Emission of Rn and CO2 From Soil at Fault Zones Caused by Seismic Waves

In the search for precursors to earthquakes, correlation has been found between geochemical characteristics of soil gases and seismic activity. In this paper we present evidence that seismic waves can trigger emission of soil radon (Rn) and carbon dioxide (CO2). An active experiment was performed in two fault zones in China, the Annighe fault in Sichuan province and the Xiadian fault in Heibei province. An active seismic source was used to generate seismic waves at 10 m depth in wells within bedrock. Rn and CO2 detectors were placed around the wells at a distance of ∼1 m for observing the effects of the seismic waves on the emission of the gases. The observations confirm that the seismic waves have a significant and direct effect on the concentration and flux of soil radon and carbon dioxide. When the seismic events were triggered, the observed concentrations of Rn and CO2 immediately increased and reached peak values within 5–50 min and 30–60 min, with corresponding increases of Rn and CO2 concentrations by 10.5%–238.7% and 3.1%–54.1%, respectively. The measured concentrations and flux of CO2 and Rn after the passage of the seismic waves showed strong correlation, confirming the suggestion that CO2 is the carrier gas for Rn. To the best of our knowledge this is the first direct, in‐situ measurement of gas emission caused by the passage of seismic waves and provides important constraints for better understanding of geochemical earthquake precursors.


