Geofluids
Volume 2020 (2020), Article ID 2359740, 14 pages
https://doi.org/10.1155/2020/2359740

Research Article

Mantle-Derived Helium Emission near the Pohang EGS Site, South Korea: Implications for Active Fault Distribution

Heejun Kim,¹ Hyunwoo Lee,¹ Jaemin Lee,2,3 Hyun A. Lee,² Nam Chil Woo,² Youn-Soo Lee,⁴ Takanori Kagoshima,⁵ Naoto Takahata,⁵ and Yuji Sano5,6

¹School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Republic of Korea
²Department of Earth System Sciences, Yonsei University, Seoul 03722, Republic of Korea
³Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea
⁴Division of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 37673, Republic of Korea
⁵Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba 277-8564, Japan
⁶Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China

Correspondence should be addressed to Hyunwoo Lee; lhw615@snu.ac.kr

Received 18 February 2020; Revised 3 July 2020; Accepted 6 July 2020; Published 24 July 2020

Academic Editor: Ondra Sracek

Copyright © 2020 Heejun Kim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

An Mw 5.5 earthquake occurred in Pohang, South Korea on November 15, 2017, resulting in a great impact on society. Despite a lot of controversy about the cause of the earthquake in relation to the enhanced geothermal system (EGS), the location of earthquake-related active faults is poorly known. Here, we first report the results of the geochemical and isotopic analyses of dissolved gases in groundwater in the Heunghae, Yeonil, and Sinkwang areas. According to the N₂-Ar-He relationship, samples from the Heunghae and Yeonil areas are contributed to the mantle, except for the Sinkwang area, where all samples are atmospheric. The Pohang samples consist mainly of N₂ and CO₂, and some samples of the Heunghae and Yeonil areas contain substantial CH₄. Stable isotope compositions of N₂ (), CO₂ (), and CH₄ () indicate that these components are derived from organic substances in sedimentary layer of Pohang Basin. On the other hand, helium isotope ratios (³He/⁴He, up to 3.83 Ra) represent the significant mantle contribution in the Heunghae and Yeonil areas. Through the distribution of high ³He/⁴He ratios, we propose that the Heunghae, Namsong, and Jamyeong faults are the passage of mantle-derived fluids. Computed ³He fluxes of the Heunghae (120 to 3,000 atoms cm⁻² sec⁻¹), Namsong (52 to 1,300 atoms cm⁻² sec⁻¹), and Jamyeong (83 to 2,100 atoms cm⁻² sec⁻¹) faults are comparable to other major active faults around the world, reflecting either high porosity or high helium flow rates. Therefore, our results demonstrate that there are active faults near the EGS facilities, which can provide the basis for future studies.

1. Introduction

Enhanced geothermal system (EGS) is a type of heat exchanger designed to improve the efficiency of geothermal energy plants. EGS is configured to enable convective production or to improve heat production. One of the main goals of EGS is to increase the permeability of reservoir rocks with high temperatures but low permeability. For this purpose, hydraulic fracturing, fluid injection (and/or extraction), and acidification can be used [1].

The correlation between EGS and seismic activity has been proposed for decades [1]. Geysers (USA), Cooper Basin (Australia), Berlin (El Salvador), Soultz-Sous-Forêts (France), and Basel (Switzerland) are well-known examples of EGS-related earthquake activities. In addition to these cases at the EGS site, for other cases such as wastewater injection, carbon capture and storage (CCS), or hydrocarbon (e.g., shale gas and oil) extraction, fluid injection, and hydraulic fracturing are often proposed as triggers of earthquakes (e.g., Keranen et al. [2]). Two mechanisms for triggering earthquakes associated with fluid injection and/or extraction are described by McGarr et al. [3]; (1) direct fluid pressure effects on injection and (2) changes in solid stress due to fluid extraction and/or injection. As we can infer from earthquake-inducing mechanisms of fluid injection, knowing locations of faults around the EGS site is important for society. For both cases, it is necessary to identify not only faults beneath the EGS site but also any potentially unknown faults. In the case of the 2019 Ridgecrest earthquake, for example, it shows how the earthquake swarm can propagate to interlocked faults [4]. This refers to the possibilities that the small induced earthquake can trigger much larger seismicity than expected, which amplifies the seismic hazards and risks around the EGS site.

Since mantle-derived fluids have been identified and quantified from the San Andreas Fault system [5] in the nonvolcanic region, noble gas studies have been conducted in several active fault zones to understand the fluid behavior related to seismicity (e.g., Sano et al. [6]). Even in some cases, helium isotope ratio distribution could detect concealed fault zones [7]. Noble gases and their isotope compositions can be used as natural tracers. They are chemically inert, retaining their properties through the water-rock systems. Therefore, the contents and isotopic compositions of noble gases allow us to trace the fluid sources into mantle, crust, and atmosphere. It is also possible to quantify the contribution of each source [8]. Mantle-derived helium and CO₂ degassing through faults in the southeastern Korean peninsula has recently been reported [9].

Here, we report new chemical and isotopic compositions of dissolved gases in groundwater, which are rarely been documented in the study area. Then, we will first discuss the general characteristics of gas compositions in this area. Based on the results, the perceptual impact on the composition of the noble gas is assessed to suggest that there are active fault zones near the EGS site, reaching the upper mantle through continental crust. In addition, helium flux through faults was calculated and compared with the characteristics of major fault zones around the world.

1.1. Geological Setting

The Pohang region consists of three subdivisions: Heunghae, Yeonil, and Sinkwang areas (Figure 1). The Heunghae and Yeonil areas are located in Pohang Basin, the Miocene sedimentary basin, and the Sinkwang area is near the Yangsan fault (Figure 1). Pohang Basin is one of the largest sedimentary basins in the Korean Peninsula [10]. The border faults in Pohang Basin are the Yeonil tectonic line, the Ocheon fault system, and segmented faults bounding to the west (Figure 1). These western boundary segment faults are almost parallel to the Yangsan fault, 2-7 km away from the west. During the early Miocene period, Pohang Basin was formed while the East Sea (Sea of Japan) which is a back-arc basin was opened, and the opening was ceased at ~15 Ma [11, 12]. The sedimentation in the Pohang Basin lasted from 17 to 10 Ma [11, 12]. The basin is filled with Paleogene volcanic rocks and granite, followed by Neogene conglomerates, sandstone, and mudstone, with a total thickness of less than 500 m [13, 14]. The basin has been cut by normal faults with the NNE-SSW strike and eastern dip. These normal faults have been formed after 15 Ma, blocked by other normal faults with the ENE-WSW strike [14]. The Sinkwang area is spread over Cretaceous biotite granite cut by the Yangsan fault covered with quaternary sediments (Figure 1, [14]). The Yangsan fault, located at the current Cretaceous sedimentary area, is a strike-slip fault formed in the early Cretaceous period as a result of tension due to the subduction of the Izanagi plate [15, 16]. After the subduction of the Pacific plate in the late Cretaceous period, compressive stresses affected the Yangsan fault. The direction of compressive stress was initially in the NW-SE direction at the end of the Cretaceous period, and the direction of subduction at the end of the Paleogene changed and moved in the NE-SW direction [15, 16].

