Variations of soil radon and thoron concentrations in a fault zone and prospective earthquakes in SW Taiwan
Introduction
Because of the noble geochemical characteristics of radon gas (mainly 222Rn decayed from 238U, with a half-life of 3.8 days), its variations of concentrations in the soil and fluid samples are considered a useful tool for earthquake monitoring and prediction in active fault zones (e.g., Chyi et al., 2002, Chyi et al., 2005, Igarashi et al., 1995, King, 1986; Liu et al., 1985, Singh et al., 1999, Toutain and Baubron, 1999, Virk et al., 2001) and for tracing neotectonic faults (e.g., Ciotoli et al., 1999, Etiope and Lombardi, 1995, Fu et al., 2005, Guerra and Lombardi, 2001, Walia et al., 2005). However, only a few articles have mentioned thoron (220Rn), one of the isotopes of radon gas decayed from 232Th, for the same applications due to its short half-life (55.6 s) (LaBrecque, 2002).
Mud volcanoes are widely distributed along the tectonic sutures in SW Taiwan. Some are located in the active fault zones. Radon is continuously being emitted from these mud volcanoes accompanied by other major gases, for example, and (Yang and Jiang, 2005, Yang et al., 2004). Recently, Yang et al. (2003b) concluded that the carrier gases ( and ) play a dominant role in controlling the transport and redistribution of radon gases towards the Earth's surface via a fault/fracture zone. Gases from Chung–lun (CL) hot springs and mud pool (Fig. 1), where the Chu-ko (CK) active fault zone cuts across, show significant temporal variations of 3He/4He ratio before and after earthquakes (Yang et al., 2005a). Hence, it was considered sensitive for earthquake surveillance. Consequently, a multi-parameter continuous gas monitoring station, mainly equipped with a radon/thoron detector and a quadrupole mass spectrometer (QMS), was installed at the CL site (Yang et al., 2005b). After continuous measurements for more than one year, this automated system was demonstrated to be feasible for long-term continuous seismo-geochemical research. In this paper, we report the variations of radon and thoron concentrations of soil gases. Some spike-like anomalous peaks can be recognized. They are closely related to the earthquake events and hence, can be considered as precursors.
Section snippets
Helium isotopic compositions
Terrestrial materials exhibit a wideranged distribution of the helium isotopic ratios from 0.01 to 40 times the 3He/4He ratio of the air (Ra). Hence, the helium isotopic ratio is widely used as a tool to trace relevant sample source domain (e.g., Porcelli et al., 2002) and could be used for tracing the gas compositions before and after an earthquake (Italiano et al., 2001, Sano et al., 1998, Yang et al., 2005a).
Soil gases from CL station and bubbling gases of CL mud pool have been regularly
Discussion
A total number of 24 peaks could be recognized and labeled as peaks a to o in the enlarged spectrum (Fig. 4A and B). They were used for further correlation with relevant earthquakes. In general, peaks occurred simultaneously in both radon and thoron spectra. However, some peaks were only found in the radon spectrum (e.g. peaks and g in Fig. 4A). In contrast, other peaks could only be found in the thoron spectrum (e.g., peaks j to n in Fig. 4B). For peak a, three sub-peaks may be suggested.
Conclusion remarks
- (1)
Consistent spectra of soil radon concentrations were obtained from two independent monitoring stations at a fault zone of southwestern Taiwan.
- (2)
Spike-like peaks of the radon spectrum is synchronous with thoron. They usually occur 1–20 days before an earthquake event, and thus can be a precursor.
- (3)
At least two gas sources are required to explain the rare gas data in this area. One is from a deeper source, and another from closer and shallower source.
- (4)
In addition to radon and thoron, helium is helpful
Acknowledgements
The authors wish to thank Messrs. K.W. Wu, C.C. Wang, D.R. Hsiao, J.H. Jiang, Y.J. Lu and C.Y. Chou for their help in setting up and calibrating the system and L.C. Huang and W.J. Lin for providing local logistic help during different stages of this project. B.W. Lin helped to measure the helium isotopic compositions. The senior author also benefited from discussions with Dr. S. Feige regarding radon/thoron measurements. Two anonymous reviewers improved the manuscript. This study was
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