Abstract
In the search for precursors to earthquakes, correlation has been found between geochemical characteristics of soil gases and seismic activity. In this paper we present evidence that seismic waves can trigger emission of soil radon (Rn) and carbon dioxide (CO 2 ). An active experiment was performed in two fault zones in China, the Annighe fault in Sichuan province and the Xiadian fault in Heibei province. An active seismic source was used to generate seismic waves at 10 m depth in wells within bedrock. Rn and CO 2 detectors were placed around the wells at a distance of ∼1 m for observing the effects of the seismic waves on the emission of the gases. The observations confirm that the seismic waves have a significant and direct effect on the concentration and flux of soil radon and carbon dioxide. When the seismic events were triggered, the observed concentrations of Rn and CO 2 immediately increased and reached peak values within 5-50 min and 30-60 min, with corresponding increases of Rn and CO 2 concentrations by 10.5%-238.7% and 3.1%-54.1%, respectively. The measured concentrations and flux of CO 2 and Rn after the passage of the seismic waves showed strong correlation, confirming the suggestion that CO 2 is the carrier gas for Rn. To the best of our knowledge this is the first direct, in-situ measurement of gas emission caused by the passage of seismic waves and provides important constraints for better understanding of geochemical earthquake precursors.
Faults and associated fractures act as channels for fluid migration because of their greater porosity and permeability compared to surrounding rocks (Claessen et al., 2007;Faulkner et al., 2010;Hunt et al., 2017;Immè et al., 2006;Muirhead et al., 2016;Sciarra et al., 2018;Walia et al., 2010;T. F. Yang et al., 2004). The upward migration of deep fluid through an active tectonic zone may weaken faults and trigger earthquakes Wiberley et al., 2008). Deep fluid likely controls aftershock distributions in the tectonic zones of rifts (Miller et al., 2004;Noir et al., 1997;Terakawa et al., 2010), subduction zones (Waldhauser et al., 2012;Z. Wang et al., 2012), and reverse and strike-slip faults (Cappa et al., 2009;Sibson, 2007). Investigations on the transport and chemical composition of soil gases within fault zones have been extensively carried out because those geochemical anomalies serve as potential precursors for seismic and fault activities.
Among soil gases, CO 2 and Rn have attracted great attention because these gases may sensitively respond to the fault and seismic activities. Rn is produced by the radium series in uranium-bearing materials. Emission of Rn through active fault zones can be enhanced by seismic, volcanic and fault activities, which is demonstrated by high concentrations and flux of soil Rn worldwide (Ambrosino, Sabbarese, Roca, Giudicepietro, & Chiodini, 2020;Ambrosino, Sabbarese, Roca, Giudicepietro, & De Cesare, 2020;İçhedef et al., 2014;Kamislioglu and Külahci, 2016;Oh & Kim, 2015;Sabbarese et al., 2020;T. F. Yang et al., 2004). Anomalous discharge of CO 2 -rich fluid was directly measured during the 1965-1967 Matsushiro earthquakes (Cappa et al., 2009). Emission of CO 2 was observed in association with earthquakes at the Lassen Peak volcano (Cascades Range, USA) (Ingebritsen et al., 2015) and the Eger Rift (Czech Republic) (Fischer et al., 2017), indicating close connections between the Earth's surface and its crust by fluid transport. 575 earthquakes occurred at Cava dei Selci (Colli Albani Volcano, Italy) from 2009 to 2021 showed that increase of CO 2 flux seemed related to extensional deep earthquakes (Tarchini et al., 2022).
Deep CO 2 plays an important role in triggering earthquake. CO 2 -rich mantle-derived fluids are considered to directly lead to fault-weakening at the San Andreas fault zone in California, USA (Kennedy et al., 1997). Aftershocks of the 1997 seismicity in Colfiorito were driven by high pressure CO 2 in the Northern Apennine island (Miller et al., 2004). Obviously increased concentration of radiogenic crustal gases indicated existence of high-pressure-fluids in the deep Earth that played an important role in generation of Appenine earthquakes (Chiodini et al., 2011). Ten years of observations (2009-2018) of CO 2 flux in the Apennines (Italy) during a period of intense seismicity, including Mw 6.3 L'Aquila earthquake (6 April 2009), Mw 6.0 Amatrice earthquake (24 August 2016), and Mw 6.5 Norcia earthquake (30 October 2016), indicated that over-pressurized CO 2 -rich reservoirs might trigger earthquakes in the deep crust .
For better exploring quantitative relationships between geochemical variations of soil gases and seismic activity and then applying gas geochemical monitoring methods to predict seismic activity, a direct and real-time close observation of effects of earthquakes on fluid emission must be carried out. However, due to difficulty in natural earthquake prediction, direct observations of seismic effects on geochemical characteristics of soil gases at the Earth surface are very rare. Most observations on seismic geochemical effects were far-away from the epicenters (a few kilometers, a dozen kilometers, dozens or hundreds of kilometers of kilometers away), and effects were inferred based on seismic parameters. To our knowledge, only one measurement has been conducted (Gresse et al., 2016) that investigated flux variations of CO 2 caused by artificial seismic vibrations at the Campi Flegrei caldera (Italy). Therefore, questions arise whether geochemical properties of soil gases are directly modified when seismic waves pass through, and whether those changes occur with a delay. At the same time, CO 2 is usually considered to be the carrier gas of Rn based on mathematical statistics (Chyi et al., 2010;Etiope & Lombardi, 1995;Yuce et al., 2017), but, related experimental evidences have not been reported. To explore those issues and the link between CO 2 and Rn emissions, we performed real-time measurements for concentrations and flux of soil CO 2 and Rn by soil gas geochemical observation methods during seismic vibrations induced by artificial seismic sources at two active fault zones in China.