Figure 1: Geological map of Pohang, South Korea (modified from Song [14]), created by GeoMapApp 3. 6. 10 [52]. Sample locations are marked with diamond symbols. Each symbol is represented by a helium isotope ratio (³He/⁴He). The location and scale (white circle) of aftershocks until 2017 are excerpts from the 2017 annual report of earthquakes published by the Korea Meteorological Agency (KMA). The gray triangle represents the EGS site, and the brown star stands for the epicenter of the Mw 5.5 earthquake that occurred on November 15, 2017.

1.2. 2017 Pohang Earthquake

The Pohang enhanced geothermal system (EGS) project was launched in November 2010 to produce 160°C geothermal water and 1.2 MW geothermal energy in a nonvolcanic area [17]. To construct the EGS facility, two boreholes (PX-1 and PX-2) were drilled through the sedimentary basin into the granodiorite basement rock. The measuring depths of the two boreholes (MD, measured along the borehole) are 4,362 m and 4,382 m, respectively. Hydraulic fracturing and fluid injection were performed from January 2016 to September 2017 to increase geothermal productivity [17]. During this period, five hydraulic stimuli were performed through PX-1 and PX-2. First, the third and fifth hydraulic stimuli were performed on the PX-2 with maximum well-head pressures of 89.2 MPa, 88.8 MPa, and 84.6 MPa, respectively. The second and fourth hydraulic stimuli were performed on the PX-1, and the maximum well-head pressures were 27.71 MPa and 25.16 MPa, respectively.

After the third hydraulic stimulation, an earthquake occurred with Mw 3.2. About two months after the cessation of the fifth hydraulic stimulus, on November 15, 2017, an earthquake with Mw 5.5 occurred (Figure 1), followed by more than one hundred aftershocks (≥ Mw 2.0). The earthquake was the second-largest earthquake in the Korean Peninsula since modern earthquake observation has begun in 1978, resulting in physical and economic damage to local residents (135 people were injured and more than 1,700 people were in emergency housing; directly USD 75 M and total economic impact USD 300 M) [17]. Due to this great impact on Korean society, it is necessary to study active faults related to potential seismic crises in this area.

2. Methods

We collected groundwater samples from groundwater wells in the Heunghae, Yeonil, and Sinkwang areas, Pohang, Republic of Korea (Table 1, Figure 1). Ranges of water temperatures and pH are 14.3 to 20.5°C and 6.0 to 9.0, respectively, and well depth ranges between 30 and 230 m from the topographic surface. The samples were stored in copper tubes and sealed with clamps, except for PH-3 that was collected in a preevacuated Giggenbach bottle. Dissolved gases were extracted from water samples by the high vacuum system and analyzed in the Atmosphere and Ocean Research Institute (AORI), the University of Tokyo. Concentrations (consist of CO₂, N₂, O₂, CH₄, Ar, and He) of dissolved gases were measured by a Pfeiffer QMS 200 quadrupole mass spectrometer (QMS). Stable isotope compositions of nitrogen for N₂ and carbon for CO₂ and CH₄ were measured by an isotope ratio mass spectrometer (Isoprime 100 by Elementar). For samples with high CH₄ concentration (>4%), coexisting CO₂ and CH₄ were separated before measurement by liquid nitrogen. To measure ³He/⁴He and ⁴He/²⁰Ne ratios, dissolved gas samples were purified by titanium getters at 400°C and charcoal traps at liquid nitrogen temperature. Neon was trapped by the cryogenic pump at 40 K after measuring ⁴He/²⁰Ne ratios via online QMS (Pfeiffer Prisma 80). Then, purified helium was injected into a noble gas mass spectrometer (Helix SFT by ThermoFisher) to measure ³He/⁴He ratios. Calibration of He isotope ratios was conducted by using the internal He standard of Japan (HESJ) [18]. Measured ³He/⁴He ratios were corrected for atmospheric helium by using measured ⁴He/²⁰Ne ratios, since ²⁰Ne is assumed to be mostly atmospheric [19]. From Sano et al. [20]:

where is the corrected ³He/⁴He ratio, and is the ⁴He/²⁰Ne ratio of air-saturated water (ASW). Analytical errors for and ³He/⁴He and ⁴He/²⁰Ne ratios are about 1% and 5% (1σ), respectively.

Table 1: Sampling information, gas composition, and ratios of the main components of the Pohang samples.

ID
Region
Latitude
Longitude
Date
Type
T
pH
Well depth
He
CH₄
N₂
O₂
Ar
CO₂
He/Ar
N₂/Ar
N₂/He

°N
°E

°C

m
ppm
(vol.%)

P-11
Heunghae
36.08608
129.321591
2019-02-21
Groundwater
15.4
8.5
120
104
0.2
94.0
—
2.1
4.1
0.005
45
9,041

P-12

36.12613
129.382112
2019-02-20
Groundwater
14.3
7.3
154
215
0.6
88.1
2.0
1.7
7.6
0.01
53
4,099

P-14

36.12574
129.360214
2019-02-20
Groundwater
16.4
7.2
110
170
36.7
32.5
0.2
0.6
30.0
0.03
51
1,909

DS-1

36.14254
129.319276
2019-02-21
Groundwater
15.4
7.9
120
279
—
63.5
—
1.3
36.6
0.02
50
2,277

PH19-01

36.11937
129.346867
2019-04-11
Groundwater
18.0
7.0
130
9
3.8
73.0
—
1.6
22.0
0.001
46
81,078

PH19-03

36.13743
129.357115
2019-04-12
Groundwater
16.5
6.0
30
14
0.2
53.8
—
1.1
45.4
0.001
51
38,400

PH19-04

36.13393
129.36331
2019-04-12
Groundwater
19.1
7.0
150
44
0.6
76.3
—
1.3
22.0
0.003
57
17,332

PH19-05

36.13235
129.366165
2019-04-12
Groundwater
17.7
7.0
150
101
1.3
61.8
—
1.3
35.7
0.01
49
6,114

PH19-06

36.12957
129.349261
2019-04-12
Groundwater
17.1
7.0
78
468
0.4
66.2
1.4
1.5
30.5
0.03
45
1,414

PH19-10

36.07346
129.326287
2019-04-13
Groundwater
15.9
7.0
120
12
0.6
62.9
—
1.4
35.6
0.001
46
52,442

PH19-11

36.07565
129.32894
2019-04-13
Groundwater
16.8
7.0
120
43
0.1
77.7
—
1.6
21.0
0.003
48
18,063

PH19-12

36.09678
129.350515
2019-04-13
Groundwater
17.3
6.0
230
701
18.7
47.5
—
1.0
33.1
0.07
49
678

PH19-07
Yeonil
36.01893
129.306846
2019-04-13
Groundwater
20.5
8.0
160
2744
34.3
57.9
—
0.9
6.9
0.3
64
211

PH19-08

36.02889
129.304796
2019-04-13
Groundwater
20.2
8.0
160
597
42.9
48.1
—
0.9
8.3
0.1
55
806

PH19-09

36.02730
129.302284
2019-04-13
Groundwater
16.1
7.0
—
7
0.1
75.2
—
1.6
23.4
0.0004
46
107,400

PH-3

36.02572
129.311889
2018-11-17
Groundwater
17.0
9.0
101
8
31.0
60.0
8.0
0.9
0.1
0.001
67
74,988

P-2
Sinkwang
36.10668
129.248979
2019-02-20
Groundwater
15.4
7.1
120
16
0.1
84.2
—
1.3
14.7
0.001
67
52,631

