**Radon as Earthquake Precursor**

Giuseppina Immè and Daniela Morelli

*Dipartimento di Fisica e Astronomia Università di Catania - INFN Sezione di Catania Italy* 

#### **1. Introduction**

142 Earthquake Research and Analysis – Statistical Studies, Observations and Planning

Walia, V., Virk, H.S. & Bajwa, B.S. (2006). Radon precursory signals for some earthquakes of

Zmazek, B., Vaupotic, J., Zivcic, M., Premru, U. & Kobal, I. (2000). Radon Measurements for

No.4, pp.711-721. ISSN 0033-4553.

1333-9133.

magnitude > 5 occurred in N-W Himalaya. *Pure and Applied Geophysics*, Vol.163,

Earthquake Prediction in Slovenia, *Fizika B*, Vol.9, No.3, pp.111-118. ISSN 1330-0016 /

Earthquake predictions are based mainly on the observation of precursory phenomena. However, the physical mechanism of earthquakes and precursors is at present poorly understood, because the factors and conditions governing them are so complicated. Methods of prediction based merely on precursory phenomena are therefore purely empirical and involve many practical difficulties.

A seismic precursor is a phenomenon which takes place sufficiently prior to the occurrence of an earthquake. These precursors are of various kind, such as ground deformation, changes in sea-level, in tilt and strain and in earth tidal strain, foreshocks, anomalous seismicity, change in b-value, in microsismicity, in earthquake source mechanism, hypocentral migration, crustal movements, changes in seismic wave velocities, in the geomagnetic field, in telluric currents, in resistivity, in radon content, in groundwater level, in oil flow, and so on. These phenomena provide the basis for prediction of the three main parameters of an earthquake: place and time of occurrence and magnitude of the seismic event.

The most important problem with all these precursors is to distinguish signals from noise. A single precursor may not be helpful, the prediction program strategy must involve an integral approach including several precursors.

Moreover, in order to evaluate precursory phenomena properly and to be able to use them confidently for predictive purposes, one has to understand the physical processes that give rise to them. Physical models of precursory phenomena are classified in two broad categories: those based on fault constitutive relations, which predict fault slip behavior but no change in properties in material surrounding the fault, and those based on bulk rock constitutive relations, which predict physical property changes in a volume surrounding the fault. Nucleation and lithospheric loading models are the most prominent of the first type and the dilatancy model is of the second type.

During the past two decades efforts have been made to measure anomalous emanations of geo-gases in earthquake-prone regions of the world, in particular helium, radon, hydrogen, carbon dioxide. Among them radon has been the most preferred as earthquake precursor, because it is easily detectable.

Radon is found in nature in three different isotopes: 222Rn, member of 238U series, with an half life of 3.8 days, 220Rn (also called thoron), member of 232Th series, with an half life of 54.5 s and 219Rn, member of 235U series, with an half life of 3.92 s.

Owing to his longer half-life, the most important of them is 222Rn, produced by 226Ra decaying. After his production in soil or rocks, 222Rn can leave the ground crust either by

Radon as Earthquake Precursor 145

When radium decays in a mineral substance, the resulting radon atoms must first emanate from the grains into the air-filled pore space. The fraction of radon that enters the pores, commonly known as *emanation fraction*, consists of two components due to recoil and diffusion mechanisms. Since the diffusion coefficient of gases in solid materials is very low, it is assumed that the main portion of the emanation fraction comes from the recoil process. From the alpha decay of radium, radon atoms possess sufficient kinetic energy (86 keV) to move from the site where radon is generated. The range of 222Rn is between 20 to 710 nm in

The emanation fraction can be strongly influenced by water content in the material, increasing with soil moisture, up to saturation in the normal range of soil moisture content.

The increase in the emanation fraction can be explained by the lower recoil range of radon atoms in water than in air. A radon atom entering a pore that is fully or partially filled with water has a very good chance of being stopped by the water in the pore. Generally, the presence of water increases the emanation fraction, but this trend may show a saturation

In addition to the moisture effect, dependence of the emanation fraction on grain size and temperature has also been observed. Small grain size soils, such as clay, display maximum emanation at about 10%-15% water content. The ratio of the maximum emanation fraction to that of a dry sample also decreases as the grain size increases. A rise in temperature also causes an increase in the emanation fraction, which is probably due to the reduced

Different types of soil show different emanation fractions for 222Rn, which are generally in

Some emanated radon atoms, after their penetration trough the pore of a material, may finally reach the surface before decaying. Radon behaves as a gas and its movement in material follows some well-known physical laws. There are essentially two mechanisms of

In diffusive transport, radon flows in a direction opposite to that of the increasing concentration gradient. Fick's law describes this process. Expressions for the radon fluence

If one assumes the earth as a semi-infinite homogeneous material, with density and porosity the fluence rate JD of radon emerging at the earth surface can be given by (*Sabol et* 

where *CRa* is the activity concentration of 226Ra in earth material (Bq/kg); *Rn* is the decay constant of 222Rn (2.1 10-6 s-1); *f* and *De* are the emanation fraction and the effective

After crossing soil-air interface radon exhales into the atmosphere. The exhalation rate, that is the amount of radon activity released from the surface, depends on meteorological parameters. In particular the exhalation of radon is positively correlated with moisture content, temperature and wind speed and negatively with pressure, so that these factors

0.5

*<sup>D</sup> JC f* (1)

*Rn*

 

*<sup>e</sup> D Ra Rn*

common materials, 100 nm for water and 63 m for air. (*Sabol et al., 1995*)

A representative estimate of the fraction of radon that leaves solid grains is 25%.

effect or the effect may even later reverse as the water content becomes greater.

radon transport in material: (1) molecular diffusion and (2) forced advection.

rate, in Bq m-2 s-1 , can be derived for specified geometric conditions.

diffusion coefficient for earth material (m2/s) respectively.

adsorption of radon.

*al., 1995*):

the range 0.01-0.5 (*Sabol et al., 1995*).

molecular diffusion or by convection and enters the atmosphere where his behavior and distribution are mainly governed by meteorological processes.

The radon decay products are radioactive isotopes of Po, Bi, Pb and Tl and they are easily attached to aerosol particles present in air. In table 1 are shown the principal decay characteristics of 222Rn and 220Rn, including properties of their respective parent radionuclides and their short-lived decay products.


Table 1. Principal decay Characteristics of 222Rn and 220Rn

The release of radon from natural minerals has been known since 1920's (*Spitsyn, 1926*) but its monitoring has more recently been used as a possible tool for earthquake prediction, because the distribution of soil-gas radon concentration is closely related to the geological structure, fracture, nature of rocks and distribution of sources. Therefore, surveying of radon concentration can prospect fracture trace, earthquake forecast, environment monitoring, etc.