Tectonic Setting of Observed Areas
Two active fault zones, the Anninghe fault in Sichuan province and the Xiadian fault in Hebei province, China ( Figure 1) were selected for the observation. The Anninghe fault is a part of the southeastward extrusive active tectonic system in the Tibetan plateau (Peltzer & Tapponnier, 1988;Tapponnier et al., 1982), controlling the 10.1029/2023EA003012 3 of 18 major seismic activities in southwest China (Wen et al., 2008). This fault connects to the Zemuhe fault near Xichang city, showing characteristics of a left-lateral strike-slip movement since late Quaternary period (Xu et al., 2003) and trending N-S for ∼200 km along the Anninghe valley (Ren, 2014). Multiple historical earthquakes have been recorded such as the 1952 Mw 6.75, 1850 Mw 7.5, 1536 Mw 7.5, and 814 Mw 7 earthquakes in the Anninghe fault zone (Department of Earthquake Disaster Prevention, 1995).
The Xiadian fault, located in the eastern part of the capital area in China, is a NNE-trending lithospheric-scale fault and also an active fault with strong earthquakes in history (e.g., the Sanhe-Pinggu Mw 8.0 earthquake in 1679) (Deng et al., 2019;He et al., 2013;Meng et al., 1983). The Xiadian fault is a normal fault with right-lateral strike-slip components and one of the main active faults in the North China plain (Bureau of Geology and Mineral Resources of Beijing Municipality, 1991; Ran et al., 1997;Xiang et al., 1988). The lithology at the observation area of the Xiadian fault zone is dominated by sandstone and sandy clay. In the observation area of Anninghe fault zone, Quaternary sediments and sandstones in a thickness of 0.5-2 m are occasionally exposed. However, all observational wells for the artificial source detonation were drilled in the granite bedrock (Chinese Academy of Geological Sciences, 2007).

The Active Seismic Source
Active seismic source based on methane gaseous detonation was employed to produce seismic waves (W. Wang et al., 2019). The device is assembled with a cylindrical steel seal container in an outer diameter of 120 millimeters

of 18
(mm), an inter diameter of 12 mm and length of 1,000 mm, mixture of methane and oxygen gases were injected in the sealed container, similar to traditional explosive source, which needs to be placed in a well before it is ignited ( Figure 2). Detonations generated by high pressure gas mixture produced elastic waves in surrounding rocks, simulating effect of seismic events. Initial gas injection pressure of methane and oxygen is 6 mega-pascal (MPa), and internal pressure can reach to 165 MPa after detonation (excitation). Pressure limiting valve is designed to be located at the bottom of vessel to ensure that main impact energy traveling downward. Intensity of the high-frequency vibration decays from 6.48 to 10.9 m/s 2 at 6 m from the epicenter to 0.38-0.84 m/s 2 at various sites 30 m from the epicenter in different directions. The effective energy to generate seismic waves from the methane detonation source is about 1.4 mega joules. The near-field vibration has a short duration of about 0.25 second (s) at a distance of 6 m and about 0.3 s at 30 m. At longer distances, the signal is coupled to the subsurface structure and its waveform. Frequency of the vibration distributed at 10-80 Hz with a peak at 20-60 Hz. Peak of the response spectrum of the active source used here at 6 m is 2.5 Gal, being close to the near-fault record during the Wenchuan Mw 7.9 and the Lushan Mw 6.7 earthquakes. Detonated products of the active source are water and CO 2 (W. Wang et al., 2019).

Measurement Design
The first criterion for selected observation sites is to be as close as possible to the fault plane. At the same time, in easy to drill rocks that provide adequate conditions to put and detonate safely explosive charges at 10 m depth. A total of 20 sites were selected: 11 sites (XC01-XC11) in the Anninghe fault zone study area, and 9 (SH01-SH09) in the Xiadian fault zone.
Measurements of Rn and CO 2 were carried out at distance of ∼1 m from the active seismic source ( Figure 3). In this way, effect of the artificial seismic event on Rn and CO 2 emission can be measured to the maximum extent and influences of other factors (such as changes of temperature and air pressure) can be minimized. Measurements of flux of Rn and CO 2 and their concentrations  started from 0.5 to 2 hr before the detonation, and then measurements for co-seismic and post-seismic were taken from the time of detonation and over the following 1-3 hr. According to measured conditions of every observed site, measuring time at a site is usually 1.5 hr after the seismic event. However, for tracking the trend of changes, long-term measurement was also carried out at some sites. Different duration at the different sites depend on the time to get stable pre-seismic results and time limitation of measurement. Similarly, duration for measurement at co-seismic and post-seismic period depend on the trend of observation results and time limitation of measurement. For ensuring reliability of the observed results, two parallel measurements were performed at each observed site. Totally, eight detected sensors, the four detectors for measuring CO 2 and four detectors for measuring Rn, were placed around the active source before the detonation at each observed site and simultaneously measured the Rn and CO 2 concentrations and flux throughout the duration of the observation.