P-3

36.19378
129.242729
2019-02-20
Groundwater
15.2
7.4
210
16
0.1
74.6
—
1.6
24.3
0.001
48
46,613

P-4

36.10299
129.245568
2019-02-20
Groundwater
15.0
6.6
88
11
0.1
69.3
0.8
1.3
28.5
0.001
53
63,000

P-5

36.11097
129.247894
2019-02-20
Groundwater
14.8
7.1
120
32
—
60.8
—
1.2
41.4
0.003
51
19,009

P-7

36.14834
129.244634
2019-02-20
Groundwater
16.0
6.5
178
14
0.1
61.5
—
1.2
37.8
0.001
52
43,921

3. Results

The measured gas compositions are reported in Table 1. N₂ is the most abundant gas component for most samples with the range of 32.5 to 94.0 vol.%. CO₂ is also observed in all samples ranging from 0.1 to 45.4 vol.%. CH₄ is also one of the main components for samples in the Heunghae and Yeonil areas (for example, P-14, PH19-12, PH19-07, PH19-08, and PH-3) with the range from 18.7 to 42.9 vol.%. Ar and He range from 0.6 to 2.1 vol.% and from 7 to 2744 ppm, respectively. Except for PH-3 (), almost no O₂ is observed in most samples, indicating minimum air contamination during sampling and analysis. Isotopic compositions of N, C, and He are summarized in Table 2. The nitrogen isotope compositions of N₂ (δ¹⁵N-N₂) for all samples range from 0.19 to 3.56‰, all heavier than air (0‰). Carbon isotope values of CO₂ (δ¹³C-CO₂) for all samples range from -27.33 to -16.01‰. There is no significant regional difference for δ¹⁵N-N₂ and δ¹³C-CO₂. The carbon isotope composition of CH₄ (δ¹³C-CH₄) is from -76.05 to -70.04‰, implying similar CH₄ sources for both Heunghae and Yeonil areas. In these areas, most samples show ⁴He/²⁰Ne ratios higher than ASW (, [21]), with ³He/⁴He ratios ranging from 0.18 to 3.83 Ra, where Ra is the ³He/⁴He ratio of air (, [22]). The ³He/⁴He ratios (0.90 to 1.22 Ra) of the Sinkwang groundwater samples are atmospheric because ⁴He/²⁰Ne ratios of the samples are similar to air.

Table 2: Analysis of stable isotopes and noble gases in the Pohang area.

ID
Area
³He/⁴He
Error
³He/⁴He
⁴He/²⁰Ne
²⁰Ne/⁴He
⁴He
²⁰Ne
δ¹⁵N
Error
N₂/³He
δ¹³C
Error
CO₂/³He
δ¹³C
Error
CH⁴/³He

R/Ra

Rc/Ra

ccSTP/g
ccSTP/g
N₂
(1σ)

CO₂
(1σ)

CH₄
(1σ)

P-11
Heunghae
0.18
0.02
0.05
1.8
0.6
4.06.E-07
2.22.E-07
2.15
0.48
3.52E+10

1.53E+09

8.98E+07

P-12

3.83
0.01
3.92
7.7
0.1
1.91.E-06
2.48.E-07
3.15
0.35
7.70E+08
-19.49
0.31
6.64E+07

5.59E+06

P-14

2.69
0.01
2.71
21.4
0.1
2.66.E-06
1.24.E-07
3.16
0.53
5.11E+08
-19.49
0.31
4.72E+08
-70.04
0.41
5.78E+08

DS-1

0.87
0.02
0.80
0.7
1.4
1.65.E-07
2.37.E-07
2.43
0.28
1.89E+09
-27.33
0.6
1.09E+09

—

PH19-01

1.30
0.01
1.38
1.2
0.9
2.04.E-07
1.75.E-07
3.12
0.3
4.55E+10
-16.01
0.35
1.37E+10

2.38E+09

PH19-03

1.35
0.01
2.57
0.3
3.1
8.59.E-08
2.67.E-07
2.62
0.3
2.03E+10
-23.08
0.31
1.71E+10

6.78E+07

PH19-04

1.99
0.01
2.12
2.2
0.5
2.99.E-07
1.38.E-07
2.42
0.26
6.21E+09
-25.29
0.32
1.79E+09

5.05E+07

PH19-05

2.49
0.01
2.54
7.1
0.1
1.32.E-06
1.87.E-07
0.96
0.25
1.77E+09
-25.77
0.31
1.02E+09

3.83E+07

PH19-06

3.38
0.01
3.45
8.9
0.1
1.75.E-06
1.98.E-07
1.80
0.25
3.02E+08
-23.14
0.37
1.39E+08

1.87E+06

PH19-10

0.79
0.01
0.29
0.4
2.8
7.22.E-08
2.03.E-07
1.78
0.27
4.99E+10
-20.78
0.32
2.82E+10

4.92E+08

PH19-11

0.36
0.02
0.06
0.8
1.3
1.58.E-07
2.01.E-07
1.25
0.22
3.60E+10
-20.66
0.38
9.74E+09

5.10E+07

PH19-12

2.25
0.01
2.26
61.9
0.02
6.84.E-06
1.11.E-07
1.57
0.26
2.17E+08
-25.71
0.3
1.51E+08
-76.05
0.47
8.51E+07

PH19-07
Yeonil
3.09
0.01
3.09
145.3
0.01
1.07.E-05
7.35.E-08
1.26
0.24
4.92E+07
-21.66
0.34
5.88E+06
-75.40
0.35
2.91E+07

PH19-08

2.32
0.01
2.33
61.9
0.02
6.96.E-06
1.12.E-07
0.19
0.23
2.50E+08
-23.70
0.46
4.32E+07
-75.13
0.33
2.23E+08

PH19-09

1.00
0.01
1.00
0.4
2.8
4.83.E-08
1.36.E-07
2.06
0.24
7.60E+10
-20.09
0.34
2.36E+10

6.06E+07

PH-3

1.26
0.03
1.31
1.5
1.3

0.33
0.01
4.07E+10

8.81E+07
-71.47
0.1
2.10E+10

P-2
Sinkwang
0.90
0.02
0.33
0.3
3.4
1.21.E-07
4.11.E-07
1.44
0.21
4.21E+10
-26.31
0.41
7.35E+09

3.00E+07

P-3

0.91
0.01
0.70
0.4
2.8
7.35.E-08
2.07.E-07
3.56
0.36
3.66E+10
-26.69
0.47
1.19E+10

2.45E+07

P-4

0.96
0.02
0.86
0.4
2.8
8.41.E-08
2.37.E-07
1.21
0.22
4.54E+10
-25.43
0.36
1.87E+10

3.28E+07

P-5

1.09
0.01
1.51
0.3
3.3
7.74.E-08
2.55.E-07
3.07
0.27
1.27E+10
-25.57
0.38
8.62E+09

—

P-7

1.22
0.01
2.30
0.3
3.3
1.35.E-07
4.47.E-07
1.91
0.29
2.57E+10
-23.86
0.32
1.58E+10

2.09E+07

Note: the standards for δ¹⁵N and δ¹³C are air and Pee Dee Belemnite (PDB), respectively. Concentrations of ⁴He and ²⁰Ne of PH-3 were not measured because the sampling method was different. The carbon isotope ratios of CO₂ in P-11 and PH-3 were not measured due to their low concentrations.

