Radiation Measurements 40 (2005) 496–502 www.elsevier.com/locate/radmeas Variationsofsoilradonandthoronconcentrationsinafaultzone andprospectiveearthquakesinSWTaiwan T.F. Yang a, b, ∗ , V. Walia b , L.L. Chyi c , C.C. Fu a , C.-H. Chen a , T.K. Liu a , S.R. Song a , C.Y. Lee a , M. Lee d a Department of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei 10699, Taiwan b National Center for Research on Earthquake Engineering, Taipei 106, Taiwan c Department of Geology, The University of Akron, OH 44325-4101, USA d Central Geological Survey, MOEA, Taipei 235, Taiwan Received 27 August 2004; accepted 7 May 2005 Abstract An automatic station for soil gas monitoring was set up on an active fault zone of SW Taiwan. After more than one year of continuous measurements, some spike-like anomalous high radon and thoron concentrations could be observed. A similar soil radon spectrum was also obtained from an independent monitoring station, which was only 100m away. These anomalous peaks usually occurred a few days or weeks before the earthquakes (M L  4.5). This indicates that variations of both soil radon and thoron can serve as useful tools for earthquake surveillance, esp. at fault zones. © 2005 Elsevier Ltd. All rights reserved. Keywords: Radon; Thoron; Earthquake monitoring; Soil gas; 3 He/ 4 He ratio; Taiwan 1. Introduction Because of the noble geochemical characteristics of radon gas (mainly 222 Rn decayed from 238 U, 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, 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, ∗ Corresponding author. Department of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei 10699, Taiwan. Tel.: +886 2 33665874; fax: +886 2 23636095. E-mail address: tyyang@ntu.edu.tw (T.F. Yang). 1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.05.017 only a few articles have mentioned thoron ( 220 Rn), one of the isotopes of radon gas decayed from 232 Th, 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, CH 4 and CO 2 (Yang and Jiang, 2005; Yang et al., 2004). Re- cently, Yang et al. (2003b) concluded that the carrier gases (CO 2 and CH 4 ) play a dominant role in controlling the trans- port 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 tempo- ral variations of 3 He/ 4 He ratio before and after earthquakes (Yang et al., 2005a). Hence, it was considered sensitive for earthquake surveillance. Consequently, a multi-parameter