Soil Gas Measurements
Concentrations of CO 2 (vol%, given as percentage of the volume of CO 2 detected relative to the total volume of gas in the detectors) and Rn were measured by connecting a stainless steel sampling tube of 3 cm in diameter inserted into the ground hole at 80 cm depth ( Figure 4). Measurements for CO 2 and Rn were performed in the field by the RAD 7 radon detector and the portable GXH-3010-E infrared CO 2 monitor, respectively. Detection limits and accuracy of the CO 2 monitor are 0.02 vol% and ±2% (Chen et al., 2020), and those of the Rn detector are 14.8 Bq/m 3 and ±4%. Measuring time for each concentration value of Rn and CO 2 were 5 min and 15 s, respectively.
The flux of CO 2 and Rn were measured by the static closed chamber method (Ciotoli et al., 1999;King et al., 1996). Employed equipment includes an inverted circular accumulation hemispherical chamber with a volume of 1.68 × 10 −2 m 3 and radius of 0.2 m here ( Figure 4). Measuring time for each flux value of Rn and CO2 were about 1 hr (10-15 concentration values) and 0.75 hr, respectively.
Flux values were calculated from the gas concentration build-up in the chamber, using the following equations (Lehmann et al., 2000;Tuccimei & Soligo, 2008;Yuce et al., 2017): where ΔC is the variation of gas (Rn or CO 2 ) concentration with time in the chamber during the measuring period Δt (minute), P std and T std are the standard barometric pressure (101.325 kPa) and temperature (273.15 K), respectively, V C is the volume of the chamber (m 3 ), A C is the surface area of the chamber base (m 2 ), P C is the atmospheric pressure (Pa), T C is the measured soil temperature (K), and dc/dt is the rate of concentration increase in the chamber. To rule out any other factors, we recorded ground temperature and air pressure at all measured sites. In addition, we measured concentrations and fluxes of Rn and CO 2 at the same observed conditions before and after detonation. Thus, we did not consider the effects of meteorological factors for the results of this work.

Results
Totally, active source detonations were performed at 20 sites including 11 times at the Anninghe fault zone (XC01-XC11) and 9 times at the Xiadian fault zone (SH01-SH09) (Figure 1). The geographic, environmental parameters and measured items are listed in Table 1. Soil Rn and CO 2 concentrations and flux were measured before, during and after those artificial seismic events and named as pre-seismic, co-seismic and post-seismic results. And the co-seismic results refer to the results that measured started at the same time of the active source detonation in this work.

The Anninghe Fault Zone
The observed results of soil Rn and CO 2 concentrations at the Anninghe fault zone are presented in Figures 5  and 6, and Measured values of Rn concentrations are in the range of 400-15,600 Bq/m 3 before the artificial seismic events at the observed area, however the highest concentration increased up to 41,400 Bq/m 3 after the seismic events. Except for two sites (XC03 and XC05), Rn concentrations of other nine sites showed obvious increase during and after the seismic events, that is, evident co-and post-seismic effects. Except for the XC08 site, Rn concentrations immediately increased after artificial seismic events, reached the peak value after 5-25 min and continuously remained at a high level for about 1.5-3 hr. The most increase of Rn concentration ranged from 18.9% to 238.7%. The Rn concentration at the XC08 site also immediately increased to peak value after the artificial seismic event, however then decreased to below the pre-seismic values.
Measured CO 2 concentrations are in the range of 0.165-1.135 vol% before the artificial seismic events in the observed area, however concentrations increase to 0.21-1.21 vol% after seismic events. After seismic events, CO 2 concentration immediately increased and reached the peak value in 30-60 min and continuously remained at high level for about 1.5-2.5 hr. The most increase of CO 2 concentration range from 4.9% to 54.1%.