4. Discussion

4.1. Gas Geochemistry

Based on the N₂-Ar-He ternary diagram (Figure 2), nonreactive gases (N₂, Ar, and He) in the Heunghae and Yeonil areas display a two-component mixing relationship between the mantle and atmospheric end members. Lee et al. [9] have shown that fault gases in the southeastern Korean peninsula are continental gases rather than subduction zone gases. Dissolved gases in groundwater from the Sinkwang area are atmospheric with low He/Ar ratios (0.003 to 0.001). Lee et al. [9] addressed that some gas samples which are severely contaminated by air in southeastern Korea are due to shallow well depths (130 to 296 m). However, in the Heunghae and Yeonil areas, it is unlikely that well depths (Table 1) and contribution from deeply derived gases are relevant. Instead, we suggest that distance to the permeable fault zones where gases released from deep sources are transported is important. In general, groundwater wells in the Heunghae and Yeonil areas contain more dissolved helium (Table 2), implying there are more permeable areas than the Sinkwang area.

Figure 2: N₂-Ar-He ternary plot. Reference data of fault-related gas samples in the southeastern part of the Korean peninsula (white diamond) is shown [9]. All samples are mixed between MORB, air, and air-saturated water (ASW) and displayed as the trend of continental gas.

CO₂ concentrations are negatively correlated with pH (), indicating that CO₂ is likely to be trapped in high pH water to be present as HCO₃⁻ and CO₃²⁻ (Figure S1a, [23, 24]). To verify this, we show that CO₂/N₂ () and CO₂/CH₄ () ratios also have negative trends with pH (Figure S1b, c). It is attributable that CO₂ removal increased those ratios because N₂ and CH₄ are not responsive to pH changes. Also, N₂ and/or CH₄-rich gases are found in alkaline springs [23, 25, 26]. Moreover, CO₂/CH₄ ratios and helium concentrations show a negative correlation (, Figure S1d). Although Lee et al. [9] suggested both CO₂ and helium are derived from the mantle source in southeastern Korea, it is plausible that their origins are decoupled in the Pohang region.

4.2. Origins of Nitrogen, Carbon Dioxide, and Methane

δ¹⁵N-N₂ values of all samples with the range of 0.19 to 3.56‰ are between the air (0‰) and sediment (7‰) end-members. By plotting δ¹⁵N with N₂/³He ratios (Figure 3(a)), we can identify contributions of the air, sediment, and mantle end-members. To quantify the contribution of each end-member, we adopted the three-component mixing model from Sano et al. [27]:

where obs is the observed value; , , and are the contributions of the mantle, sediments, and air; δ¹⁵Nmantle, δ¹⁵Nsediment, and δ¹⁵Nair are , , and 0‰; N₂/³Hemantle, N₂/³Hesediment, and N₂/³Heair are , , and , respectively [21, 27, 28, 29 and references therein]. The results are summarized in Table S1. Air is the most dominant source for N₂ in the Pohang region with the fA range of 49.1 to 97.0%. Also, sediment is another main source for N₂ with the fS range of 2.8 to 50.9%. The mantle contribution is very minor (), indicating N₂ is the primarily sedimentary origin and is contributed by air at shallow depths. Moreover, the sediment-derived N₂ in the Pohang region is of shallow origin rather than the recycled nitrogen through subduction as discussed in section 4.1.

(a)

(b)

(c)

Figure 3: (a) N₂/³He versus δ¹⁵N diagram. Each end-member and mixing lines are described in section 4.2. (b) δ¹³C of CO₂ versus 1/CO₂ diagram (modified from Lee et al. [53]). Mantle value of δ¹³C () is from Sano and Marty [30]. The biogenic CO₂ area ranges between δ¹³C values of C3 plant and C4 plant [31]. The air value of δ¹³C and CO₂ concentration are from Lewicki and Brantley [54]. (c) CH₄/³He versus δ¹³C of CH₄ diagram (modified from Sano et al. [55]). Reference data from northeastern Asia is shown together (Sano et al. [55]).

The Pohang region has no mantle-derived CO₂ which has been reported in Gyeongju and Ulsan areas, southeastern Korea (Lee et al., 2019). δ¹³C values (-27.33 to -16.01‰) of CO₂ in all samples are lighter than the MORB value (, [30]) and lie approximately between the mean δ¹³C-CO₂ values of C3 (-27‰) and C4 (-13‰) plants [31] (Figure 3(b)). The results are similar to those of most fault gases previously reported in southeastern Korea () as well as δ¹³C-CO₂ values (−11.9 to −24.0‰) in global fault zones without magma activity, such as San Andreas Fault and North Anatolian Fault [9 and references therein]. On the δ¹³C-CO₂ vs CO₂/³He plot (Figure S2), a majority of samples are outside the mixing curve between the biogenic and mantle end-members due to their low CO₂/³He ratios. As discussed in section 4.1, the decrease in CO₂/³He ratios can be attributed to CO₂ loss in this area under the influence of pH. Moreover, the negative correlation () between ⁴He concentrations and CO₂/³He ratios is displayed well (Figure S1e). Gilfillan et al. [32] have shown the same trend for natural gas fields in North America. They argued that noble gases are unlikely involved in increasing or decreasing CO₂/³He ratios. However, the trend between ³He/⁴He and CO₂/³He ratios (Figure S1f) is also negatively correlated (). Thus, in the Pohang region, we argue that not only CO₂ loss but also external helium was introduced to reduce CO₂/³He ratios in the local groundwater layer (see section 4.3-4.5). This further supports that CO₂ source derived from shallower sediments than the mantle.

The origin of CH₄ in Pohang is relatively uniform. In the measured samples (Table 2), δ¹³C values of CH₄ range from -76.05 to -70.04‰, indicating a typical microbial origin [33]. Considering δ¹³C of CO₂, the mechanism of production of CH₄ is resulted by carbonate reduction with slight oxidation after the process of methanogenesis (Figure S2b, [34]). Therefore, the isotope separation factors (εCCO₂-CH₄) which are approximately from 54.36 to 58.12 allow us to estimate the growth temperature of CH₄ at ~40°C [34].

4.3. Helium Isotope Geochemistry in the Pohang Area

In the Pohang region, higher ³He/⁴He ratios (up to 3.83 Ra) than ASW/air are found in the Heunghae and Yeonil areas. Although elevated ³He/⁴He ratios can be resulted by the ³H-derived ³He, it is unlikely because the samples with high ³He/⁴He ratios also have high ⁴He/²⁰Ne ratios (up to 145.29). Helium is also remobilized from old igneous rocks [35]. It is known that Pohang Basin is filled by sediments on the granodiorite basement rocks with some Tertiary basaltic rocks (Daljeon basalt) which erupted at 13.8 Ma [14 and references therein]. However, the basaltic rocks show a limited distribution (<1 km²) in the Yeonil area [14 and references therein]. Considering the typical ⁴He contents in inclusion bearing olivine (10⁻⁸ to 10⁻⁹ ccSTP/g, [8, 35]) and the ⁴He concentrations of high ³He/⁴He ratio samples from the Yeonil area ( to ccSTP/g), the amounts of trapped ³He in the Daljeon basalt is insufficient to be the source of ³He in the Yeonil area. Furthermore, in consideration of general Li concentration of igneous and sedimentary rock, radiogenic ³He by decay reaction ⁶Li(n, α)³H(β-)³He cannot affect ³He/⁴He ratio of groundwater [35 and references therein]. Therefore, the excess ³He in the Pohang region originates from the mantle as well as ⁴He [23, 36].

