belt in the south. The basement rocks of the NCC consist of biotite-hornblende gneisses and Trondhjemite, Tonalite, Granodiorite (TTG) [34]. Overlying sedimentary layers with a thickness of several thousand meters, mainly carbonate rocks and clastic rocks. During the Yanshan tectonic period, the NCC experienced destruction and thinning, accompanied by a series of volcanic tectonic processes [35,36,37,38,39].

Beijing is located in the northern margin of NCC, high in the northwest and low in the southeast. Tectonic movement is active in the area [40]. The main faults include the Yanqing Fault, Dahuicang Fault, Liangxiang Fault, and Zhangjiakou-Bohai Fault (Figure 1). It is a seismic activity zone in eastern China. In history, there has been the M_L7.8 Tangshan earthquake (28 July 1978), the M_L7.4 Bohai earthquake (18 July 1969), and the M_L8.0 Shanhe-Pinggu earthquake (2 September 1679) [41].

Beijing area is enriched in geothermal resources [42]. At present, there are 10 geothermal fields: (1) Yanqing, (2) Xiaotangshan, (3) Houshayu, (4) Northwest district, (5) Tianzhu, (6) Lisui, (7) Southeast district, (8) Shuangqiao, (9) Liangxiang and (10) Fengheying geothermal field (Figure 1) [43]. The total geothermal resources are about 9.94 × 10¹⁶ KJ, equivalent to 3.39 × 10⁹ t of standard coal (Data from Beijing Geological Archive).

Figure 1

(a) A simple map of China. (b) Schematic map showing the distribution of geothermal fields and location of sampling points in the Beijing area, modified after Liu et al. [43]. 1: Yanqing, 2: Xiaotangshan, 3: Houshayu, 4: Northwest district, 5: Tianzhu, 6: Lishui, 7: Southeast district, 8: Shuangqiao, 9: Liangxiang and 10: Fengheying geothermal field. The size of the symbol of the earthquake label indicates the magnitude of the earthquake.

[Figure omitted. See PDF]

The terrestrial heat flow in Beijing ranges from 16.45 to 383.97 mW/m², with an average value of 65.95 mW/m². The geothermal gradient in the central area of the geothermal field is more than 3.0 °C/100 m. In some areas (Figure 1b, 5: Tianzhu and 6: Lishui), the geothermal gradient is more than 5 °C/100 m. The geothermal resources in Beijing belong to the medium and low-temperature hot water, and the temperature range is 25.0–118.5 °C. The geothermal water is Na-HCO₃∙SO₄ type water with a salinity between 500 and 700 mg/L, with a high content of F and SiO₂, containing a small amount of trace elements, which is used for medical treatment and health care.

Beijing’s climate is a warm, temperate, semi-humid, semi-arid monsoon climate, with an average annual temperature of 9 to 19 °C and annual precipitation of 600 mm. The seasonal distribution of precipitation is very uneven, with 80% of the annual precipitation concentrated in summer. The natural rivers of Beijing run through five major river systems from west to east: the Juma River, the Yongding River, the Beiyun River, the Chaobai River, and the Jiyun River. Most of them originated from the northwest mountain, meandered through the plain to the southeast, and finally merged into the Bohai Sea at the Haihe River.

3. Sampling and Analytical Methods

3.1. Geothermal Water Samples Collection and Analysis

Twenty-six samples of water were collected in Beijing, including hot springs and geothermal wells. All samples were analyzed for anions, cations, trace elements, hydrogen, and oxygen isotopes at the Beijing Institute of Geology of the Nuclear Industry. Detailed sample collection and testing methods can be found at Luo et al. [44]. In short, the waters were collected in a 50 mL clear polyethylene bottle, and the pH and temperature were recorded. Two water samples need to be collected at each geothermal water sampling site, one with ultrapure Nitric acid for cation analysis and the other for hydrogen and oxygen isotopes and anion analysis. Each sample is filtered with a 0.45 μm filter membrane before being tested. The cation and anion were analyzed by Dionex ICS-900 ion chromatograph (Thermo Fisher Scientific Inc., Bremen, Germany), and SiO2 was analyzed by inductively coupled plasma emission spectrometer Optima-5300 DV (PerkinElmer Inc., Waltham, MA, USA). HCO₃⁻ and CO₃²⁻ was determined by acid-base titration with a ZDJ-100 potentiometric titrator. Trace elements were analyzed by Element XR ICP-MS. Multielement standard solutions (IV-ICPMS 71A, IV-ICP-MS 71B and IV-ICP-MS 71D, iNORGANIC VENTURES) were used for quality control (the analytical error margin of major cations and trace elements were less than 10%). MAT 253 was used to analyze hydrogen and oxygen isotopes (reported as δD and δ¹⁸O relative to Vienna Standard Mean Ocean Water (V-SMOW)).