The Xiadian Fault Zone
Our observed variations of Rn and CO 2 concentrations at the Xiadian fault zone are presented in Figure 7, Figure 8 and Table S1 in https://data.mendeley.com/datasets/x3stxdd4pz/1. The concentrations of Rn are in the range of 6,500-47,500 Bq/m 3 before the artificial seismic events, however concentrations increased to the highest value of 61,500 Bq/m 3 after the active seismic events (Figure 7). Compared with the pre-seismic results, all the Rn concentrations obtained during and after the artificial seismic events obviously increased. After seismic events, the Rn concentrations immediately increased and reached the peak value in 5-50 min, and continuously remained at high level for about 1.5-2 hr. The maximums of increase of Rn concentrations ranged from 10.5% to 135.7%.
Concentrations of CO 2 that we measured before artificial seismic events on the Xiadian fault zone are in the range of 0.32-2.8 vol% (Figure 9), however the concentrations increased to 0.365-3.0 v% after seismic events. CO 2 concentrations measured at six sites immediately increased and then reached peak values in 30-50 min and the maximums of increase ranged from 3.1% to 23.0% after seismic events. The measured values increased throughout the observation periods at three sites (SH04, SH06, and SH08) and the increase were 24.5%-128.6%. Except the site SH09, CO 2 concentrations measured at the co-and post-seismic period are higher than those of pre-seismic results. The CO 2 at the SH09 site, its concentrations also immediately increased to peak values after the seismic event, and then decreased to below the pre-seismic values.

Gas Flux
CO 2 and Rn flux were measured before, during and after the seismic events. Here, measurements of Rn and CO 2 flux started from 1 to 2 hr before the seismic events; measurements of co-seismic results started at the same time of the active source detonation; for the post-seismic results, the measurements started within 5-10 min when the co-seismic measurements finished. The measured time for each Rn and CO 2 flux was 1 and 0.75 hr, respectively. And the total measured time at a site is about 3.5-5 hr.
Observed flux of Rn and CO 2 at the Anninghe fault zone are presented in Figure 9 and Table S2 in https://data. mendeley.com/datasets/x3stxdd4pz/1. Since observed flux results of the two parallel measurements are very consistent, the average of the parallel measured results are shown in Figures 9 and 10. Because the measured time for each site is slightly different, we did not list the time scale in Figures 9 and 10. From pre-earthquake to co-and post-earthquake, values of CO 2 flux changed from 11.9 to 95.3 g/m 2 /d to 16.1-125.6 g/m 2 /d and then 12.6-116.5 g/m 2 /d, and Rn flux changed from 19.8-100.9 mBq/m 2 /s to 27.4-170.4 mBq/m 2 /s and then 22.4-150.9 mBq/m 2 /s, respectively. Obviously, the flux of Rn and CO 2 increased from pre-earthquake to post-earthquake. Except for the Rn flux at the XC04 site, the measured flux during post-earthquake are always lower than those of the co-earthquake. The maximums of increase of Rn and CO 2 flux at the co-earthquake are 104% and 71%, respectively. Figure 10 and Table S2 in https://data.mendeley.com/datasets/x3stxdd4pz/1 summarize the results for the flux of Rn and CO 2 that were measured at the Xiadian fault Zone. From pre-earthquake to co-and post-earthquake, the values of CO 2 changed from 3.9 to 42.8 g/m 2 /d to 8.8-63.9 g/m 2 /d and then 7.4-62.8 g/m 2 /d; and the Rn flux changed from 55.8 to 349.4 mBq/m 2 /s to 91.6-406.0 mBq/m 2 /s and then 61.6-345.9 mBq/m 2 /s. It is evident that flux of Rn and CO 2 increased from pre-earthquake to post-earthquakes time. Except for the Rn flux at the SH01 site and CO 2 flux at the SH06 and SH08 sites, flux values measured during post-earthquake time were lower than those of co-earthquake. The maxima of increase of Rn and CO 2 flux in the co-earthquake time were 84% and 172%, respectively.

