Increased radon-222 in soil gas because of cumulative seismicity at active faults
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This study demonstrates how the radon-222 (222Rn) concentration of soil gas at an active fault is sensitive to cumulative recent seismicity by examining seven active faults in western Japan. The 222Rn concentration was found to correlate well with the total earthquake energy within a 100-km radius of each fault. This phenomenon can probably be ascribed to the increase of pore pressure around the source depth of 222Rn in shallow soil caused by frequently induced strain. This increase in pore pressure can enhance the ascent velocity of 222Rn carrier gas as governed by Darcy's law. Anomalous 222Rn concentrations are likely to originate from high gas velocities, rather than increased accumulations of parent nuclides. The high velocities also can yield unusual young gas under the radioactive nonequilibrium condition of short elapsed time since 222Rn generation. The results suggest that ongoing seismicity in the vicinity of an active fault can cause accumulation of strain in shallow fault soils. Therefore, the 222Rn concentration is a possible gauge for the degree of strain accumulation.
KeywordsRadon-222 concentration Nonequilibrium condition Gas velocity Darcy's law Total earthquake energy Western Japan
Radon-222 (222Rn) concentrations in soil gas and groundwater have been known to increase abruptly before and/or during large earthquakes in many parts of the world (Hauksson 1981; King 1984/85; Heinicke et al. 1995; Igarashi et al. 1995; Pérez et al. 2007; İnan et al. 2008; Kuo et al. 2010). The sensitivity of 222Rn concentration has been confirmed by its response to earthquakes a few hundred kilometers away (Fleischer 1981; Toutain and Baubron 1999; Cicerone et al. 2009; Ghosh et al. 2009). 222Rn is therefore thought to be a promising precursory gauge for large earthquakes. The observed spike-like increases in Rn concentration are similar to groundwater-level changes induced by earthquakes, and seismically enhanced crustal strain and pore pressure in saturated soils and rocks are the most plausible factors leading to this change (Manga et al. 2012).
Another widely recognized characteristic of 222Rn is that concentrations tend to be high at faults (King 1978; King et al. 1996; Baubron et al. 2002; Walia et al. 2009; Neri et al. 2011). Therefore, 222Rn surveys have been used to map faults and, in particular, to detect active faults. However, 222Rn enhancement does not necessarily occur at all faults, and the mechanism for this enhancement is still disputed. Three possible causes are (i) the magnitude of fault displacement velocities during the late Quaternary, (ii) the accumulation of parent nuclides caused by historical large earthquakes, and (iii) recent seismicity at nearby faults. By undertaking a radioactivity survey at seven active fault areas in western Japan, this study aimed to identify the most rational cause of the 222Rn concentration enhancement. The results could enable discrimination between faults that are continually active and faults that are occasionally active, i.e., faults experiencing stationary fracturing at short time scales or faults experiencing intermittent large fracturing over long time intervals.
Details of the active faults studied
Rank of activity
30 to 40
A set of E-W normal sub-faults
Normal fault with right-lateral slip
Number of lines
Number of survey points at each line
AT-1 (9), AT-2 (5), AT-3 (5), AT-4 (4)
AR-1 (15), AR-2 (13)
NJ-1 (26), NJ-2 (12), NJ-3 (7), NJ-4 (4), NJ-5 (4)
BH-1 (9), BH-2(8)
FG-1 (10), FG-2 (4), FG-3 (7)
HN-1 (25), HN-2 (33)
IM-1 (9), IM-2 (10), IM-3 (11)
Total survey points
Number of soil samples at each line
AR-1 (10), AR-2 (8)
NJ-1 (4),NJ-2 (3), NJ-3 (2), NJ-4 (1), NJ-5 (2)
FG-1 (10), FG-2 (4), FG-3 (7)
HN-1 (11), HN-2 (33)
IM-1 (8), IM-2 (3), IM-3 (5)
Total soil samples
Nov. to Dec. 2008
Oct. to Dec. 2004
Oct. 2005, Jan. and Oct. 2006, Oct. 2009
S (FG-2), V
M (HN-1), V (HN-2)
G (IM-3), S
Number of nonequilibrium points (%)
The latest large earthquakes and fault movements for the faults studied are well constrained by the evaluation reports of active faults (The Headquarters for Earthquake Research Promotion 2013). An 1858 earthquake (magnitude M 7.0 to 7.1) has been ascribed to movement of a portion of AT. The latest large earthquake on AR, which occurred in 1586, is thought to have been about M 7.8. A destructive earthquake of M 7.3 occurred in January 1995 on the northern part of NJ at a depth of 16 km. The most recent movements on BH, which is composed of a set of normal sub-faults forming grabens in an active volcanic area, range from 3.9 ka to the sixth century AD in the north and from after the thirteenth century AD in the south. The latest remarkable movements of FG, HN, and IM are estimated from trenching investigations to be in the ranges of 6.9 to 2.4 ka, 8.4 to 2.2 ka, and 7.3 to 2.4 ka, respectively. A large earthquake of M 6.4 in 1997 was located on the south end of IM, where many small earthquakes had occurred along two orthogonal segments (Figure 1). The IM-3 survey line was positioned in this zone.