The range of ³He/⁴He ratios (0.18 to 3.83 Ra) in the Heunghae area is wide, indicating that both mantle and crustal helium sources are present in a short range, up to 10 km (Figures 1 and 4). Even though some samples (PH19-09 and PH-3) in the Yeonil area are atmospheric (Figures 1 and 4), two samples (PH19-07 and PH19-08) have high ³He/⁴He (3.09 Ra and 2.32 Ra) and ⁴He/²⁰Ne (145.29 and 61.86) ratios, implying the presence of mantle-derived fluids. All samples in the Sinkwang area show atmospheric ³He/⁴He (0.90-1.34 Ra) and ⁴He/²⁰Ne (0.27-0.36) ratios (Figures 1 and 4). A ³He/⁴He ratio of up to 5.69 Ra was reported for the Yangsan fault zones [9]. The absence of mantle signatures for all samples in the Sinkwang area can be explained by the distance from the main fault line (Figure 1), which means the influx of external helium is quite low.

Figure 4: ³He/⁴He versus ²⁰Ne/²²Ne diagram. The reference values for MORB, ASW, and Crust are shown. Each line represents mixing lines between ASW and external helium sources with different MORB contributions.

The highest ³He/⁴He ratio (3.83 Ra) represents about 50% of the mantle contribution to the fluid (Figure 4). It is known that mantle-derived helium can be actively released to the surface through magmatism [37, 38]. However, the Pohang region is located hundreds of kilometers away from the active volcanoes of the Japanese arc (Figure 1). A low-velocity zone beneath Ulleungdo has been proposed by Chen et al. [39] and references therein, which is also 200 km away from Pohang. Moreover, magma activity in this area has been ceased after 9,300~6,300 BP [40 and references therein].

The appropriate model for the occurrence of mantle-derived helium in this region is that there are permeable fault zones like the release of mantle fluids in the San Andreas Fault zones [5]. According to Song [14], there are some faults in the Pohang region, such as Heunghae, Gokgang, Hyeongsan, and Ocheon faults (Figure 1). Also, Westaway and Burnside [41] named the new fault as the Namsong fault on the basis of the aftershock distribution of the Pohang earthquake (Figure 1) and proposed that the fault has been already critically stressed before the EGS project [41]. In addition, according to the Korea Meteorological Administration (KMA), the depths of the 2017 Pohang earthquake and aftershocks are less than 16 km, which is shallower than the Moho depth (~28 km) of the region [42]. Kennedy and Van Soest [37] suggested that the mantle fluids of the San Andreas Fault penetrate the brittle-ductile boundary based on helium isotope ratios and strain rates measured by GPS. To explain the mantle-derived helium in the Gyeongju and Ulsan areas, southeastern Korea, Lee et al. [9] also proposed that mantle helium migrates along the ductile shear zone underneath the brittle regime.

From the above information, in the Pohang region, we suggest that there have been already tectonically active areas that have developed from the ductile shear zone to the brittle fault zone. This condition enabled the inflow of the mantle-derived fluids through the lower crust into the permeable faults. Furthermore, in this area, relatively high temperatures, heat contents, and heat flows have been reported [43 and references therein], supporting that ³He came from the mantle through active faults (e.g., Umeda et al. [44]).

4.4. Distribution of Active Faults

As discussed in section 4.3, we identified the existence of permeable faults in the Heunghae and Yeonil areas. The locations of the faults can be constrained based on the geographical distribution of helium isotope ratios [7]. Since the latitude variation in the ³He/⁴He ratios is the most prominent to specify fault locations, we can show the relationship between the ³He/⁴He ratios and latitude (Figure S3). Although it is known that the location of the Heunghae fault is still ambiguous, we found that the distribution of higher ³He/⁴He ratios are well consistent with the fault striking EW at 36.126°N (Figure 1). Also, there is a relationship between higher ³He/⁴He ratios and distance from the fault line (Figure 5(a)), which has been observed in the San Andreas, North Anatolian, and Karakoram faults [5, 8, 37, 45]. In Figure 5(a), we could observe a sample (PH19-12) with a high ³He/⁴He ratio (2.25 Ra), which is about 3 km away from the Heunghae fault to the south (Figures 1, S3). The sampling location of PH19-12 is still in the area where aftershocks have frequently occurred (Figure 1), suggesting there can be another ³He discharge in the Heunghae area. To confirm the pathway of mantle-derived helium, we calculated the distance of all samples from the Namsong fault (Figure 5(b)). Based on the mainshock strike (N34°E) and dip (51°NW), we assumed that the easternmost boundary of the aftershock occurrences (from 2017 Annual report of Earthquake) with the N34°E strike is the uppermost line of the fault. By using this uppermost line, the distance from the closest fault and ³He/⁴He ratios are displayed (Figure 5(c)), showing a better correlation with exponential distribution () than Figure 5(a). To validate the relationship between the distance from faults and ³He/⁴He ratios, we selected six samples at latitudes higher than P12 (Figure 5(a), red circles). These samples are well correlated with distance from the Heunghae fault exponentially (), validating the model of the Heunghae-Namsong fault system as shown in Figure 5(c). Samples with lower ³He/⁴He ratios than 1 Ra indicate that the inflow of ³He into the aquifer is less than crustal or atmospheric contributions (Figure 4). Although it is not well known about the exact fault locations in the Yeonil area, previous studies have reported the presence of faults [14 and references therein]. Based on some samples with high ³He/⁴He ratios (Figure 1), we suggest that there can be a highly permeable fault zone. In this study, we propose to name the Jamyeong fault considering the name of the village called Jamyeong-ri in this area.

(a)

(b)

(c)

Figure 5: (a) ³He/⁴He ratios to distance from the Heunghae fault line. Samples at latitudes higher than P-12 are marked with red circles. (b) ³He/⁴He ratios to distance from the Namsong fault line, which is not prominent. (c) ³He/⁴He ratios to distance from closer fault lines between the Heunghae and Namsong fault lines. Samples closer to the Namsong fault than the Heunghae fault are marked with blue circles.

4.5. Helium Flux from the Faults

To compare fluid dynamics with other fault systems in the world, helium flow rates and ³He flux were estimated. We calculated helium flow rates from the most reliable mantle helium source from each fault (P-12 for Heunghae fault, PH19-07 for Jamyeong fault, PH19-12 for Namsong fault), following Menzies et al. [46]:

where is the helium flow rate (cm/yr); is the crust thickness; and are the density of the crust and the fluid, respectively; is the current production rate of ⁴He from crust; is the concentration of helium in the original mantle fluid; , , and represent helium isotopic ratios (³He/⁴He) of the sample, crust, and mantle, respectively. The crust below the Pohang region was assumed to be a double layer with thicknesses for the upper and middle crust of 12.3 and 15.6 km, respectively [42, 47]. We used the average density values of the middle crust (2.72 g/cm³) and the upper crust (2.65 g/cm³) as and the water density (1 g/cm³) as [48 and references therein]. From Menzies et al. [46], P(He) (ccSTP g⁻¹ yr⁻¹) can be obtained as:

where ⁴He production rates of U and Th are ccSTP μg⁻¹ yr⁻¹ and ccSTP μg⁻¹ yr⁻¹, respectively; and are concentrations for U and Th in ppm; is the porosity of the material. By using the average concentration of each layer for U (middle crust: 0.7 ppm and upper crust: 2.7 ppm) and Th (middle crust: 0.63 ppm and upper crust: 10.5 ppm) [48 and references therein], Hcρc P(He) for each crust layer (in equation (3)) was calculated and were added all up. The [He]F.m for each sample was calculated using Rs (0.02 Ra), Rm (8 Ra), and helium concentration of each sample.