Spatial Characters of the Observation Concentration and Flux
At  In conclusion, the observation Rn and CO 2 concentration and flux at sites that closer to the fault show higher value (Figure 1), the results are consistent with previous measurement (Ciotoli, 2014;Li et al., 2013;Y. Yang et al., 2021;Yuce et al., 2017).

Relationship Between Gas Discharge and Seismic Waves
Our data document a dramatic and obvious increase of concentrations and flux of CO 2 and Rn after the seismic events and demonstrate a striking co-and post-seismic effect. After the seismic event, Rn concentrations immediately increased and reached the peak value in 5-25 min. The values remain at a high level during the entire post-seismic observation within about 1.5-3 hr at the Anninghe fault zone; and Rn concentrations reach to the peak value in 5-50 min and continuously remained high for about 1.5-2 hr at the Xiadian fault zone. Similarly, CO 2 concentrations increase to peak values in 30-60 min and keep high value during post-seismic observation within Figure 9. Variation of the flux of Rn and CO 2 at the Anninghe fault zone. The pre-seismic measurements of Rn and CO 2 flux started from 1 to 2 hr before the active source excitation; the co-seismic results were measured starting synchronously with the active source detonation; for the post-seismic results, the measurements start within 5-10 min when the co-seismic measurement finished. about 1.5-2.5 hr at the Anninghe fault zone; and CO 2 concentrations increase to the peak values in 30-50 min and continuously remained high for about 1.5-2.5 hr at the Xiadian fault zone. The maxima of increase of Rn and CO 2 flux are 104% and 71% at the Anninghe fault zone and 84% and 172% at the Xiadian fault zone during co-seismic observation, respectively. In general, the artificial seismic event has similar influence on the observed Rn and CO 2 emission, however the measured results show slight differences at the two fault zones. The Xiadian fault is a right-lateral normal fault and the Anninghe faults show characteristics of a left-lateral strike-slip (Ran et al., 1997;Xiang et al., 1988;Xu et al., 2003). The rocks at the observation area of Xiadian fault zone are mainly sandstone and sandy clay, and the rocks are Quaternary sediments and sandstone and granite at the observation area of Anninghe fault zone (Chinese Academy of Geological Sciences, 2007). Those lithological and tectonic differences between the two fault zones may explain the variations of the measured results. The two measured Rn concentrations of the XC03 and XC05 sites ( Figure 5) at the Anninghe fault zone did not show obvious changes before and after the artificial seismic events. The Rn concentrations at the XCO3 and XC05 sites respectively are 3,400-3,800 and 400-700 Bq/m 3 . Those values are much lower and mean small gases storage in the two observed sites. So, no obvious co-and post-seismic effects show in the two observed sites. Similarly, Gresse et al. (2016) investigated variations of CO 2 flux caused by artificial seismic vibrations (active seismic truck) at the Campi Flegrei caldera (Italy). They recognized that vibrations led to a temporary sharp increase in CO 2 flux with a maximum increase of 2-3 times its pre-vibrational level. Measurements of CO 2 , Rn, and Hg concentration show that seismic rupture zones were continuously intensely degassing after the Wenchuan Mw 7.9 earthquake. Short-term CO 2 degassing increase after 2006 seismic swarm was observed at Solfatara (Chiodini et al., 2010). Rn concentrations showed well correlation with the total earthquake energy within a 100-km radius of fault (Koike et al., 2014). Girault et al. (2018) observed the spectacular non-volcanic CO 2 emissions following the 2015 Mw 7.8 Gorkha earthquake at the front of the Nepal Himalayas. Peaks of CO 2 flux were observed during intense seismicity and decayed as the number and energy of earthquakes decreased . These previous observations are consistent with our new findings.
The previous artificial seismic observation (Gresse et al., 2016) showed that temporary and drastic increase in CO 2 flux reach a peak in 3-11 s after the onset of the vibrations, and effects of vibrations on CO 2 flux last for about 15 s. The post-seismic flux was always lower than the pre-seismic results. The measurements of Gresse et al. (2016) were conducted at the Earth surface using an active seismic truck. No steady increase of soil permeability during the artificial seismic vibrations was detected. Therefore, the effects of those vibrations on the CO 2 degassing was restricted. During our observations, most of CO 2 and Rn concentrations immediately increased and then attained peaks 5-50 min after the artificial seismic events. The increase lasted about 1.5-3 hr and most of the observed concentrations and flux of CO 2 and Rn were higher than the pre-seismic results. Our data unequivocally show the effect of seismicity on degassing of the Earth on a longer time scale.
Different mechanisms have been proposed for explaining the gases/fluid emission from the deep Earth, including fluid pressurization (Précigout et al., 2017;Sibson, 2014;Xue et al., 2018), emplacement of magma within the crust (Di Luccio et al., 2018) and passing of seismic waves Cyltankhodzhaev, 1977), etc. Cyltankhodzhaev (1977 first proposed an ultrasonic mechanism of the Rn seismic precursor that adsorption energy would be weakened under ultrasonic treatment, and then Rn adsorbed in rock pore transformed to a free state and exhaled to surrounding space by diffusion. Feng et al. (1981) carried out research on Rn release of artificial rocks and found Rn anomaly releasing from microcracks caused by ultrasonic vibration. Seismic vibrations would lead to sharp increase of pre-existing gas bubbles and new gas exsolution from CO 2 -rich solution . Coseismic thermal and mechanical process would lead to changes of hydraulic property of fault zone (Manga et al., 2012;Miller et al., 2004), and those changes lead to degassing or release of the natural fluid (Toutain & Baubron, 1999). Furthermore, dissociation of carbonate caused by seismic wave would produce enormous CO 2 (Koeberl and MacLeod, 2002;Ryder et al., 1996). Besides, change of stress of Earth interior and rupture or grinding of rock also would release or produce Rn and CO 2 (Italiano et al., 2008;Martinelli & Plescia, 2005;Mollo et al., 2011;Scarlato et al., 2013).
We performed an active source detonation at the 10 m deep well in bedrock. Those artificial seismic activities produce seismic waves and rock fracture. Increase of permeability in the experimental zones may be observed by new fractures produced by the active source excitation (Manga et al., 2012;Manning & Ingebritsen, 1999) or by release of stored gas in pores in the soil and rocks caused by changes of pore stress induced by passing seismic waves (Gresse et al., 2016). Those are the main factors controlling the Rn and CO 2 emission during the artificial seismic events at our observed field.
The detonation of an active source used in the present work released water and CO 2 (W. Wang et al., 2019). Therefore, released CO 2 could directly lead to increase of concentration and flux of CO 2 at the observed sites. Since it is not possible to assess migration of CO 2 released by the detonation in the well and surrounding soil and rock, it is problematic to evaluate extent of influence of this CO 2 on the observed post-seismic CO 2 concentration and flux.