Methods of radioactivity survey
Measurement lines were established near trench investigation sites (FG-1, HN-1, and IM-1 and IM-2), in concentrated epicenter zones (all AT lines, all BH lines, FG-2 and FG-3, HN-2, and IM-3), near fault outcrops (all AR lines), and near the surface displaced by the latest large earthquake (all NJ lines). Survey points were placed at 1- to 10-m intervals along a line or at 100- or 500-m intervals on a grid pattern applied to HN-2 (Figure 2). Because it was difficult to identify the position of the Hinagu Fault in the flat terraced landscape of the HN-2 area, the epicenters of M ≥ 2 earthquakes were used to establish the survey points there. At each survey point, soil gas was sampled from a hole with a depth of 60 cm and a diameter of 3 cm. The total number of α particles per minute (cpm) resulting from the decay of 222Rn and 220Rn and their daughter nuclides, 218Po and 216Po, were counted successively for 10 to 20 min using a portable α-scintillation detector, RDA-200 (Scintrex, Vaughan, Canada) or AB-5 (Pylon Electrics, Ottawa, Canada). The long count time was used to ascertain whether the gas was at nonequilibrium or equilibrium when judgment was difficult.
Soil samples were taken from the bottom of the borehole after the soil gas survey, dried at 80°C for 24 h, and arranged for samples with particle sizes smaller than 0.25 mm and mass of 150 g. A γ-ray spectrometer using a Ge semiconductor detector (GMX-25190-P: EG&G ORTEC, Oak Ridge, USA) was used to measure the γ-ray intensity (counts) of the 12 γ-decay nuclides in the uranium and thorium series. The concentration of nuclides was assumed to be proportional to the γ-ray intensity. Only the γ-ray intensity of 226Ra (I226 122 data in total) was used in conjunction with N222 (230 data in total) because of the isotope's long half-life (1,601 years) and because it is a direct parent nuclide of 222Rn. We used the conventional definition of I226 in becquerels per gram from the original γ-ray channel data in counts by the following transformation. First, the net spectral peak count of 226Ra per second was measured using Covell's method (Covell 1959), and then it was divided by three factors: the detector efficiency of 226Ra (0.065%), sample weight (g), and γ-ray yield from a standard source.
Results and discussion
Identifying the controlling factor for 222Rn concentration
If we assume the equilibrium condition between 222Rn and 226Ra, then λ222C222(z) should be equal to λ226C226(z). It follows that ϕ = λ226C226(z), where C226(z) and λ226 are the 226Ra concentration at z and the decay constant of 226Ra, respectively. C222(z) at z = 60 cm (the bottom depth of the borehole) is calculated by assigning the above value D = 5 × 10−6 m2/s and a fixed ϵ of 0.3. In normal soils, v is smaller than 1 × 10−5 m/s (Kristiansson and Malmqvist 1982; Schery and Siegel 1986). The value 1 × 10−3 m/s is its upper limit according to the study of Brown (2000). Using two values for high and low velocity, the ratio of C222(z) at v = 5 × 10−5 and 5 × 10−6 (m/s) is 3.1 for the same C226(z). This means that the 222Rn concentration is highly variable by a factor of 3 in this case even though the 226Ra concentration is constant. For a pair of velocities with less contrast, v = 5 × 10−5 and 1 × 10−5 (m/s), the ratio is 2.3. In loose deposits, D ranges from 1.5 × 10−6 to 6.8 × 10−6 m2/s (Gasparini and Mantovani 1978). When D is halved (2.5 × 10−6 m2/s), the ratios for the above two v pairs become 2.4 and 1.7. Consequently, the absence of a correlation between 222Rn and 226Ra can be attributed to the changeability in gas ascent velocity in the soils, not to the heterogeneous distribution of 226Ra concentration.