Each variable is measured or obtained except , resulting in that the helium flow rate is a function of porosity (). The porosity of Yeongnam Massif granodiorite underneath the study area is 0.48% [49]. The porosity measured by wireless logging along PX2 at depth is 5.2% [49], which includes the void volume of fractures. The porosity of the fault zone itself should be higher than that of the fractured basement rock. Therefore, we assumed four different porosity conditions with which are about 0.2, 1, 2, and 4 times the porosity of the basement rock.

Calculated helium flow rates (qHe) for a given porosity range from 26.95 cm/yr to 128 cm/yr (Heunghae fault), 5.34 cm/yr to 25.37 cm/yr (Namsong fault), and 4.00 cm/yr to 19.02 cm/yr (Jamyeong fault, Table S2). Compared with other global fault zones (1.7 to 12.7 cm/yr of San Andreas Fault; 0.87 cm/yr of North Anatolian Fault; 55 cm/yr of Alpine Fault, New Zealand) [46, 50], faults in the Pohang region show relatively high helium flow rates. To assume that faults in the Pohang region have helium flows of the approximately same magnitude, the porosity of the faults need to be higher than other fault systems. Thus, in the fault zones of the Pohang region, it is believed that the porosity is high, or the helium flow rate is high. With the calculated helium flow rates and measured helium concentration of each sample, we were able to calculate the ³He flux for each sample site by using measured ³He/⁴He ratios. The ³He flux per unit area () can be calculated as:

where is measured ⁴He concentration of each sample; is helium isotopic composition (³He/⁴He). The calculated ³He flux values are 120 to 3,000 atoms cm⁻² sec⁻¹ (Heunghae fault), 52 to 1,300 atoms cm⁻² sec⁻¹ (Namsong fault), and 83 to 2,100 atoms cm⁻² sec⁻¹ (Jamyeong fault, Table S2).

In Figure 6, samples related to the Heunghae fault show the correlation between ³He flux and fault. Considering the size of the study area (<10 km from the fault), this trend can be compared with the results near the Futagawa fault (<40 km from the fault) in the Kumamoto area, Kyushu, Japan, where an earthquake of magnitude 7.3 occurred on April 16, 2016 (Sano et al. [6], Table S3, Figure 7). The Heunghae fault zone shows the sharper pattern than the Futagawa fault zone because samples from this area are collected in a smaller area. The maximum flux of the Futagawa fault is 5,600 atoms cm⁻² sec⁻¹, which is higher than that of the Heunghae area because the sample is close to the fault as well as Mt Aso to supply helium from the nearby magma [6].

Figure 6: ³He flux values displayed according to the distance from the Heunghae fault in the Pohang region (this study) and the Futagawa fault in the Kumamoto region, Japan [6]. The assumptions about the calculations are described in section 4.5.

Figure 7: ³He flux of the Heunghae (P-12), Namsong (PH19-12), and Jamyeong faults (PH19-07). Reference data from San Andreas Fault Zone (SAFZ, [50]), North Anatolian Fault[50], Alpine Fault [46], Eastern Morogongo basin [56], North-Central Sicilly thermal basin (NCStb) [57], extensive tectonics settings [22], and Northern Taiwan [51].

Like the helium flow rate, this ³He flux is relatively high compared to other major fault systems, such as 1.7 to 34 atoms cm⁻² sec⁻¹ of the San Andreas Fault, 75 atoms cm⁻² sec⁻¹ of the North Anatolian Fault, and 170 atoms cm⁻² sec⁻¹ of the Alpine Fault, New Zealand [46, 50], showing high porosity in the fault zone or high ³He flux per unit area (Figure 7). Also, this ³He flux is several orders of magnitude higher than the continental ³He flux in steady-state (3.9 to 7.2 atoms cm⁻² sec⁻¹, from Sano et al. [51]). Therefore, the helium flux results suggest that the Pohang region may have faults comparable to other active fault zones around the world (Figures 1, 6).

5. Conclusions

We first analyzed dissolved gases in groundwater in the Pohang region, South Korea, where the Mw 5.5 earthquake occurred on November 15, 2017. The N₂-Ar-He relationship shows that there is the contribution of the mantle component in the Heunghae and Yeonil area samples, which is similar to that previously reported in fault zones of the southeastern Korean peninsula [9]. However, the dissolved gases in the Sinkwang area are mostly close to atmospheric components. N₂ (32. 5 to 94.0 vol.%) and CO₂ (0.1 to 45.4 vol.%) are present in all areas of the Pohang region, and CH₄ (18.7 to 42.9 vol.%) is observed as a major component in some samples of the Heunghae and Yeonil areas. The results of the stable isotope analysis indicate that N₂ (), CO₂ (), and CH₄ () in the Pohang region are derived from organic material sources at shallow depths. Helium isotope ratios (³He/⁴He) with mantle signatures (up to 3.83 Ra) are observed in the Heunghae and Yeonil areas except in the Sinkwang area, where atmospheric ³He/⁴He ratios are mainly observed. Helium originates from the mantle, but the Pohang region, a sedimentary basin formed during the Miocene period, is believed to contain a large amount of organic matter that can be the source for N₂, CO₂, and CH₄. The distribution of the helium isotope ratio seems to be related to the locations of faults which are permeable passage. Based on the observation, we suggest that the Heunghae, Namsong, and Jamyeong faults in the Pohang region are active faults that release the mantle fluids. Although the Heunghae and Namsong faults are close to the EGS facilities, considering the depths (<10 km) of the 2017 earthquake and aftershocks in the area, the Moho depth (~28 km) is far below, which is similar to the Gyeongju and Ulsan areas [9]. Thus, we propose that there were already active faults extending into the ductile shear zone to release the mantle helium. In order to show that the faults in this area are active, we computed ³He flux () for the Heunghae (120 to 3,000 atoms cm⁻² sec⁻¹), Namsong (52 to 1,300 atoms cm⁻² sec⁻¹), and Jamyeong (83 to 2,100 atoms cm⁻² sec⁻¹) faults. These values are comparable to those in the regions known as active faults around the world, which may be due to either high porosity or high helium flow rates. Therefore, our results demonstrate that there are active faults in Pohang, especially around the EGS facilities, and will provide important information for future research.

Data Availability

All the data in this study is contained in the tables of both the main manuscript and the Supplementary Materials.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

This study was financially supported by the fundamental research project of Korea Institute of Geoscience and Mineral resources (KIGAM) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2019R1G1A1002297). We are grateful to Myungok Jung and Kwangtae Kim for their support on the sampling.

