Measuring radon flux across active faults: Relevance of excavating and possibility of satellite discharges

doi:10.1016/j.radmeas.2010.01.019

Radiation Measurements

Volume 45, Issue 2, February 2010, Pages 211-218

https://doi.org/10.1016/j.radmeas.2010.01.019 Get rights and content

Abstract

Searching for gas exhalation around major tectonic contacts raises important methodological issues such as the role of the superficial soil and the possible long distance transport. These effects have been studied on the Xidatan segment of the Kunlun Fault, Qinghai Province, China, using measurement of the radon-222 and carbon dioxide exhalation flux. A significant radon flux, reaching up to 538 ± 33 mBq m⁻² s⁻¹ was observed in a 2–3 m deep trench excavated across the fault. On the soil surface, the radon flux varied from 7 to 38 mBq m⁻² s⁻¹, including on the fault trace, with an average value of 14.1 ± 1.0 mBq m⁻² s⁻¹, similar to the world average. The carbon dioxide flux on the soil surface, with an average value of 12.9 ± 3.3 g m⁻² day⁻¹, also remained similar to regular background values. It showed no systematic spatial variation up to a distance of 1 km from the fault, and no clear enhancement in the trench. However, a high carbon dioxide flux of 421 ± 130 g m⁻² day⁻¹ was observed near subvertical fractured phyllite outcrops on a hill located about 3 km north of the fault, at the boundary of the large-scale pull-apart basin associated with the fault. This high carbon dioxide flux was associated with a high radon flux of 607 ± 35 mBq m⁻² s⁻¹. These preliminary results indicate that, at the fault trace, it can be important to measure gas flux at the bottom of a trench to remove superficial soil layers. In addition, gas discharges need to be investigated also at some distance from the main fault, in zones where morphotectonics features support associated secondary fractures.

Introduction

Radon-222 is a radioactive inert gas, with a half-life of 3.8 days, ubiquitous in natural settings as a member of the uranium-238 chain (Tanner, 1964). The radon exhalation flux at the soil surface depends on the transport properties of the medium and the concentration of its parent nucleus, radium-226. Except in the presence of uraniferous formations, the flux remains generally close to the world average of 22 mBq m⁻² s⁻¹ (Nazaroff, 1992). However, the flux can be much larger if the physical properties of the subsurface medium allow for a higher permeability to gas of the subsurface, or in the presence of a carrier gas, such as carbon dioxide, methane or water vapour, advectively driven from depth towards the surface. A radon flux larger than 3000 mBq m⁻² s⁻¹, for instance, has been reported at several locations near hot springs in Central Nepal (Girault et al., 2009, Perrier et al., 2009). Increased radon transport to the atmosphere also leads to higher radon concentrations in the soil. Concentrations larger than 20,000 Bq m⁻³, significantly larger than local background values, have been reported on volcanoes (Baubron et al., 1991, Heiligmann et al., 1997) and in tectonically active areas such as along the San Andreas Fault system in California (King et al., 1996). Mapping radon concentration and radon flux anomalies, thus, has been proposed as a way to detect hidden faults (Burton et al., 2004). A few studies also suggested that monitoring the radon concentration in the soil (Richon et al., 2007) or its exhalation to the atmosphere as a function of time could be useful in order to forecast impending earthquakes (Toutain and Baubron, 1999, Richon et al., 2003).

The pattern of radon exhalation in the vicinity of major tectonic faults remains confusing, however (Table 1). While clear anomalies were detected in some cases, such as the San Andreas Fault (King et al., 1996), or in the Dead Sea rift in Israel (Steinitz et al., 1992), negligible enhancement of the radon signal was reported near major faults such as the North Anatolian Fault in Turkey (Inceoz et al., 2006) and the Levant fault in Jordan (Atallah et al., 2001, Al-Bataina et al., 2005). Most of these studies, however, relied on profiles with few data points (Giammanco et al., 2009), and where more extensive studies were undertaken, for example, in California (King et al., 1996), they were discontinued. On the other hand, clear correlation of radon anomalies with Quaternary faults was reported in Italy (Tansi et al., 2005), in Germany (Kemski et al., 1992), and in France, in the Pyrenees (Baubron et al., 2002), where seismic activity and tectonic strain rates are at least one order of magnitude smaller. The correlation between radon concentration anomalies and tectonic structures, which is at present elusive but potentially promising, should thus be reexamined in a more systematic manner in active tectonic zones.

Before such comprehensive studies can be undertaken, some methodological issues need to be clarified, in particular concerning the role of the superficial soil layers. In addition, potentially interesting locations may need to be investigated away from the known fault trace. In this paper, to contribute to these methodological issues, we present preliminary results of radon-222 and carbon dioxide flux measurements along one of the major continental strike-slip faults of Asia, the Kunlun Fault (KF, Fig. 1).

