Elsevier

Journal of Asian Earth Sciences

Volume 149, November 2017, Pages 64-77
Journal of Asian Earth Sciences

Full length Article
Spatial and temporal anomalies of soil gas in northern Taiwan and its tectonic and seismic implications

https://doi.org/10.1016/j.jseaes.2017.02.032Get rights and content

Highlights

  • Soil gas anomalies appearing in specific sites coincide with their structural settings.

  • Anomalies in the time series of soil gas can be linked to earthquakes.

  • A longer precursory time of a radon anomaly is attributed to a larger magnitude of an impending earthquake.

Abstract

In this paper, we study (1) the spatial anomalies and (2) the temporal anomalies of soil gas in northern Taiwan. The spatial anomalies of soil gas are related to tectonic faults, while the temporal anomalies of soil gas are associated with pre-earthquake activities. Detailed soil gas sampling was systematically performed, and the analysis of the collected gas species shows that high helium and nitrogen concentrations appear in samples from specific sites, which coincide with the structural setting of the area studied. This analysis indicates the possibility of using these soil gases to determine fault zones in the studied area. Based on the soil gas data, a station (Tapingti) for automatic soil gas monitoring was constructed on an appropriate site at the fault zone. Some anomalous high radon concentrations at certain times can be identified from the dataset, which was generated by the continuous monitoring of soil gas for over a year. Notably, many of these anomalies were observed several hours to a few days before the earthquakes (ML > 3) that occurred in northern Taiwan. By combining the information of epicenters and fault plane solutions of these earthquakes, we find that the shallow earthquakes (<15 km) were mainly strike-slip and normal-type earthquakes, and concentrated within a distance of 30 km to the monitoring site (Group A). The deep earthquakes (>20 km) were mainly thrust-type earthquakes and distributed in greater distances (>45 km) east of the monitoring site (Group B). Such focal mechanisms of earthquakes suggest an extensional and compressional structural domain in the continental crust for Group A and Group B earthquakes, respectively. It is suggested that the pre-earthquake activities associated with the seismicity of Group B may be transmitted along the major decollement in the region below the Tapingti station, leading to the observed soil gas enhancements.

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)

  • V.S. Iakovleva et al.

    Spatial and temporal variations of radon concentration in soil air

    Radiat. Meas.

    (2003)
  • G. Igarashi et al.

    Groundwater radon anomalies associated with earthquakes

    Tectonophysics

    (1990)
  • K. Kafadar et al.

    An alternative to ordinary Q-Q plots: conditional Q-Q plots

    Comput. Stat. Data Anal.

    (1986)
  • A. Kumar et al.

    Earthquake precursory studies in Kangra valley of North West Himalayas, India, with special emphasis on radon emission

    Appl. Radiat. Isot.

    (2009)
  • T. Kuo et al.

    Anomalous decrease in groundwater radon before the Taiwan M6.8 Chengkung earthquake

    J. Environ. Radioact.

    (2006)
  • K. Mair et al.

    3D numerical simulations of fault gouge evolution during shear: grain size reduction and strain localization

    Earth Planet. Sci. Lett.

    (2008)
  • K.D. McIntosh et al.

    Crustal-scale seismic profiles across Taiwan and the western Philippine Sea

    Tectonophysics

    (2005)
  • A. Minissale et al.

    Fluid geochemical transect in the Northern Apennines (central-northern Italy): fluid genesis and migration and tectonic implications

    Tectonophysics

    (2000)
  • S.A. Pulinets et al.

    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

    (2007)
  • D.V. Reddy et al.

    Groundwater electrical conductivity and soil radon gas monitoring for earthquake precursory studies in Koyna, India

    Appl. Geochem.

    (2011)
  • D.V. Reddy et al.

    Soil gas radon emanometry: a tool for delineation of fractures for groundwater in granitic terrains

    J. Hydrol.

    (2006)
  • Y. Sano et al.

    Helium degassing related to the Kobe earthquake

    Chem. Geol.

    (1998)
  • J.C. Sibuet et al.

    East Asia plate tectonics since 15 Ma: constraints from the Taiwan region

    Tectonophysics

    (2002)
  • C. Tansi et al.

    Interpretation of radon anomalies in seismotectonic and tectonicgravitational settings: the south-eastern Crati graben (northern Calabria, Italy)

    Tectonophysics

    (2005)
  • J.P. Toutain et al.

    Gas geochemistry and seismotectonics: a review

    Tectonophysics

    (1999)
  • V. Walia et al.

    Spatial variations of radon and helium concentration in soil gas across Shan-Chiao fault, Northern Taiwan

    Radiat. Meas.

    (2005)
  • V. Walia et al.

    Continuous temporal soil-gas composition variation for earthquake precursory studies along Hsincheng and Hsinhua faults in Taiwan

    Radiat. Meas.

    (2009)
  • V. Walia et al.

    Soil-gas monitoring: a tool for fault delineation studies along Hsinhua Fault (Tainan), southern Taiwan

    Appl. Geochem.

    (2010)
  • V. Walia et al.

    Temporal variation of soil gas compositions for earthquake surveillance in Taiwan

    Radiat. Meas.

    (2013)
  • T.F. Yang et al.

    Exhalation of radon and its carrier gases in SW Taiwan

    Radiat. Meas.

    (2003)
  • T.F. Yang et al.

    Variations of soil radon and thoron concentrations in a fault zone and prospective earthquakes in SW Taiwan

    Radiat. Meas.

    (2005)
  • S.B. Yu et al.

    Velocity field of GPS stations in the Taiwan area

    Tectonophysics

    (1997)
  • J.S. Andrews

    Radiogenic and inert gases in groundwater

  • B.A. Bolt

    Earthquakes

    (1999)
  • C. Chiarabba et al.

    Pore-pressure migration along a normal-fault system resolved by time-repeated seismic tomography

    Geology

    (2010)
  • L.L. Chyi et al.

    Continuous radon measurements in faults and earthquake precursor pattern recognition

    Western Pacific Earth Sci.

    (2001)
  • L.L. Chyi et al.

    Soil gas radon spectra and earthquakes

    Terrest., Atmosph. Oceanic Sci.

    (2005)
  • G. Ciotoli et al.

    Soil gas survey for tracing seismogenic faults: a case study in the Fucino basin, Central Italy

    J. Geophys. Res.

    (1998)
  • G. Ciotoli et al.

    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.

    (2004)
  • L. Claesson et al.

    Hydrogeochemical changes before and after major Earthquake

    Geology

    (2004)
  • Cited by (56)

    View all citing articles on Scopus
    1

    Deceased.

    View full text