Nucl. Tracks Radiat. Meas., Vol. 19, Nos 1--4, pp. 417-420, 1991 Int. J. Radiat. Appl. lnstrum., Part D Printed in Great Britain

0735-245X/91 $3.00 + .00 Pergamon Preu plc

SUBSURFACE SOIL GAS RADON CHANGES ASSOCIATED WITH EARTHQUAKES

MANWINDER SINGH, R.C.RAMOLA, BALJINDER SINGH,SURINDER SINGH and H.S.VIRK

Department of Physics, Guru Nanak Dev University Amritsar - 143 005, India

Abstract

Radon concentration within the ground but close to the surface are being measured continuously from 1984 for earthquake prediction. During this period several radon anomalies considerably larger than meteorologically induced variations are recorded. These anomalies are followed by an earthquake of magnitude above 3.8 occured within 3 to I0 days, at an epicentral distance I00 to 400 km.

KEYWORDS

Radon; soil-gas; earthquake; integrated; anomaly; deviation; track.

INTRODUCTION

Ulomov and Mavashev (1967) reported that radon concentrations in deep aquifer had increased starting from at least one year before a M=5.3 earthquake at Tashkent USSR. Numerous and widespread investigations have provided evidence of groundwater or soil-radon anomalies precursory to earthquakes of moderate- to-large magnitudes (King,1978, Teng, 1980, Fleischer, 1981, Ramola et ai.,1990 ) The observed anomalies are generally, increases of rad~ c-oncentratlon upto 2 or 3 times the normal range of seasonal, monthly or yearly variations. A few cases have been reported where radon concentration in aquifers (Birchard and Libby, 1980) and soil gases (Talwani et al., 1980) decreased before seismic activity. Since 1984, we have been mon~t~ing the soil-gas radon concentration continuously at non-mineralized site at Amritsar. The radon monitoring in soll and water has been extended to earthquake prone areas in Kangra valley, Himachal Pradesh from August, 1989.

RADON MONITORING TECHNIQUES

The short term fluctuations in radon concentration was continuously monitored with alphameter-400 (AlphaNUCLEAR Company, Canada) which contains silicon diffused junction detector for the detection of alpha particles with its associated electronic circuit. Its sensitiveity is such that a 24 hrs exposure gives sufficient counts in most of the soils (Singh et el., 1988a)

An integrated soll radon concentration over monthly time interval is determined through the use of Radon - Thoron discriminator (Ramola et al., 1990) with alpha track detector (LR-II5 Type2) which indicate the-- a~ha activity level resulting from radioacitive decay of gaseous radon -222 emanating from the soil. After exposure for monthly period the films are chemically processed and analyzed.

A closed circuit alpha sclntilation technique has been used to measure the radon concentration in water (Ghosh and Bhalla, 1966) .The air is circulated in closed circuit bubbling through the water sample and de-emanating in the process for about I0 minutes. The counts are recorded after 4 hours during which the equilibrium is established between Rn-222 and its decay products.

OBSERVATIONS

The continuous monitoring of soil gas radon at Amritsar was initiated in February, 1984 with Alphameter-400 and plastic track detectors. The observed radon variation patterns are quite complicated. There is seasonal variation

417

418 M. SINGH et al.

in most record, which tend to be high at the end of summer and low at the end of winter. Daily variations caused by changes in meteorolgocial conditions such as temperatures, atmospheic pressure, wind velocity, humidity and rainfall were also observed (Singh et al., 1988b). The variation in radon concentration due to various meteorol~i~l parameters are well below X + I~ value. There does not exist any general rule about how to select earthquakes that possibly can correlate with radon data. Based upon experimental evidence, the empirical magnitude-distance relationship was applied (Haukson and Goddard , 1981);

M = 2.5 log D - 0.43 (I) I0

such that an earthquake of magnitude M could be preceded by radon anomaly at a distance of less than or equal to D(Km). Thus, it will often be stated whether an earthquake is within the distance range of radon station or not.