CO 2 Migration Transporting Rn
Due to the short half-life period of 222 Rn (3.82 days), its mobility in the ground is limited to a few meters. Except the diffusion driven by concentration difference, carrier gases are necessary to help the Rn transport from the deep to shallow levels in the Earth (Etiope & Martinelli, 2002;T. F. Yang et al., 2003). Due to the similar emission behavior of soil Rn, He and CO 2 from depth in the Earth, the CO 2 is thought to be the carrier of the Rn and He (Chyi et al., 2010;Etiope & Lombardi, 1995;Walia et al., 2010;Yuce et al., 2017). Detonated products of active seismic sources used in this work are water and CO 2 (W. Wang et al., 2019). Here we have opportunity to verify this hypothesis.
Rn and CO 2 are released from pores of soil and rocks when seismic waves pass, produced by artificial seismic events as listed above. Release of CO 2 by the detonation leads to immediate increase of observed CO 2 concentration. Increases of CO 2 concentrations of 4.9%-54.1% at the Anninghe fault zone and 3.1%-128.6% at the Xiadian fault were measured during the observed period. Increases of the CO 2 flux during co-seismic period are 71% at the Anninghe fault zone and 172% at the Xiadian fault zone. At the same time, increases of Rn concentration are 18.9%-238.7% at the Anninghe fault zone and 10.5%-135.7% at the Xiadian fault zone. Increases of Rn flux during co-seismic period are 104% at the Anninghe fault zone and 84% and the Xiadian fault zone. Generally, increase of Rn concentration and flux are higher than those of CO 2 . Duration of the Rn emission to attain the maximum is usually shorter than the CO 2 emission. Those results show that emission of CO 2 clearly promote the transport of Rn.
For further analyzing relationships between CO 2 emission and Rn emission, we calculated the correlation of concentration and flux of CO 2 and Rn. CO 2 concentration is measured every 15 s and measured time for one Rn concentration is 5 min. Because of this different measured frequency for CO 2 and Rn concentration, we recal culated the 5-min averages of CO 2 concentration from the time of active seismic source detonation. Because measurement of concentration of CO 2 and Rn after the active seismic events at a site nearly started at same time, so correlation between the concentration of CO 2 and Rn can be calculated. The calculated correlation coefficients of CO 2 and Rn concentration are 0.66-0.87 at the Anninghe fault zone, and 0.68-0.92 at the Xiadian fault zone. And, we also calculated the correlation of CO 2 and Rn flux. In order to highlight the influence of seismic waves, we firstly calculated flux increase caused by the active seismic events and then correlation of the increase of CO 2 and Rn flux are analyzed (Figure 11). Correlation coefficients of co-seismic value and post-seismic values are Those results indicate that there is a good correlation between the observed emission of CO 2 and Rn after the artificial seismic event. Here, our observations provide direct evidence that CO 2 is the carrier gas for Rn. Furthermore, we point out that checking isotopic composition of Rn and CO 2 gases gathered from tubes that are connected to the Rn and CO 2 detectors would help to trace the two gases and provide more information and evidences for this conclusion. We plan to make related measurements in the future work.

