Ground Gases (Seismogenic)

Primary reference(s)

IVHHN, 2020. The health hazards of volcanic and geothermal gases: A guide for the public. International Volcanic Health Hazard Network (IVHHN). Accessed 15 October 2020.

USGS, no date. Volcano hazards programme. United States Geological Survey (USGS). Accessed 14 October 2020.

Additional scientific description

Volcanogenic gases escape from magma as a consequence of the pressure relief that occurs as the magma rises to the surface. These gases are also released via geothermal systems and fault systems activated by earthquakes. King et al. (2006) found elevated concentrations of soil gases such as carbon dioxide, helium, hydrogen, mercury vapour and radon in fault zones associated with earthquakes. These gases are released in combination with water vapour and particulate matter during volcanogenic events, or via fumaroles, and hydrothermal systems, as well as faults activated by earthquake events. It has been suggested that radon monitoring might be used for earthquake early warning systems.

Earthquakes can also trigger the release of soil gases derived from other sources, such as the chemical or biological processes that generate ground gases, including the breakdown of uranium-bearing minerals releasing radon from granite or by oxidation and/or biogenic reduction (releasing hydrogen sulphide) as well as the release of anthropogenic stores of gas. For example, rupture of tanks and pipes (WHO, 2018), as well as landfill gas, a product of the largely biogenic decomposition of anthropogenic waste. Its composition reflects that of the waste, but is dominated by methane and carbon dioxide, becoming more carbon dioxide rich as the waste ages, and with a small amount of non-methane organic compounds. Methane is a potent greenhouse gas (US EPA, no date a).

Ground gases from material decay (natural or anthropogenic) typically include radon, methane, carbon dioxide, and hydrogen sulphide, but may also include the breakdown products of other compounds, such as nitrogen, alcohols, alkanes, cycloalkanes and alkenes, aromatic hydrocarbons (monocyclic or polycyclic); esters and ethers, as well as halogenated compounds and organosulphur (US EPA, no date b; USGS, no date).

Ground gases are a hazard owing to the risk to human health and/or their flammability. As an example, the UK limits for several gases are summarised below from sources other than earthquake triggered gases:

Methane is a colourless, odourless flammable gas. When the concentration of methane in air (oxygen 20.9% by volume, % v/v) is between the limits of 5% v/v and 15% v/v, an explosive mixture is formed. The Lower Explosive Limit (LEL) of methane is 5% v/v, which is equivalent to 100% LEL. The 15% v/v limit is known as the Upper Explosive Limit (UEL), but concentrations above this level cannot be assumed to represent safe concentrations, owing to the potential for dilution to the UEL (PHE, 2015).

Carbon dioxide is a colourless, odourless gas, which, although non-flammable, is both a toxic gas and an asphyxiant. As carbon dioxide is denser than air, it will collect in low points and depressions, which can be an extreme hazard during foundation construction and earth movements on development sites. the Long-Term Exposure Limit (LTEL, 8-hour period) and the Short Term Exposure Limit (STEL, 15-minute period), are 0.5% v/v and 1.5% v/v carbon dioxide, respectively (HSE, no date).

Radon is a colourless, odourless radioactive gas derived from the radioactive decay of radium, itself from radioactive decay of uranium. The UK target level for homes is 100 Bq/m3 (PHE, no date).

Levels of hydrogen sulphide of 100 ppm and higher are considered immediately dangerous to life and health (WorkSafe BC, no date).

Radon species, concentration and flux emitted in soil gas in active fault zones near Beijing have been reported by Chen et al. (2018), with a maximum flux of 334.56 mBq/m2/s being observed in the Fengnan district located at the epicentre of the 28 July 1976 earthquake. Chen et al. (2018) reported that these concentrations warrant mitigation measures and advised that fault zones in earthquake regions should be monitored as part of the pre-development land planning procedure.

Another source of ground gas with a potential for release by earthquake is methane hydrates associated with continental margins (Geology.com, 2005-2020).

Metrics and numeric limits

No globally agreed limits for ground gases (earthquake trigger).

Examples of drivers, outcomes and risk management

Ground gases are a hazard in terms of risk to human health, flammability and climate change (greenhouse gases). For these reasons, where possible, ground gas is monitored and controlled. Where buildings may come into contact with ground gases, specialist construction techniques are deployed to protect human health (e.g., Claire, 2021).

In the case of earthquake-triggered gases, consideration should also be given to the associated particulate matter. Landfill gas management has been a focal point for national-scale reductions in carbon dioxide emissions. For example, in 2018 waste management-related carbon dioxide formed 4.6% of UK carbon dioxide emissions (BEIS, 2020).

Ground gases occur in mining environments, for example, in the mining of coal (carbon dioxide and methane), potash (methane and nitrogen) and shale gas (BGS, no date). In the UK, control measures in these environments are guided by the Health and Safety Executive.