0

,3 2 [ - i ~ =

I ' t i Y i - /-

~. 4 .4

2.

i

q*3

-~+2 o-

-X + Io-

"-x

I I I I I.I. I H I 1,71 ;,,/ I I 03/84 04/84 07/85 04/86 07/86 03/87 04/87

Fig. I. Radon time series in soil indicating the recorded earthquake events at Amritsar measured with Alphameter-400.

Daily radon records at Amritsar with mean value of radon activity and the range of one and two standard deviataion above the mean are shown in Fig.l.An empirical criterion is here adopted to define the radon anomaly as the positive deviation that exceeds the mean radon level by more than twice standard deviation. Six positive deviations above the normal range were identified out of which four fulfil equation (I).

The first radon anomaly occured in last week of March 1984. The peak value generally three times the average flux was recorded on March 23 1984, which is well above the X + 2o value (Fig.l). This increase was followed by an earthquake of magnitude 5.0 on March 29, 1984 whose epicentre was about 150 km away in North-East direction from the recorded station Amristar.

The 6.8 earthquake occured on July 29, 1985 (II) whose epicenter was 400 km away in North-East direction (lat 35.3N and long 71.5E in Hindu Kush area) from recording station at Amritsar. Daily monitoring of radon recorded the peak below X + 2o level as the seasons heaviest rainfall occurred before the earthquake (Fig.l). But the peak value almost twice the average radon flux above level is observed with Track Etch Method (Fig.2) which reocrds the signal from the greater depth and also smooths the short term noise.

Other impulsive changes in radon concentration were recorded on April 21, 1986; July I0, 1986 and March 17, 1987. These anomalies were followed by the earthquakes of magnitude 5.7, 3.8 and 4.3 on April 26, 1986 (III) July 17, 1986 (IV) and March 17, 1987 (V), respectively with epicenters in Kangra

GAS RADON CHANGES AND EARTHQUAKES 419

valley at a distance of about 150 km from the recording station at Amritsar The observed radon anomalies are three to four times greater than the average values (Figs.l and 2).

The sixth noticeable radon anomaly occured in Hindu Kush area. The impulsive increase was recorded on May 1,1987 (Fig.2) followed by an earthquake of magnitude 5.0 on May 5, 1987 whose epicenter is 400 Km away in Hindu Kush area from recording station. The peak is estimated to be roughly thrice the average level and well above the mean radon level by more then twice the standard deviation (Figs.l and 2).

6 . 6 6

6 .29

5 .92 - j ,

X

~_5.55 E O"

~ 5.1e . _

~ 4.81 !

C

4.q4

,¢0

t,,,, I I

10 12 02 04 06 08 ~-198~.- • 1985 .-m-

=.

I I 3E

i o

=£

Ir I 17

X +2o"

~..

- - - | a i |

O~ 04 06 08 10 12 02 Oq, 06 08 • 1986 =' ~ 1987 •

Fig. 2. Radon time series in soil indicating the recorded earthquake events at Amritsar measured with Alphameter-400.

~ _ 5 , 7 x

IE ~.4.3

0

~ 2 . 9 ._c ¢ -

1 . 5 I I // 12189 0119 0

I 12189

J b

UJ

- 9 . 0 ~ E

m

- 7 .6 = O 3=

6.2

. °

01/90

Fig. 3. Impulsive radon increase in soll and water recorded at Palampur in Kangra valley.

Fig 3. shows the daily variation of radon content in soll and water recorded at Palampur in Kangra Valley (Hlmachal Pradesh). As a matter of chance there have been only one impulsive increase of radon recorded on December 23

I(T 1,9:114-0

420 M. SINGH et al.

1989 in both soil and water. No unusual meteorological variation was recorded at that time but an eathquake of moderate intensity occured on December 24,1989 with epicentre in Chamba area at a distance of 40 Km from recording station.