Supplementary Materials

Figure S1: (a) relative concentration of CO₂ and pH plot. (b) CO₂/N₂ and pH plot. (c) CO₂/CH₄ and pH plot. (d) CO₂/CH₄ and concentration of ⁴He plot. (e) CO₂/³He and concentration of ⁴He plot. (f) CO₂/³He and ³He/⁴He ratio plot. Figure S2: (a) CO₂/³He and δ¹³C plot. Dashed arrow indicates CO₂/³He and δ¹³C decrease trend due to CO₂ trap, which described in Section 4.2. (b) δ¹³C of CO₂ and δ¹³C of CH₄ plot (modified from Wfhiticar [34]). Figure S3: helium samples from the Heunghae area (diamond symbol) and the helium isotope and latitude plot. Each symbol is colored by its helium isotope ratio (³He/⁴He). The locations of Heunghae (HF) and Namsong (NF) fault lines (dashed line) are described in Section 4.4. The gray triangle indicates the location of the EGS site, and the brown star indicates the location of the Mw 5.5 earthquake. Table S1: δ¹⁵N, N₂/³He, and the contribution of three nitrogen endmembers on the Pohang samples: the mantle, sediment, and the air. The δ¹⁵N and N₂/³He of each endmember and the mixing model are described in Section 4.2. Table S2: the corrected Helium isotope ratio, the concentration of helium in original mantle fluid, the helium flow rate, and the ³He flux of each fault system. Table S3: the ⁴He concentration, corrected ³He/⁴He ratio, and ³He flux of each sample and their distance from related faults, respectively. The data of the Futagawa fault is from Sano et al. [6]. (Supplementary Materials)

References

E. L. Majer, R. Baria, M. Stark et al., “Induced seismicity associated with Enhanced Geothermal Systems,” Geothermics, vol. 36, no. 3, pp. 185–222, 2007.

K. M. Keranen, H. M. Savage, G. A. Abers, and E. S. Cochran, “Potentially induced earthquakes in Oklahoma, USA: links between wastewater injection and the 2011 Mw 5.7 earthquake sequence,” Geology, vol. 41, no. 6, pp. 699–702, 2013.

A. McGarr, D. Simpson, and L. Seeber, Case Histories of Induced and Triggered Seismicity, in: International Handbook of Earthquake and Engineering Seismology, Academic Press LTD, 2002.

Z. E. Ross, B. Idini, Z. Jia et al., “Hierarchical interlocked orthogonal faulting in the 2019 Ridgecrest earthquake sequence,” Science, vol. 366, no. 6463, pp. 346–351, 2019.

B. M. Kennedy, “Mantle fluids in the San Andreas fault system, California,” Science, vol. 278, no. 5341, pp. 1278–1281, 1997.

Y. Sano, N. Takahata, T. Kagoshima, T. Shibata, T. Onoue, and D. Zhao, “Groundwater helium anomaly reflects strain change during the 2016 Kumamoto earthquake in Southwest Japan,” Scientific Reports, vol. 6, no. 1, 2016.

K. Umeda and A. Ninomiya, “Helium isotopes as a tool for detecting concealed active faults,” Geochemistry, Geophysics, Geosystems, vol. 10, no. 8, 2009.

S. L. Klemperer, B. M. Kennedy, S. R. Sastry, Y. Makovsky, T. Harinarayana, and M. L. Leech, “Mantle fluids in the Karakoram fault: helium isotope evidence,” Earth and Planetary Science Letters, vol. 366, pp. 59–70, 2013.

H. Lee, H. Kim, T. Kagoshima, J.-O. Park, N. Takahata, and Y. Sano, “Mantle degassing along strike-slip faults in the Southeastern Korean Peninsula,” Scientific Reports, vol. 9, no. 1, article 15334, 2019.

S. K. Chough, S.-T. Kwon, J.-H. Ree, and D. K. Choi, “Tectonic and sedimentary evolution of the Korean peninsula: a review and new view,” Earth-Science Reviews, vol. 52, no. 1-3, pp. 175–235, 2000.

J.-M. Kim, “Early Neogene biochemostratigraphy of Pohang Basin: a paleoceanographic response to the early opening of the Sea of Japan (East Sea),” Marine Micropaleontology, vol. 36, no. 4, pp. 269–290, 1999.

S. Yi and H. Yun, “Miocene calcareous nannoplankton from the Pohang Basin, Korea,” Palaeontographica Abteilung B, vol. 237, pp. 113–158, 1995.

T. J. Lee, Y. Song, D.-W. Park, J. Jeon, and W. S. Yoon, “Three dimensional geological model of Pohang EGS pilot site, Korea,” Proceedings of the World Geothermal Congress, vol. 19, pp. 19–25, 2015.

C. W. Song, Study on the Evolution of the Miocene Pohang Basin Based on its Structural Characteristics, Pusan National University, Busan, Korea, 2015, (In Korean with English abstract).

S. S. Chun and S. K. Chough, “Tectonic history of Crataceous sedimentary basins in the southwestern Korean Peninsula and Yellow Sea,” The Sedimentary Basins in the Korean Peninsula and Adjacent Seas, pp. 60–76, 1992.

D. W. Lee, “Strike–slip fault tectonics and basin formation during the Cretaceous in the Korean Peninsula,” Island Arc, vol. 8, no. 2, pp. 218–231, 1999.

K. K. Lee, “Summary report of the Korean government commission on relations between the 2017 Pohang earthquake and EGS project,” The Geological Society of Korea, 2019.

J. Matsuda, T. Matsumoto, H. Sumino et al., “The ³He/⁴He ratio of the new internal He Standard of Japan (HESJ),” Geochemical Journal, vol. 36, no. 2, pp. 191–195, 2002.

H. Craig, W. B. Clarke, and M. A. Beg, “Excess ³He in deep water on the East Pacific Rise,” Earth and Planetary Science Letters, vol. 26, no. 2, pp. 125–132, 1975.

Y. Sano, N. Takahata, and T. Seno, “Geographical distribution of ³He/⁴He ratios in the Chugoku District, Southwestern Japan,” Pure and Applied Geophysics, vol. 163, no. 4, pp. 745–757, 2006.

Y. Sano and H. Wakita, “Geographical distribution of ³He/⁴He ratios in Japan: implications for arc tectonics and incipient magmatism,” Journal of Geophysical Research: Solid Earth, vol. 90, no. B10, pp. 8729–8741, 1985.

M. Ozima and F. A. Podosek, Noble Gas Geochemistry, Cambridge University Press, 2002.

H. Lee, T. P. Fischer, J. D. Muirhead et al., “Incipient rifting accompanied by the release of subcontinental lithospheric mantle volatiles in the Magadi and Natron basin, East Africa,” Journal of Volcanology and Geothermal Research, vol. 346, pp. 118–133, 2017.

J. Zhang, P. D. Quay, and D. O. Wilbur, “Carbon isotope fractionation during gas-water exchange and dissolution of CO₂,” Geochimica et Cosmochimica Acta, vol. 59, no. 1, pp. 107–114, 1995.

K. Suda, Y. Ueno, M. Yoshizaki et al., “Origin of methane in serpentinite-hosted hydrothermal systems: the CH₄-H₂-H₂O hydrogen isotope systematics of the Hakuba Happo hot spring,” Earth and Planetary Science Letters, vol. 386, pp. 112–125, 2014.

Y. Taran, D. Morán-Zenteno, S. Inguaggiato, N. Varley, and L. Luna-González, “Geochemistry of thermal springs and geodynamics of the convergent Mexican Pacific margin,” Chemical Geology, vol. 339, pp. 251–262, 2013.