Conclusion
An active seismic source based on methane gas detonation was employed to produce seismic events for exploring the effect of seismic waves on emission of Rn and CO 2 from soil in fault zones. The active seismic experiments were performed 20 times including 11 at the Anninghe fault zone, Sichuan and 9 at Xiadian fault zone, Hebei, China. After active source detonation, Rn concentrations immediately increased and reached to peak in 5-50 min and concentrations remained high during our post-seismic observation for about 1.5-3 hr. CO 2 concentrations also immediately increased and reached a peak in 30-60 min and remained high during our post-seismic observation for about 1.5-2.5 hr. The increase of Rn and CO 2 concentrations are 10.5%-238.7% and 3.1%-54.1%, respectively. The flux of Rn and CO 2 increased from pre-earthquake to co-earthquake and post-earthquake times, and most of the post-seismic flux values were lower than the co-seismic results. The highest increase of the Rn and CO 2 flux during the co-seismic period are 104% and 172%, respectively. The main factors that control Rn and CO 2 emission in this work are increase of permeability cause by the new fractures produced by the artificial seismic event and release of stored gas in the pores of soil and rocks caused by the passing seismic waves.
Our work provide first-hand in-situ observations of seismic effects on emission of soil Rn and CO 2 . The observed results confirm that seismicity has an important and direct effect on geochemical characteristics of soil Rn and CO 2 at fault zones. The observed emissions of CO 2 and Rn after the artificial seismic events show strong correlation and suggest that CO 2 is the carrier gas for Rn. This is the first direct field measurement of gas emission in a fault zone caused by the artificial seismic events. It is not only of great significance for monitoring fault activity and predicting aftershock, but also for understanding the genetic mechanism of seismic fluid geochemical precursors.
At the same time, degassing caused by seismic activity will continue to emit CO 2 , Rn and other gases into the atmosphere. The use of active sources with different energy levels to evaluate the degassing caused by seismic activity of different magnitudes, so as to evaluate the environmental and climate changes caused by seismic activity is also the focus of our future research.

Data Availability Statement
Data sets for this research are available in these in-text data citation references: Liu (2022