DISCUSSION

The several spike like radon anomalies were recorded , which incidently preceede the earthquakes occured in the region. In consideration of the possible relationship, no correlation was observed, except that generally lower radon concentrataion during and after the period in which heaviest rainfalls occured (Singh et al., 1988b). If the changes in weather conditions were the main cause of the observed radon anomalies, then one would expect to see signficant seasonal component in the radon variations= and this component would be synchronous throughout the study area because the climatic conditions of the area are quite uniform. The observed radon anomalies are mainly earthquake related. The increase of radon content is connected with the amount of cracking of rock and therefore is sharply increased and then after decrease before the earthquake due to the closure of small cracks. The observed radon patterens during the earthquake events are similar and may be explained with the IPE model (Mjachkin et ai.,1975)

CONCLUSIONS

Analysis of the temporal variations in radon emanation suggests that radon activity in soil gas is controlled by several physical mechanisms. The influence of meteorological parameters and seasonal changes in radon concentration was observed. However, the effects of these factors did not obscure an evident relationship between radon emanation and seismicity. The results point out some potential problems in the precision of prediciting earthquake time (before and after the radon peak), location and magnitude. Finally, we conclude that radon anomalies are sometimes associated with earthquake activity. However, more data are needed especially from areas in dfiferent tectonic settings to establishe whether radon measurements will frequently be helpful in allowing the prediction of earthquakes.

Acknowledgements: The authors acknowledge the financial assistance by the Council of Scientific and Industrial Research (CSIR) and Department of Science and Technology, New Delhi.

REFERENCES

Birchard, G.F.and W.F.Libby (1980).Soii radon concentration changes preceding and four magnitude 4.2-4.7 earthquakes on the San Jacinto fault in Southern California.J.Geophys. Res.,85,3100-3106.

Fleischer,R.L. (1981). Dislocation model fo-'~a~-6n responseto distant earthquakes. Geophys. Res. Lett.,8,477-480.

Ghosh P.C. and N.S.Bhalla---[-f9~6-[?-.-.-~losed circuit technique for radon measurement in water and soil, with some of its applications.Proc. All Ind. Symp. on Radioactivity and Meteorology of Radionucli--~ A---E-ET~Bom aa~, pp. 226-239.

Haukson,E. and J.G.Goddrad (1981). Radon earthquake precursor studies in Iceland. J. Geophys. Res.,86,7037-7054.

King,C.Y. (i-g78). Radon emana-6ion on San Andreas fault.Nature,271, 516-519 Mjachkin,V.l., W.F.Brace, G.A.Sobolev and J.H.Dieter--~(l--975). Two

models for earthquake forerunner. Pageoph,ll3, 169-180. Ramola,R.C., M.Simgh, A.S.Sandhu, S.Singh and H.S~-.'.Virk (1990). The use of

radon gas as earthquake precursor. Nucl. Geophys., 4, 275-287. Singh,M., R.C.Ramola,N.P.Singh, S.Singh an-'~H.S.Virk (I~88a). Measurement

of soil gas radon at Amritsar. Geophys. Res. Bull.,26, 8-12. Singh,M, R.C.Ramola, S.Singh and H.S.Virk-----[19-~.--The influence of

meteorological parameters on soil gas radon. J. Assoc. Explor. Geophys., 9, 85-90.

Talwani,P, -W.S.Moore and J.Chiang (1980). Radon anomalies and microearthquakes at lake Jocasse. J. Geophys. Res.,85, 3079-3088.

Teng,T. (1980). Some recent studies on--ground wate~ac~-6n content as an earthquake precursor. J. Geophys. Res., 85,3089-3099.

Ulomov,V.l. and B.Z.Mavas~-6v (1967). Aprecursor of strong tectonic earthquake. Dokl. Acad. Sci. USSR~ Earth Sci. Sect., 176, 9-11.