3.2. Geothermal Gas Samples Collection and Analysis

Between April 2022 and April 2023, we collected geothermal gas samples five times at the No. 17 geothermal well. 500 mL glass bottles were used to collect gas by drainage gas collection method [44]. During transportation and storage, glass bottles are kept sealed to prevent contamination by air. The chemical composition of geothermal gas samples was measured using the Agilent Macro 490 portable gas chromatograph with a measurement accuracy of better than 5%. He concentration in hot spring gas samples was analyzed using the Noblesse noble gas isotope mass spectrometer by the Northwest Institute of Eco-Environmental Resources, Chinese Academy of Sciences.

4. Results and Discussion

4.1. Hydrochemistry of Geothermal Waters

The physical properties and chemical and isotopic compositions of geothermal waters are shown in Table S1. The temperature of water varies from 13 to 92 °C. In this study, we divided the samples into three groups according to the sampling sites (Figure 2). The first group of geothermal waters was distributed in the Beijing urban area, and the second group of geothermal waters was collected in the Yanqing basin. In particular, we also classified the geothermal water in group 3, which is similar to that in group 1 in terms of collection location but obviously different from that in group 1 in terms of hydrochemical characteristics. This will be discussed in detail below.

The δ¹⁸O and δD of waters of Beijing are −16.2‰ to −9.6‰ and −92.4‰ to −69.2‰ respectively, which is close to the local meteoric water line (LMWL) of the Beijing δD = 7.0181 δ¹⁸O + 3.5231 (R² = 0.86, n = 36) (Figure 3) [45], suggesting they originated in meteoric. Group 2 is more enriched in light isotope composition than groups 1 and 3. The δ¹⁸O value of a few waters went off the LMWL, indicating that the isotopic exchange of ¹⁸O occurs during the water-rock reaction.

From Figure 2 and Table S1, the geothermal waters are Na∙Ca∙Mg-HCO₃ (group 1), Na-SO₄∙HCO₃ (groups 2) and Na-Cl∙HCO₃ (groups 3) types. Groups 2 have significantly higher concentrations of Na⁺ (84.50–151 mg/L) but lower Ca²⁺ (3.16–46.90 mg/L) and Mg²⁺ (0.04–14.8 mg/L) than group 1(Na⁺ (5.41–135 mg/L), Ca²⁺ (19.20–57.50 mg/L) and Mg²⁺ (2.30–37.10 mg/L)), which may reflect the reaction between groundwater and silicate rocks (Figure 4). It is consistent with the fact that group 2 waters are located in the granite thermal reservoir of the Yanqing basin. Analogously, there were also differences between group 2 and group 3. The anions of group 2 are HCO₃⁻ (17.70–303 mg/L) and SO₄²⁻ (29.70–177 mg/L), while Group 3 contains more Cl⁻ (225–325 mg/L) and HCO₃⁻ (648–1022 mg/L). In the Paleogene period, the gypsum salt layer was widely distributed in the North China Plain [46]. The elevated concentration of SO₄²⁻ could be caused by the dissolution of sulfate minerals, such as anhydrite (CaSO₄) and mirabilite (Na₂SO₄). In addition, the high concentrations of Cl⁻ in geothermal water probably originated from brine or the mixing with a deep fluid [44,47,48]. However, in the Beijing area, the brine has almost no effect on Cl⁻. Because group 3 is well water located near the fault zone. The depth of 3588 m has exceeded the thickness of the sedimentary and reached the top of the magmatic batholith. Moreover, the Cl⁻ versus Na⁺ also indicates that the fluid is non-brined (Figure 5). Therefore, deep Cl⁻-rich magmatic fluids rise along faults and then mix with groundwater to form group 3 of geothermal waters with high Cl⁻ concentration.

