Gas samples were collected from the hot springs and five deep soil holes from the NMYB Fault on 23 December 2022 (see Table 2). The gas samples were immediately sent to the Key Laboratory of the Institute of Earthquake Forecasting, China Earthquake Administration. Measurements of the samples were finished within 30 days. The compositions of the gas samples were analyzed using an Agilent 490 Gas Chromatograph, which measured H₂, He, CO₂, N₂, O₂, and CH₄. The measurement accuracy was estimated as follows: the relative standard deviation was <0.5% when the content was 1%–100%, and the relative standard deviation was <1% when the content was 0.01%–1% (Zhou et al., 2015; Zhou et al., 2020). Helium and neon isotopes were measured using the Noblesse noble gas isotope mass spectrometer. When the R-value in helium isotope measurement was above 1 × 10⁻⁷, the test error was ±10%, and the measurement data error was at 1 × 10⁻⁸–1 × 10⁻⁷ were ±15% (Cao et al., 2018). The results were normalized to standard atmospheric values. The carbon isotope ratio was analyzed using the DeltaPlusXL mass spectrometer (Thermo Finnigan, United States), consisting of an HP 6890 Gas Chromatograph, a combustion/conversion furnace, and an interface connecting to the DeltaPlusXP mass spectrometer. The stable carbon isotope composition was expressed by δ¹³C; the accuracy of ¹³C/¹²C was 0.6‰ (Li et al., 2014).

Based on the above concentration characteristics of the soil gas measurement, five new continuous gas monitoring stations were built along the NMYB Fault (Figure 1); these stations aim to monitor the continuous changes in fault gases (H₂ and CO₂), analyze the spatial differences of the gas characteristics in different fault segments, and observe their changes over time. Furthermore, the five gas stations can monitor the stress adjustments and seismicity of the Beijing area. The five geochemical monitoring sites along the Northern Margin Fault form high-density gas geochemical networks for automatic and continuous gas sampling and observation (Figure 1 shows the distribution of the stations).

In the past, traditional soil/fault gas observations were subject to many disturbing factors, mainly the shallow depth of gas extraction and its susceptibility to biogenic gases and atmospheric disturbances. In order to minimize the influence of these surface factors, the quality of monitoring was improved, the influence of gases produced by biogenic layers and humus in the soil was avoided as much as possible, and the seasonal interference of the ground temperature was avoided. Fault gas fixed monitoring sites were constructed this time, reaching approximately 50–60 m deep into the bedrock. The depth was well beyond the variable temperate zone of the region (the region’s variable temperature zone is generally in the range of 20 m).

Hot spring well stations and faulted soil gas stations make the monitoring of additional tectonic activity possible. The monitoring environment is very stable. There is no industrial or mining activity around the monitoring stations. Instead, the fault gas is introduced at a depth of 50–60 m below ground level using sieve-perforated gas collection cylinders. Both hot spring and fault gases are accumulated (Figure 2) and introduced using polytetrafluoroethylene (PTFE) tubes (length: 33–55 m, outer diameter: 3 cm, and inner diameter: 2 cm). Hydrogen concentrations were monitored every 2 h using an ATG-6118H autoanalyzer in the range of 0.01–1,000 ppm with an accuracy of ±5%, and CO₂ concentrations were monitored every 2 h using an ATG-C600 CO₂ continuous monitor in the measurement range of 0%–60% with a resolution of 0.001%. Both instruments were calibrated and maintained by the instrument manufacturing company (Wen et al., 2018; NOA Certification, 2019). All gas piping was well sealed from the effects of weather, air, and temperature, as well as external humidity and rainfall. The data obtained from this system were transmitted in real time via 5G.

The gas background values of the new deep-bore fault gas wells in the NMYB Fault correlate with the historical earthquakes. According to the curves of the background H₂ observation values of the stations when the station was being constructed (Figure 3), it could be seen that the background values of the hydrogen concentration in the area between Yaojiaying (YJY) and Huangbaisi (HBS), where more historical earthquakes had occurred, were higher. In addition, the historical earthquakes of Baicaowa (BCW) and Huangyukou (HYK), located in the northeast of the rupture, were also fewer, and their H₂ concentration background values were low at the same time (Sugisaki and Sugiura, 1986; Cheng et al., 1995).

Gases (isotopic ratios of CO₂, He, Ne, and other gas compositions) collected from Yanqing soil gas wells from 1 January 2023 to 31 January 2024 are shown in Table 2. The N₂ concentration of the gas samples was ≥79 vol%. Helium and H₂ concentrations in the gases were relatively low (≤500 ppm). O₂ concentrations in most fugitive gas samples were less than 20%. Methane concentrations were below 3%. Most CO₂ concentrations were below 5%.

The source of H₂ is most likely attributable to (A) H₂ produced by water–rock reactions. Abiotic H₂ is produced under near-surface conditions when water reacts with ultramafic rocks or serpentinization and migrates from deeper reservoirs (Lollar et al., 2014; Parnell and Blamey, 2017; Irfan et al., 2019; Wang et al., 2019; Wang et al., 2020); (B) water interaction with the newly exposed rock surface (Hirose et al., 2011); and (C) mixing with large amounts of microbial H₂ produced by bioactivity and organic matter decomposition (Prinzhofer et al., 2019; Myagkiy et al., 2020).

