Full length ArticleSpatial and temporal anomalies of soil gas in northern Taiwan and its tectonic and seismic implications
Graphical abstract
Introduction
Taiwan is a young and active mountainous island formed by collision between the Philippine Sea plate and the Eurasian plate, and is thus densely faulted. The 1999 Chichi earthquake (Mw 7.6) that occurred in Central Taiwan induced faulting and remarkable surface rupture along the Chelungpu Fault, resulting in severe casualties as well as property loss. Hence, the major pursuits in disaster prevention in Taiwan are to better understand the distribution of active faults, and to assess potential earthquake hazard areas.
Active fault zones usually have higher permeability than surrounding strata, and can therefore provide conduits for gases originating from the deep crust or mantle to migrate towards the surface. It is also common for mantle-derived gases to exhibit higher concentrations and higher flux rates due to greater strains (Trique et al., 1999, Pulinets and Dunajecka, 2007). Northern Taiwan was chosen for this study because it has been proven that it is an area where mantle-derived gases are still emanating (Lin et al., 2004, Yang et al., 2005a).
General overviews of the geochemical, structural, and seismic features in tectonically active areas have shown some evidence of correlation between soil gas anomalies and tectonic activities (Toutain et al., 1992, Ciotoli et al., 1998, Toutain and Baubron, 1999, Fu et al., 2008). For example, it has been found that blind active faults, which are usually difficult to identify at the surface, can be outlined by soil gas surveys (Fu et al., 2005, Walia et al., 2005a, Walia et al., 2008). Useful soil gases include noble gases, radon, and helium, which play important roles in fault delineation and earthquake precursory studies (Claesson et al., 2004, Chyi et al., 2005, Walia et al., 2005b, Yang et al., 2006, Kumar et al., 2009a, Reddy and Nagabhushanam, 2011, Fu et al., 2017).
The famous precursory anomalies in the radon concentration of groundwater have observed prior to the Izu-Oshima-kinkai earthquake in 1978 and Kobe earthquake in 1995 (Wakita et al., 1980, Igarashi et al., 1995, United Nations, 2000). Changes in gas compositions have also proposed as potential precursors (Sugisaki, 1978, Sano et al., 1998). Pre-, co-, and post-seismic signals can be observed by soil gas anomalies (King, 1986, Wakita et al., 1989, Fu et al., 2008, Kumar et al., 2009b), although they are sometimes unrelated to seismic events (Heinicke et al., 1995). Some earlier investigations have revealed that these anomalies may also be affected by meteorological parameters, such as atmospheric pressure, humidity, temperature, and rainfall (Washington and Rose, 1992, Bunzl et al., 1998, Iakovleva and Ryzhakova, 2003, Fu et al., 2017). Therefore, meteorological effects are also taken into consideration when analyzing soil gas anomalies.
In a compilation of gas emissions to catalog observed earthquake precursors (Cicerone et al., 2009), only earlier works on water radon in Taiwan were mentioned (Liu et al., 1983, Liu et al., 1985). In recent years, there have been an increasing number of studies focusing on the use of gas measurements in Taiwan. For example, radon anomalies have been observed in soil gas and water prior to earthquakes in different areas; hence, their potential as an earthquake precursor has been mentioned or recognized (Chyi et al., 2005, Fu et al., 2008, Fu et al., 2009, Fu et al., 2017, Kumar et al., 2009a, Kumar et al., 2009b, Kuo et al., 2006a, Kuo et al., 2006b, Walia et al., 2009, Walia et al., 2010, Walia et al., 2013, Yang et al., 2005b).