Figure 3

δD and δ¹⁸O (relative to V-SMOW) values for waters collected from the Beijing area. The GMWL is a global meteoric water line [49]. The LMWL is a local meteoric water line [45]. Arrows indicate enhanced water-rock reactions.

[Figure omitted. See PDF]

The sedimentary layer in the North China Plain is several thousand meters high, including carbonate and clastic rocks, which well explains the high Ca²⁺ and Mg²⁺ concentrations in group 1 (Table S1) [42,43,46]. Relatively, the sedimentary layer in the granite thermal reservoir of the Yanqing basin is thinner, and the Ca²⁺ and Mg²⁺ concentrations in group 2 are lower. It can be seen from Figure 4 that the weathering characteristics of the silicate rocks of group 2 geothermal waters are significantly greater than those of group 1. Rubidium (Rb) occurs preferentially in K-containing minerals, while Strontium (Sr) occurs preferentially in Ca-containing minerals, and Nickel (Ni) is an extremely compatible element. Using Ni as the regional background value to normalize Rb and Sr, the source of ions in geothermal water can be distinguished. As can be seen from Figure 4, groups 2 and 3 are characterized by the reaction of silicate rocks with water, while group 1 is characterized by the reaction of carbonate rocks with water.

Carbonate rocks, including limestone and dolomite, can be further distinguished by the variation of Ca²⁺ and Mg²⁺ content. The Mg²⁺/Ca²⁺ molar ratio of the geothermal water in the dolomite area is near one, while it is much lower than one in the limestone area [50]. Meanwhile, the Na⁺/Ca²⁺ molar ratio can distinguish the carbonate rock and silicate rock area. As shown in Figure 6, the geothermal water in Beijing mainly comes from dolomite and silicate rock, or a mixture of them, with almost no contribution from limestone.

4.2. Characteristics of Heat Reservoir

The geothermal resources in Beijing belong to typical medium-low temperature geothermal resources of the sedimentary basin. The temperature varied from 25.0 to 118.5 °C, and the heat reservoirs are carbonate rocks [42]. The good thermal conductivity of carbonate rock results in a geothermal gradient in the study area (3–3.5 °C/100 m). All geothermal waters are plotted in the immature water field or partially equilibrated or mixed water (Figure S1). Therefore, pay attention to the applicability of the geothermometer when selecting the temperature scale. Previous studies have shown that the accuracy of the Na-Li geothermometer is higher than that of other thermometers in the carbonate rock region [44,51]. Therefore, the heat storage temperature of geothermal water in the study area was estimated by Na-Li geothermometer, and the results are shown in Table S2. Furthermore, quartz thermometers are also used as a reference [52]. The reservoir temperature and circulation depths of geothermal waters in the Beijing area calculated based on Na-Li and SiO₂ geothermometers are 65–240 °C and 1592–6597 m, respectively [44].

4.3. Origin of High He, H_2, and CH₄ Concentrations in Geothermal Gases

Yang et al. [40] observed that the No. 17 geothermal well has high concentrations of H₂ (330 ppm), He (5993 ppm), and CH₄ (volume ratio = 27.6%), and indicated that it may contain important information. Therefore, we have made a more in-depth study of No. 17 geothermal wells. Five samples were collected from the No. 17 geothermal well from April 2022 to April 2023. The chemical compositions of the geothermal gas samples are shown in Table S3 and Figure 7. N₂ and CH₄ account for more than 93% of No. 17 geothermal wells. He concentration (4243–6049 ppm) is significantly higher than other geothermal gases in the Beijing area (150–1851 ppm). What is the genesis of these high abnormal concentrations of these gas components? We will discuss this in detail below.

4.3.1. He

As discussed earlier, the No. 17 geothermal water belongs to Group 3 and has a high Cl⁻ concentration, which probably reflects the intensity of deep fluid activity. Helium is also a geochemical indicator of tectonic activity and earthquakes. Both tectonic activity and earthquakes release large amounts of He [44,53]. However, He suddenly descends into No. 17 geothermal water in response to a significant earthquake, which is different from the traditional understanding [54]. In fact, even if the He concentration was reduced from 6049 ppm to 4243 ppm, the He concentration at the No. 17 geothermal water was still much higher than that of other geothermal water in the Beijing area. The reason for the decrease in He concentration before the earthquake may be that the faulting activity leads to the mixing of more air, diluting He in the geothermal gas. Therefore, the high He concentration of No. 17 geothermal gas should originate from deep fluid activity.