Section snippets

Geological setting

The Kunlun fault is a left-lateral fault (Fig. 1) that extends roughly over a distance of about 1600 km along an east–west direction (Tapponnier et al., 2001). With a mean slip rate of about 10–12 mm per year over the last 40,000 years (Van der Woerd et al., 2000, Van der Woerd et al., 2002, Haibing et al., 2005), this fault is currently in an active cycle, with at least six earthquakes with M ≥ 7 having successively ruptured various segments of the fault or nearby branches between 1879 and 2000 (

Experimental methods

Radon exhalation was measured using the accumulation chamber method (Wilkening and Hand, 1960, Ferry et al., 2001, Ielsch et al., 2001). First, 5–10 cm of soil was scrubbed. Then an accumulation chamber of 18 L volume and 0.125 m² surface area was installed on the scrubbed area. Leakage was reduced by plastering the sides of the container with wet soil. After 1 h, the gas in the container was sampled by a Lucas scintillation flask (Lucas, 1957) with a volume of 125 mL, previously evacuated to a

Results of measurements across the fault

The results across the fault are shown in Fig. 5. Along the soil surface profile, the radon flux remained almost constant with an average value of 14.1 ± 1.0 (tot.) mBq m⁻² s⁻¹. The mean radon flux value along the segment of profile −100 m to 100 m (18.2 ± 1.1 mBq m⁻² s⁻¹) was significantly higher than the mean value measured between 100 m and 200 m (11.5 ± 0.9 mBq m⁻² s⁻¹). However, measurements at distances larger than 100 m along this profile were performed one day after the other measurements. Consequently,

Results of measurements on the KA hill

On the KA hill, a significant carbon dioxide flux, larger than 80 g m⁻² day⁻¹ was observed consistently on both profiles over a distance of about 20 m. The peak values are 421 ± 130 (tot.) g m⁻² day⁻¹ for the KA1 profile and 179 ± 45 (tot.) g m⁻² day⁻¹ for the KA2 profile. Flux measurements were repeated eight times over a 48 h period at the maximum of KA1 profile (20 m north of origin), and values varied from 65 ± 9 g m⁻² day⁻¹, significantly larger than the background level, up to 421 ± 77 g m⁻² day⁻¹, with a mean of

Discussion

The observed magnitude of the radon flux was close to the world average of 22 mBq m⁻² s⁻¹ (Nazaroff, 1992) and to annual mean values ranging from 5.9 to 59.6 mBq m⁻² s⁻¹, with an average of 24 mBq m⁻² s⁻¹, reported for soils in East Asia (Zhuo et al., 2005). An average of 37 mBq m⁻² s⁻¹ was reported for Quaternary deposits (Ielsch et al., 2001), larger than the mean value of 14.1 ± 1.0 (tot.) mBq m⁻² s⁻¹ observed near the trench. Soil gas was also sampled at a depth of 15 cm, 1 m east of the trench, and the radon

Conclusion

In this paper, we show that a significant soil radon flux was measured in a trench across the KF in Xidatan. However, since no signal was observed along a parallel soil surface profile, the radon anomaly associated with the fault would have been missed without the opportunity provided by the trench. This implies that soil, especially rich in clay and with high water content, can provide an efficient barrier to the escape of radon gas to the atmosphere. This may explain why, in several

Acknowledgments

The authors thank the members of the Chinese Earthquake Administration from Beijing, Lanzhou and Golmud for assistance during fieldwork. We thank E. Pili for the fruitful discussions and support. This work was supported by funding from ANR-05-CATT-0006. This is IPGP contribution 2589.

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Measuring radon flux across active faults: Relevance of excavating and possibility of satellite discharges

Abstract

Introduction

Section snippets

Geological setting

Experimental methods

Results of measurements across the fault

Results of measurements on the KA hill

Discussion

Conclusion

Acknowledgments

Radiat. Meas.

Earth Planet. Sci. Lett.

Chem. Geol.

Appl. Geochem.

Appl. Radiat. Isot.

J. Environ. Radioact.

Earth Planet. Sci. Lett.

J. Volcanol. Geoth. Res.

J. Environ. Radioact.

Radiat. Meas.

Appl. Radiat. Isot.

Appl. Geochem.

Earth Planet. Sci. Lett.

Radiat. Meas.

Earth Planet. Sci. Lett.

Tectonophysics

Tectonophysics

Radiat. Meas.

J. Asian Earth Sci.

Int. Congr. Ser.

Radon emanation along the Dead Sea transform (rift) in Jordan

Environ. Geol.