There are three objectives in this work. First, soil gas concentrations of He, Rn, N2, CO2, and CH4 that might have originated from great depths were analyzed. Geochemical data obtained from the soil gas method were compared with other geological and geophysical information on the known faults in the studied area, to see if they are useful for detecting buried faults or fractures. Second, a station equipped with a seismograph and facilities for the automatic measuring of soil Rn, Th, and CO2 concentrations, as well as meteorological factors (atmospheric pressure, temperature, humidity, and rainfall), was installed on a geochemically sensitive site for continuous monitoring. Results of continuous monitoring on multiple parameters were used to delineate the relationship between soil gas variations and regional earthquake events. Third, data regarding relevant earthquakes were synthesized to formulate a structural configuration that can account for the proposed seismic velocity model in the context of the recent tectonics in Taiwan. The potential for using soil radon as an earthquake precursor in northern Taiwan was assessed on this basis.
Section snippets
Geological background of northern Taiwan
Tectonics of northern Taiwan is characterized by its location at the junction of two subduction sectors pertaining to the interactions between the Eurasian and the Philippine Sea plates, namely, the E-W trending Ryukyu subduction zone resulting from the Philippine Sea plate subducting northwardly beneath the Eurasian plate, and the N-S trending Manila Trench system and its extension to the island of Taiwan related to the Eurasian plate that subducts eastwardly beneath the Philippine Sea plate (
Principle
Gases in the air and gases derived from the deep crust and mantle have characteristic compositions that enable them to be easily distinguishable. This is an application of the fundamental basics of gas geochemistry. Soil gas compositions commonly possess the mixed characteristics of air and deep crust gases, because higher He, Rn, N2, CO2, and CH4 concentrations in deeper crust often diffuse upwards to accumulate in the near-surface soil layers, and mix with air (Ciotoli et al., 1999, Tansi et
Results and discussions
Our results revealed that argon and oxygen did not show significant variation in all testing spots, and cannot be used as indicator gases for tracing fault distribution. Furthermore, limited sampling sites had observable methane concentrations. Therefore, discussion here only includes the results of the major gases, nitrogen and carbon dioxide, and the trace gases, helium and radon. The concentration of these gases vary greatly in all sampling sites: N2 ranges from 57.20 ± 3.88 to 96.75 ± 4.50%; CO2
Conclusions
The major findings of this study are listed below:
- (1)
Soil gases (helium, nitrogen, and carbon dioxide) anomalies are distributed along the NE-SW trending faults in the study area, and coincide with geological traces.
- (2)
Many anomalously high soil radon and thoron concentrations detected during continuous monitoring were observed several hours to a few days before earthquakes.
- (3)
Based on relevant earthquakes, two types of earthquakes (Group A and Group B) can be identified. Group A earthquakes have
Acknowledgments
This paper is dedicated to co-author Prof. Tsanyao Frank Yang (TFY), who passed away on 12 March 2015. Born in 1961, TFY worked for many years at the Department of Geosciences, National Taiwan University (NTU), Taipei, Taiwan. His passion for research set new standards for all who worked with him. He will be greatly missed within the Earth science community. We thank Mr. B.W. Lin and D.R. Hsiao for help with sample collection and analysis. We also thank Mr. S.J. Lin and K.W. Wu for help in the
References (80)
- et al.
Soil gas profiles as a tool to characterise active tectonic areas: the Jaut Pass example (Pyrenees, France)
Earth Planet. Sci. Lett.
(2002) - et al.
Temporal and small-scale spatial variability of 222Rn gas in a soil with a high gravel content
Sci. Total Environ.
(1998) - et al.
The separation of geochemical anomalies from background by fractal methods
J. Geochem. Explor.
(1994) - et al.
A systematic compilation of earthquake precursors
Tectonophysics
(2009) - et al.
The detection of concealed faults in the Ofanto Basin using the correlation between soil gas fracture surveys
Tectonophysics
(1999) - et al.
Migration of carrier and trace gases in the geosphere: an overview
Phys. Earth Planet. Inter.
(2002) - et al.
Constraints on backstop geometry of the southwest Ryukyu subduction based on reflection seismic data
Tectonophysics
(2001) - et al.