4.3.2. CH₄

Most of the world’s methane is biotic CH₄, which is produced either by microbial processes or by thermogenic degradation of organic matter in sedimentary rocks [55,56]. However, there is another origin of CH₄, called abiotic CH₄. It can formed by chemical reactions that do not directly include organic matter [57,58,59,60,61,62,63,64]. Abiotic CH₄ is extremely important in a wide range of scientific fields, including the origin of life, hydrocarbon synthesis, astrobiology, and planetary exploration [56].

During the Paleogene, oil shales were widely distributed in the Beijing area [46]. Does the high concentration of CH₄ in No. 17 geothermal gas originate from the thermogenic degradation of oil shales? Yes, but not entirely! Because well No. 17 had already cut through the sediment (3588 m, Table S1), and both Cl⁻ and He indicate that No. 17 geothermal is polluted by deep fluid. In addition, deep tectono-magmatic activity, magma cooling, and gas-water-rock reactions can produce abiotic CH₄ [56]. Hence, the gas of the No. 17 geothermal well should be coming from deeper and contain abiotic CH₄. Although the δ¹³CH₄ of No. 17 is −36.4 [40], which shows the characteristics of biotic CH₄ [56], it may be a mixed value. The mixture of abiotic CH₄ from deep and biotic CH₄ released by thermogenic degradation of oil shales formed geothermal gas No. 17.

What is the genesis of abiotic CH₄ in geothermal gas No. 17? We propose that there are three ways:

• (1). Carbonate reacts with water in the presence of Fe (500–1500 °C) [56]:

  (1)$8\mathrm{F}\mathrm{e}\mathrm{O}+{\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3+2{\mathrm{H}}_2\mathrm{O}=4{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3+{\mathrm{C}\mathrm{H}}_4+\mathrm{C}\mathrm{a}\mathrm{O}$

• (2). CO₂ evolution to CH₄ during magma cooling (<500 °C) [56]:

  (2)${\mathrm{C}\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}={\mathrm{C}\mathrm{H}}_4+{\mathrm{O}}_2$

• (3). The Sabatier reaction (25–500 °C) [56]:

  (3)${\mathrm{C}\mathrm{O}}_2+{4\mathrm{H}}_2={\mathrm{C}\mathrm{H}}_4+{2\mathrm{H}}_2\mathrm{O}$

The sedimentary layer in the NCC is several thousand meters high, including carbonate and clastic rocks, which can provide sufficient CaCO₃ for (1). The (2) benefits from magmatic rocks produced by extensive Yanshanian magmatic activity [35,36,39]. The high concentration of H₂ and CO₂ in the geothermal gas of No. 17 provided the conditions for (3).

4.3.3. H₂

A large number of experiments and natural observations have shown that hydrogen can be produced by faulting movements [54,65,66,67,68,69]. The origin of H₂ is usually attributed to a chemical reaction between crushed silicate minerals and water (e.g., serpentinization produces molecular hydrogen (4)) [67,70], which enables H₂ to reflect the activity of the fault to a certain extent [40,54,71].

(4)${\left(\mathrm{M}\mathrm{g},\mathrm{F}\mathrm{e}\right)}_2{\mathrm{S}\mathrm{i}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{M}\mathrm{g}}_2{\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_5{\left(\mathrm{O}\mathrm{H}\right)}_4+{\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4+{\mathrm{H}}_2$

Olivine + fluid→serpentine + magnetite + hydrogen

Hydrogen content varies greatly in No. 17 geothermal gas. In particular, in the 3 February 2023 sample, a concentration of 17,426 ppm of H₂ was recorded, which probably reflects a precursory pulse of seismic activity (Figure 7). Sure enough, on 12 February 2023, the ninth day after the signal was detected, 11 earthquakes were detected in the same place; the maximum magnitude was M_L3.4, at a maximum depth of 14 km (Table S4).

Beijing area is located on the northern margin of NCC. During the Yanshan tectonic period, the NCC experienced destruction and thinning under the influence of Pacific subduction [33,35,36,39]. The resulting magmatic rocks and fault zones provide the material sources and ascending channels for hydrogen generation. The hydrogen production is controlled by the activity of the fault zone. Therefore, the H₂ concentration in No. 17 geothermal gas can be used for monitoring fault activity and earthquake warnings.