Y. Sano, N. Takahata, Y. Nishio, T. P. Fischer, and S. N. Williams, “Volcanic flux of nitrogen from the Earth,” Chemical Geology, vol. 171, no. 3-4, pp. 263–271, 2001.

B. Marty, “Nitrogen content of the mantle inferred from N₂–Ar correlation in oceanic basalts,” Nature, vol. 377, no. 6547, pp. 326–329, 1995.

B. Marty and L. Zimmermann, “Volatiles (He, C, N, Ar) in mid-ocean ridge basalts: assesment of shallow- level fractionation and characterization of source composition,” Geochimica et Cosmochimica Acta, vol. 63, no. 21, pp. 3619–3633, 1999.

Y. Sano and B. Marty, “Origin of carbon in fumarolic gas from island arcs,” Chemical Geology, vol. 119, no. 1-4, pp. 265–274, 1995.

W. Cheng, “Measurement of rhizosphere respiration and organic matter decomposition using natural ¹³C,” Plant and Soil, vol. 183, no. 2, pp. 263–268, 1996.

S. M. V. Gilfillan, C. J. Ballentine, G. Holland et al., “The noble gas geochemistry of natural CO₂ gas reservoirs from the Colorado Plateau and Rocky Mountain provinces, USA,” Geochimica et Cosmochimica Acta, vol. 72, no. 4, pp. 1174–1198, 2008.

M. Schoell, “The hydrogen and carbon isotopic composition of methane from natural gases of various origins,” Geochimica et Cosmochimica Acta, vol. 44, no. 5, pp. 649–661, 1980.

M. J. Whiticar, “Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane,” Chemical Geology, vol. 161, no. 1-3, pp. 291–314, 1999.

L. Hoke, S. Lamb, D. R. Hilton, and R. J. Poreda, “Southern limit of mantle-derived geothermal helium emissions in Tibet: implications for lithospheric structure,” Earth and Planetary Science Letters, vol. 180, no. 3-4, pp. 297–308, 2000.

T. Torgersen, “Continental degassing flux of ⁴He and its variability,” Geochemistry, Geophysics, Geosystems, vol. 11, no. 6, 2010.

B. M. Kennedy and M. C. van Soest, “Flow of mantle fluids through the ductile lower crust: helium isotope trends,” Science, vol. 318, no. 5855, pp. 1433–1436, 2007.

Y. Sano, Y. Nakamura, H. Wakita, A. Urabe, and T. Tominaga, “Helium-3 emission related to volcanic activity,” Science, vol. 224, no. 4645, pp. 150-151, 1984.

C. Chen, D. Zhao, Y. Tian et al., “Mantle transition zone, stagnant slab and intraplate volcanism in Northeast Asia,” Geophysical Journal International, vol. 209, no. 1, pp. 68–85, 2017.

K. B. Kim and G. D. Lee, “A study on volcanic stratigraphy and fault of Ulleung-do, Korea,” The Journal of Engineering Geology, vol. 18, no. 3, pp. 321–330, 2008, (In Korean with English abstract).

R. Westaway and N. M. Burnside, “Fault “corrosion” by fluid injection: a potential cause of the November 2017 5.5 Korean earthquake,” Geofluids, vol. 2019, Article ID 1280721, 23 pages, 2019.

T. W. Chung, M. Z. Iqbal, Y. Lee, K. Yoshimoto, and J. Jeong, “Depth-dependent seismicity and crustal heterogeneity in South Korea,” Tectonophysics, vol. 749, pp. 12–20, 2018.

T.-K. Hong, S. Park, and S. E. Houng, “Seismotectonic properties and zonation of the far-eastern Eurasian plate around the Korean Peninsula,” Pure and Applied Geophysics, vol. 173, no. 4, pp. 1175–1195, 2016.

K. Umeda, Y. Sakagawa, A. Ninomiya, and K. Asamori, “Relationship between helium isotopes and heat flux from hot springs in a non-volcanic region, Kii Peninsula, southwest Japan,” Geophysical Research Letters, vol. 34, no. 5, pp. 1–5, 2007.

T. Doğan, H. Sumino, K. Nagao, K. Notsu, M. K. Tuncer, and C. Çelik, “Adjacent releases of mantle helium and soil CO₂ from active faults: Observations from the Marmara region of the North Anatolian Fault zone, Turkey,” Geochemistry, Geophysics, Geosystems, vol. 10, no. 11, 2009.

C. D. Menzies, D. A. H. Teagle, S. Niedermann et al., “The fluid budget of a continental plate boundary fault: quantification from the Alpine Fault, New Zealand,” Earth and Planetary Science Letters, vol. 445, pp. 125–135, 2016.

K. Y. Kim, J. M. Lee, W. Moon, C. E. Baag, H. Jung, and M. H. Hong, “Crustal structure of the southern Korean peninsula from seismic waves generated by large explosions in 2002 and 2004,” Pure and Applied Geophysics, vol. 164, no. 1, pp. 97–113, 2007.

B. R. Hacker, P. B. Kelemen, and M. D. Behn, “Continental lower crust,” Annual Review of Earth and Planetary Sciences, vol. 43, no. 1, pp. 167–205, 2015.

S. Kwon, L. Xie, S. Park et al., “Characterization of 4.2-km-deep fractured granodiorite cores from Pohang Geothermal Reservoir, Korea,” Rock Mechanics and Rock Engineering, vol. 52, no. 3, pp. 771–782, 2019.

J. T. Kulongoski, D. R. Hilton, P. H. Barry, B. K. Esser, D. Hillegonds, and K. Belitz, “Volatile fluxes through the Big Bend section of the San Andreas Fault, California: helium and carbon-dioxide systematics,” Chemical Geology, vol. 339, pp. 92–102, 2013.

Y. Sano, H. Wakita, and C. W. Huang, “Helium flux in a continental land area estimated from ³He/⁴He ratio in northern Taiwan,” Nature, vol. 323, no. 6083, pp. 55–57, 1986.

W. B. F. Ryan, S. M. Carbotte, J. O. Coplan et al., “Global multi-resolution topography synthesis,” Geochemistry, Geophysics, Geosystems, vol. 10, no. 3, 2009.

H. Lee, J. D. Muirhead, T. P. Fischer et al., “Massive and prolonged deep carbon emissions associated with continental rifting,” Nature Geoscience, vol. 9, no. 2, pp. 145–149, 2016.

J. L. Lewicki and S. L. Brantley, “CO₂ degassing along the San Andreas Fault, Parkfield, California,” Geophysical Research Letters, vol. 27, no. 1, pp. 5–8, 2000.

Y. Sano, N. Kinoshita, T. Kagoshima et al., “Origin of methane-rich natural gas at the West Pacific convergent plate boundary,” Scientific Reports, vol. 7, no. 1, article 15646, 2017.

J. T. Kulongoski, D. R. Hilton, and J. A. Izbicki, “Source and movement of helium in the eastern Morongo groundwater Basin: the influence of regional tectonics on crustal and mantle helium fluxes,” Geochimica et Cosmochimica Acta, vol. 69, no. 15, pp. 3857–3872, 2005.

A. Caracausi and A. Sulli, “Outgassing of mantle volatiles in compressional tectonic regime away from volcanism: the role of continental delamination,” Geochemistry, Geophysics, Geosystems, vol. 20, no. 4, pp. 2007–2020, 2019.