Laboratory experimental data showed that tectonic stress caused by the reaction of broken minerals and groundwater can produce H₂. The observed significant increase in H₂ in the high concentration of anomalies before and after many large earthquakes suggests that there is a close relationship between the changes in the concentration of H₂ and the earthquakes (Sugiyaki et al., 1983; Zhou et al., 2021). A background investigation on soil was carried out by Li et al. (2014) and Chen et al. (2022) over a larger area including the study area. The results show that there are low-velocity bodies in the upper crust of the eastern part of the Yanqing–Huailai Basin (including the Yanqing Basin), with strong tectonic activity. The investigated gases, including Hg and He, mainly originate from the deeper part of the fault zones, contributing more to the soil gas. H₂ also originates from the deeper part but was influenced by the redox environment in the soil deposits, which results in a wide range of variations in the H₂ content (Li et al., 2014).

Prior to earthquakes from 1 January 2023 to 1 January 2024, there were significant short-term (120–720 h) seismic precursor anomalies in the H₂ concentration of fault escape gases at HYK, HBS, and SBQ (Figure 7). Earthquakes occur when stress increases to a state of sub-instability stress on faults (Ma, 2016). The hydrogen concentration in the escaping gas is very sensitive to the increase in stress, which may enhance the openness of microfractures in the fractures along the Northern Margin of the Yanqing Basin.

The anomaly information was reported to the Beijing Earthquake Agency prior to the 5 August 2023 Shandong Pingyuan M5.5 and 1 January 2024 Japan M7.3 earthquakes.

Prior to these two earthquakes, there were significant and persistent precursor anomalies in the H₂ concentrations of soil gas in each of the HYK, HBS, and SBQ stations. When the stress increases to a state of sub-instable stress on the faults, the faults misalign to produce earthquakes (Ma, 2016), so the mechanism of precursor anomalies may have been caused by the sustained stress increase prior to the natural seismic events.

The hydrological response to earthquakes depends on the earthquake’s magnitude and epicenter (Wang and Manga, 2010; Cox et al., 2015). Therefore, systematic identification of short-term seismic precursor anomalies based on multiple earthquakes using certain criteria is difficult. By drawing on and analyzing the collection of many natural earthquake cases in the same region, the equation of geochemical gas precursor anomalies in relation to magnitude and distance from the epicenter can be summarized, and some criteria can be provided.

Chen et al. (2022) investigated the relationship between the gas release characteristics of the major active fault zones within the North China Craton and the regional tectonic framework. The results show that in the Zhang-Bo tectonic belt, the release of soil gas Rn and CO₂ and the contribution of crustal or mantle sources in the escaping gas from hot springs are significantly higher than in other tectonic zones in the region. A comprehensive analysis combining the results of the shear strain rate field, GPS horizontal velocity field, and S-wave velocity imaging reveals that the region was characterized by strong tectonic activities, and the development of nascent fractures may be widespread in the subsurface, and mantle-source fluids may be present in the subsurface of the Zhang-Bo tectonic belt for 20–40 km. It was also found that the peak region of helium from the mantle source material in the Zhang-Bo belt in the Yanqing Basin region showed increased deep fluid upwelling.

The NMYB Fault is located in the northern part of the central orogenic belt of the North China Craton, which consists of two east–west grade blocks of the North China Craton collided and spliced in the Paleoproterozoic (Liu and Zhao, 2012). After experiencing many geologic effects, a large number of faults were developed under the stresses of Cenozoic tensile-slip, and basin–mountain coupling tectonics were formed in the adjacent basins blocked by the mountain system (Xu et al., 1998), and the Yanqing Basin is one of them (Yang et al., 2022).

Subduction of the paleo-Pacific plate sheet is the geodynamic mechanism leading to the thinning and destruction of the lithospheric mantle of the North China Craton (Zhu et al., 2017). The destruction of the North China Craton occurred mainly in the area east of the Taihang Mountains in North China, resulting in the intrusion of magma, earthquakes, and volcanism in the area, including the Zhang-Bo belt, which are closely related to the geologic activities. The Yanhuai Basin, located in the Zhang-Bo Belt, is one of the areas with the potential danger of strong earthquakes in the eastern part of mainland China. The mechanism of earthquake occurrence in the Yanhuai Basin and basin–mountain interaction, the monitoring and prediction of future strong earthquakes, and other scientific and applied issues have also been attracting the attention of earthquake scientists.

Hence, in the broader context of tectonically active and fracture development in the Zhang-Bo belt region, combined with gas observation anomalies in this region, a model was designed to explain the anomalies. The model assumes that the NMYB Fault is a gas source system capable of responding to external stress loading and that during the anomalous time period, there was stress loading from the periphery of the fault, and anomalous gas rises, along with increases in the H₂ concentration, were observed at the observatory stations (see Figure 8).

This can be explained by the fact that in the fracture system, the hydrogen concentration in the escaping gas is very sensitive to the increase in stress, which has the potential to enhance the openness of the microfracture under the Yanqing Fracture, leading to an increase in the concentration of the gas originating from the depth.

At the same time, during stress loading, hydrogen released from rock fragmentation and hydrogen generated by water–rock chemistry at the active fault site make hydrogen more concentrated near the active fault, so some hydrogen anomalies can be observed at the location of the active fault at the surface.