4.4. Geothermal Water Cycle Model and Genesis of Geothermal Field

As discussed above, the geothermal water in the Beijing area can be divided into three groups. Group 1 is located in the sedimentary area, dominated by Na∙Ca∙Mg-HCO₃, and group 2 is located in the silicate rock area, dominated by Na-SO₄∙HCO₃. In particular, although group 3 is located in the sedimentary area, the depth of 3588 m has exceeded the thickness of the sedimentary and reached the top of the magmatic rock batholith, so that group 3 has the characteristics of deep fluid with high Cl⁻, He, H₂ and CH₄. Combined with geochemical and isotopic composition, we propose that the geothermal water in the Beijing area originated from atmospheric precipitation. The precipitation flows into the ground along the fault and reacts with the surrounding rock while being heated. Eventually, they go up well along the fault to form hot springs (Figure 8). The geothermal resources in Beijing belong to typical medium-low temperature geothermal resources of the sedimentary basin, and some areas are controlled by deep fault activity (e.g., Xiji geothermal well (No. 17)). The heat sources are upper mantle heat, radioactive heat in granite and magmatic cooling residual heat. The heat reservoir is carbonate rock.

4.5. The Promotion of Geothermal Resources to Promote the Earth’s Energy Release

The way the earth releases energy can be geothermal energy or earthquakes. A large number of studies on oil and gas extraction, wastewater treatment, and geothermal exploitation have shown that fluids can promote seismic activity [14,15,17,23,24,25,26,27,28,29,30,31,32]. The geothermal development in the Beijing area includes the extraction and injection of water. So, what is the relationship between geothermal fluid activity and earthquakes?

We have been collecting earthquake records in the Beijing area since 1970. Considering that earthquakes with smaller magnitudes may not have been recorded due to insufficient coverage area of the seismic network in the early stage, we only conducted statistics on earthquakes with magnitudes above M_L 2, and the results are shown in Figure 9. The earthquakes are distributed near the Zhangjiakou-Bohai fault zone, which is similar to the geothermal field (Figure 1). The magnitude is mainly 2–4, while earthquakes above 4 are rare (Figure 9). Since 1970, the number of earthquakes in Beijing has shown a slow upward tendency, but the total amount of energy released by earthquakes has not increased significantly (Figure 10). This reflects the fact that in the absence of a significant change in fault activity, the rise in the number of earthquakes results in less energy being released each time, i.e., a smaller magnitude. In fact, since 2013, the fault activity in the Beijing area has been weakening [41]. Exploitation of geothermal resources, on the other hand, has steadily increased. Statistically, from 1971 to 2013, the total amount of geothermal resources developed in Beijing was 2.87 × 10⁸ m³, and the injection water was 3.02 × 10⁷ m³. Subsequently, the annual production has been maintained at 600–800 × 10⁴ m³/y (Figure 10).

Significantly, the high overlap of the earthquake and geothermal field location and the positive correlation between injection water and earthquakes indicates that geothermal resource development will promote the occurrence of earthquakes (Figure 9, Figure 10 and Figure S2). The extraction and injection of water change the fluid pressure of the geothermal water, which leads to a change in rock stress, releasing its elastic potential energy and triggering earthquakes [24,26]. Due to the continuous exploitation of geothermal water, the elastic potential energy of the rock cannot be accumulated excessively, which effectively reduces the occurrence of destructive earthquakes. Therefore, we can reduce the occurrence of destructive earthquakes by rational use of geothermal resources.

5. Conclusions and Outlook

In this contribution, we perform a detailed elemental and isotopic analysis of geothermal waters and gases collected from the Beijing area. By integrating geochemical results of geothermal waters and gases, we propose that the geothermal resources in Beijing belong to typical medium-low temperature geothermal resources of the sedimentary basin, and some areas are controlled by deep fault activity (e.g., Xiji geothermal well (No. 17)).

The H₂ and CH₄ in the geothermal water/gas of the No. 17 geothermal well are sensitive to deep structural activities. By monitoring the elements and isotopes of geothermal well No. 17, the deep fluid activities can be reflected and thus forewarn earthquakes.