Variations of soil-gas composition around the active Chihshang Fault in a plate suture zone, eastern Taiwan
Radiat. Meas.
(2009) - et al.
Nitrogen as the carrier gas for helium emission along an active fault in NW Taiwan
Appl. Geochem.
(2010) - et al.
Kinematics of convergence, deformation and stress distribution in the Taiwan collision area: 2-D finite-element numerical modeling
Tectonophysics
(1996)
Spatial and temporal variations of radon concentration in soil air
Radiat. Meas.
Groundwater radon anomalies associated with earthquakes
Tectonophysics
An alternative to ordinary Q-Q plots: conditional Q-Q plots
Comput. Stat. Data Anal.
Earthquake precursory studies in Kangra valley of North West Himalayas, India, with special emphasis on radon emission
Appl. Radiat. Isot.
Anomalous decrease in groundwater radon before the Taiwan M6.8 Chengkung earthquake
J. Environ. Radioact.
3D numerical simulations of fault gouge evolution during shear: grain size reduction and strain localization
Earth Planet. Sci. Lett.
Crustal-scale seismic profiles across Taiwan and the western Philippine Sea
Tectonophysics
Fluid geochemical transect in the Northern Apennines (central-northern Italy): fluid genesis and migration and tectonic implications
Tectonophysics
Specific variations of air temperature and relative humidity around the time of Michoacan earthquake M8.1 Sept. 19, 1985 as a possible indicator of interaction between tectonic plates
Tectonophysics
Groundwater electrical conductivity and soil radon gas monitoring for earthquake precursory studies in Koyna, India
Appl. Geochem.
Soil gas radon emanometry: a tool for delineation of fractures for groundwater in granitic terrains
J. Hydrol.
Helium degassing related to the Kobe earthquake
Chem. Geol.
East Asia plate tectonics since 15 Ma: constraints from the Taiwan region
Tectonophysics
Interpretation of radon anomalies in seismotectonic and tectonicgravitational settings: the south-eastern Crati graben (northern Calabria, Italy)
Tectonophysics
Gas geochemistry and seismotectonics: a review
Tectonophysics
Spatial variations of radon and helium concentration in soil gas across Shan-Chiao fault, Northern Taiwan
Radiat. Meas.
Continuous temporal soil-gas composition variation for earthquake precursory studies along Hsincheng and Hsinhua faults in Taiwan
Radiat. Meas.
Soil-gas monitoring: a tool for fault delineation studies along Hsinhua Fault (Tainan), southern Taiwan
Appl. Geochem.
Temporal variation of soil gas compositions for earthquake surveillance in Taiwan
Radiat. Meas.
Exhalation of radon and its carrier gases in SW Taiwan
Radiat. Meas.
Variations of soil radon and thoron concentrations in a fault zone and prospective earthquakes in SW Taiwan
Radiat. Meas.
Velocity field of GPS stations in the Taiwan area
Tectonophysics
Radiogenic and inert gases in groundwater
Earthquakes
Pore-pressure migration along a normal-fault system resolved by time-repeated seismic tomography
Geology
Continuous radon measurements in faults and earthquake precursor pattern recognition
Western Pacific Earth Sci.
Soil gas radon spectra and earthquakes
Terrest., Atmosph. Oceanic Sci.
Soil gas survey for tracing seismogenic faults: a case study in the Fucino basin, Central Italy
J. Geophys. Res.
A multidisciplinary, statistical approach to study the relationships between helium leakage and neotectonic activity in a gas province: the Vasto basin Abruzzo-Molise (central Italy)
Am. Assoc. Pet. Geol. Bull.
Hydrogeochemical changes before and after major Earthquake
Geology
Cited by (56)
Applying machine learning to model radon using topsoil geochemistry
2023, Applied GeochemistryDiffuse emission of CO<inf>2</inf> from the Langjiu Geothermal Field, Western Tibet
2023, Journal of Geochemical Exploration
- 1
Deceased.