Effect of earthquake energy and mechanism of 222Rn enhancement
The mechanism causing the strong correlation described above can be interpreted by a simplified dislocation model (Fleischer 1981). This model assumes the Earth to be an infinite isotropic elastic solid containing a circular dislocation loop of radius r and slip vector b. As long as the ratio r/b is constant, the shear strain change Δγ at d can be related to M as Δγ ∝ 101.44M/d3. For the same d, this equation derives Δγ ∝ 10 M and, further, Δγ ∝ E from the Gutenberg-Richter law. Therefore, the shear strain change becomes proportional to the earthquake energy, and the increase in strain causes a corresponding increase in stress, Δσ.
In shallow soils, the carrier gas velocity of 222Rn (the above v) is thought to be governed by Darcy's law (Kristiansson and Malmqvist 1982; Schery and Siegel 1986; Ioannides et al. 2003). Assuming that the intrinsic permeability of soil and gas viscosity are constant with time and the gas flow is laminar with Reynolds number <1, then v becomes proportional to the pressure gradient, v ∝ ∂p/∂z. If the gradient is constant over the depth range, then v can be further simplified as v ∝ p(s) − p(g), where p(s) and p(g) denote the pore pressures at the 222Rn source depth (the maximum depth from which 222Rn can reach the detector) and the ground surface, respectively; p(g) is almost constant (=1 atm) despite small fluctuations with weather conditions, and p(s) is variable. It is reasonable to consider that the Δσ can enhance p(s). This enhancement induces an increase in v and consequently an increase in 222Rn concentration because of the sensitivity of the concentration to v. At active fault areas with many recent earthquakes (e.g., IM, HN, and AR), p(s) is considered to be at a higher level than in inactive seismic areas because of frequent ground motions that enhance strain and stress. Additionally, the motions may have generated preferred pathways in shallow soils for the carrier gas, such as cracks and connections between pores. This pathway generation can also enhance v and cause a nonequilibrium condition of soil gas. These states essentially enhance 222Rn concentration without the accumulation of 226Ra.
Through the radioactivity survey at seven active fault areas in western Japan and the data analyses, three points were clarified by this study: the radioactive equilibrium condition for soil gas, the reason for enhanced 222Rn concentrations in soil gas, and the differences in 222Rn concentrations in soil gas associated with active faults. The cause of the nonequilibrium gas patterns at 21% of the points, indicating young gas of short elapsed time since 222Rn generation, was interpreted as high ascent velocity of 222Rn carrier gas. Therefore, the radioactive equilibrium condition of the soil gas can help to specify the locations at which the formation of passages along which the gas can ascend rapidly has occurred. The high velocity can generate anomalous 222Rn concentrations without accumulations of its parent nuclides.
A new finding of this study is that the observed concentrations are correlated with the summation of recent earthquake energy within a 100-km radius of the fault. From Darcy's law and the dislocation model, this phenomenon was interpreted by the increase of pore pressure around the source depth of 222Rn in shallow soils caused by frequently induced strain. The increase in pore pressure can enhance the gas ascent velocity and, consequently, the 222Rn concentration. This correlation suggests that terrestrial gas is sensitive to small changes in strain. Despite being associated with typical active faults with historically large earthquakes, the 222Rn concentrations measured at the study sites in Japan are not necessary high if they are not accompanied by recent seismicity. Carrier gas velocity is the dominant factor affecting concentration.
In conclusion, 222Rn concentration in an active fault area, in addition to being an earthquake precursor, acts as a capable gauge for evaluating the conditions of steady seismicity and strain accumulation around the fault. The 222Rn concentration may contribute to the identification of regions within which the crust is repeatedly ruptured.
The authors wish to express their gratitude to the graduate students, who gave devoted assistance in this study, from the geophysical prospecting laboratory of Kumamoto University, Emeritus Professor Michito Ohmi of Kumamoto University for his cordial instructions, and Mr. Jitsuya Uemura of the Radioisotope Center of Kumamoto University for the instruction regarding the measurement of γ-ray spectra. Sincere thanks are extended to Dr. Kenji Amano, Dr. Masakazu Niwa, and Dr. Koji Shimada of JAEA for their cooperation with the field surveys at the Atotsugawa and Atera faults and to two anonymous reviewers for their valuable comments and suggestions that helped improve the clarity of the manuscript.
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