The extraction and injection of water will promote the release of Earth’s energy. The energy is differentiated into multiple releases and avoids the excess accumulation of one-time energy, resulting in damaging earthquakes (M_L ≥ 5). On the one hand, the exploitation of geothermal resources may be one way to reduce destructive earthquakes; on the other hand, the utilization of geothermal resources can reduce the consumption of fossil energy, which is of great significance for tackling global warming.

We propose that the exploitation of geothermal resources may be one of the means to reduce destructive earthquakes. However, given the complex thermal structure of the Earth’s crust, the conversion mechanism between geothermal and seismic energy release is not known. Geothermal water links to earthquakes and to earthquake stress release are not established (no physical robust, statistical, or quantitative analyses). In addition, the study area is limited. Therefore, the contribution of this paper is that we provide a new research idea for earthquake and geothermal research, and more in-depth and systematic research is needed in the future.

Author Contributions

Conceptualization, Z.L. (Zebin Luo) and X.Z.; methodology, Z.L. (Zebin Luo); software, Z.L. (Zebin Luo); validation, X.Z. and J.L.; formal Analysis, X.Z.; investigation, M.Y., Z.L. (Zhe Liu), P.H., J.M., L.H., X.S., B.C., Z.W. and Y.C.; resources, M.Y.; data Curation, M.Y.; writing—Original Draft, Z.L. (Zebin Luo); writing—Review and Editing, Z.L. (Zebin Luo); supervision, X.Z.; project Administration, M.Y. and G.L.; funding acquisition, M.Y. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Water and gas of hot springs data were measured by experimental instruments. Seismic records and geothermal resource data will be made available on request.

Acknowledgments

We would like to thank the Associate Editor and anonymous reviewers for their constructive comments, suggestions, and corrections. We also thank Lincheng Jiang and Tian Liang for the discussion.

Conflicts of Interest

The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

Footnotes

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Figures

Figure 2. Piper diagram of geothermal waters in Beijing. These waters are Na∙Ca∙Mg-HCO3, Na-SO4, and Na-Cl types.

Figure 4. (Na+ + K+)/(HCO3− + SO42−)meq/L versus (Ca2+ + Mg2+)/(HCO3− + SO42−) meq/L (a) and Rb/Ni ppm versus Sr/Ni ppm (b) for geothermal waters of Beijing area. Group 1 is characterized by the reaction of carbonate rock with water, while groups 2 and 3 are characterized by the reaction of silicate rock with water.

Figure 5. Na+ versus Cl− for Beijing area geothermal waters.

Figure 6. Mg2+/Ca2+ versus Na+/Ca2+ for geothermal waters of the Beijing area. The Dolomite, silicate, and limestone areas are from [50].

Figure 7. Characteristics of gas composition with time in Xiji (No. 17) geothermal well. Data of 19 January 2022 from Yang et al. [40].

Figure 8. The water cycle model of the geothermal waters and gases in the Beijing area. The geothermal water in the Beijing area originated from atmospheric precipitation. The precipitation flows into the ground along the fault and reacts with the surrounding rock while being heated. Eventually, upwell along the fault to form hot springs.

Figure 9. Earthquake records from 1970 in the Beijing area. The triangle shows the distribution of seismic stations, and their locations are from the China Earthquake Administration.

Figure 10. Temporal variations of earthquake frequency (time) (a), energy release (J) (b), and geothermal production (104 m3) (c). The conversion formula of magnitude and energy: lgE = 4.8 + 1.5 M, E is energy (J). M is magnitude (ML > 2), earthquake data from the China Earthquake Administration. Geothermal production data from the Beijing Hydrogeological Engineering Team (2014–2019 are estimates).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16040622/s1, Table S1: Physical properties, hydrogeochemistry and isotopic compositions of geothermal waters from Beijing area; Figure S1: Na-K-Mg ternary diagram of geothermal waters in Beijing area; Table S2: Temperature results obtained with empirical chemical geothermometers (values in °C) and depths (m) of origin for Beijing area geothermal waters; Table S3: Geothermal gases chemistry compositions of Xiji geothermal well (No. 17) from Beijing area; Table S4: Earthquake records at 12 February 2023 in Beijing area; Figure S2: Relationship between volume of injection water (10⁴ m³) and number of earthquakes (>M_L = 1) in Beijing area.

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