Download MEASUREMENT OF SOLUBLE RADON IN JOOSHAN SPA (SE OF

ENVIRONMENTAL PHYSICS
MEASUREMENT OF SOLUBLE RADON IN JOOSHAN SPA
(SE OF IRAN) AND STUDY ITS PERFORMANCE
IN EARTHQUAKE FORECASTING PROCESS
MOJTABA∗ NAMVARAN 1,*, ALI NEGARESTANI 2,3
1
Kerman Graduate University of Technology, Geophysics department, P.O.Box:76315-115, Kerman,
Iran, E-mail: [email protected]
2
Kerman Graduate University of Technology, Electronic Engineering department, P.O.Box:76315115 Kerman, Iran, E-mail: [email protected]
3
Earthquake Research Center, Kerman, Iran
Received June 25, 2012
Measuring variations in gas level such as radon in soil and groundwater is an acceptable
technique for tracing the seismotectonic activities. Uranium decayed in the lower crust
and upper mantle and produced radon and its progenies. They can track a long distance
and reach to the surface and report a lot about deep earth crust activities.
In this study, Jooshan spa located in the SE of Iran nearby the Golbaf-Sirch fault system
as one of active faults all over the country has been studied. So in this research we
study the variations in radon concentration related to earthquake occurring as a
sensitive and accurate precursor. Measured data indicate clear decline in radon
concentration a few days before the main event.
Key words: Radon measurement, Jooshan spa, Golbaf-Sirch fault system, D/R factor
1. INTRODUCTION
Measuring variation in gas levels in soil and groundwater is a proven
technique for tracing the changes in stressses due to seismotectonic activities;
which are well documented and are used regularly in physics of the earth studies
[1]. Geochemical behavior of gases usually depends on their chemical properties.
Inert gases rarely combine with other materials during migration; therefore, these
gases that are released from the depths of earth can provide useful information
about movements of the crust infrastructures and mantle [2].
Radon is a radioactive nuclide with a half-life of approximately 3.83 days
that is produced through natural decay-chain of 238U isotopes. Regarding the halflife of 222Rn nucleus, a fraction of radon gas produced as a result of the decay
* Corresponding Author: End of 7Bagh highway, Mahan knowledge paradise, Kerman
Graduate University of Technology.
Rom. Journ. Phys., Vol. 58, Nos. 3–4, P. 373–382, Bucharest, 2013
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Mojtaba Namvaran, Ali Negarestani
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inside the soil can track a long distance and reach the surface of the earth and enter
the air [3]. So, we can use it as a precursor to locate active faults and also it can be
used to discover “Uranium Mines”. In normal conditions, radon radioactive
progeny (218Po and 214Po) which are solid particles [4] float in the air although
222
Rn is a gas [2].
High concentration of radon is often found in regions overlying highly
fractured rocks such as geologic faults and active volcanoes [5, 6]. According to a
worldwide survey some of radon anomalies associated with earthquakes show
increases in radon concentration precursory to a rupture while few anomalies
manifested decreases in radon. The mitigation and transport velocity of radon and
its progeny into soil near the surface and thermal spas depends upon the properties
of faults and tectonic movements, distribution of cracks within the beneath layers
of soil and rock, soil porosity and permeability and its moistening and
meteorological factors of study area [7].
2. STUDY REGION
2.1. GEOLOGICAL SETTING OF MONITORING STATION
‘Kerman province’ is located to the SE of Iran. It has diverse morphological
features. There are vast deserted territories in eastern and southeastern part of
province. A rather small part of Zagros range is exposed in west and southwest of
Kerman province. In general, Kerman province can be a part of Central Iran zone
in structural units and extent of sedimentary basins viewpoint [8]. Also it can
be related to Tabas block and Tabas-Kerman ranges in respect to tectonically units
[9]. The main Quaternary faults in this area are the Gowk fault and Golbaf-Sirch
fault system with almost S-N trending, the Kuhbanan fault with NW-SE direction
and the Nayband fault with a length of about approximately 400 [km] running
north to south [10]. In contrast to the Zagros regions; in this region, strong
earthquakes are often associated with well recognizable surface faulting [11] (Fig. 1).
2.2. SITE DESCRIPTION
All our experiments were performed in Jooshan spa complex which located
in Jooshan village, about 60 [km] east of Kerman city (φ=30° 09' 10.1'N; λ=57° 38'
16.3''E). Jooshan geothermal system comprises of 6 thermal springs outlet. These
spas have different water temperatures ranging during the year. The presence of
many faults in studied area, the alignment of all springs along the Golbaf-Sirch
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fault system and near the Gowk fault and also the similar chemical composition of
all springs in combination with the hydrogeological setting and geochemistry of
water samples indicate that these springs are associated with deep circulation of
meteoric water. According to this heating mechanism, meteoric waters infiltrate
through fault openings to depth and after heating by geothermal gradient come
back to the ground surface due to the hydraulic and buoyancy forces, a mechanism
which is common in the southern parts of Iran.
Fig. 1 – Epicenter of few large earthquakes and location of faults and measuring station
in Kerman province, SE of Iran [12].
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3. MATERIALS & METHODS
3.1. RADON MEASUREMENT
The system for Radon measurement installed and started to measure from
December 29, 2011 until March 3, 2012. The main device in this research is RAD7
detector coupled with a measuring toolbox in which the air is driven by an air
pump have been sucked from the water container into the trap, then into the
detector as depicted in Fig. 2. The RAD7 Radon monitor (DURRIDGE Company
Inc, USA) is a commercial model and it is widely used in many applications
involving continuous Radon activities measurement. The device counts
continuously and the data are stored every 10 minutes and indicate the αdisintegrations occurred in this time interval. During measurement of Radon,
concentration level varies widely which most of these fluctuations are significantly
interpretable. At the minimum state, data decreased to about 35 [kBq/m3] and at the
maximum state, it is increased to about 130 [kBq/m3]. The RAD7 sensitivity to
Radon is 0.01281 [CPM/(Bq/m3)] with 2% calibration uncertainty which all
determined by DURRIDGE Company Inc. Also, bimonthly laboratory calibration
checking has been done in KGUT-lab by three different exposure chambers.
Fig. 2 – Schematic model of radon measurement system which executed in Jooshan spa.
3.2. SEISMIC DATA
Seismic data used in present research is obtained from earthquake catalogues
of the Iranian Seismological Center (IRSC) and International Institute of Earthquake
Engineering and Seismology (IIEES) seismic observatory with instrumentation
similar to the World Wide Standard Seismograph Network (WWSSN). In these
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catalogues, all necessary parameters such as geographical coordinate of epicenter
(latitude & longitude), magnitude [Richter], depth [km], magnitude type and
azimuth gap are accessible.
Fig. 3 – The continuous radon monitoring station in Jooshan spa.
3.3. DOBROVOLSKY EQUATION
Dobrovolsky et al., in 1979 suggested a theoretical – empirical relationship
between size of the effective precursor manifestation zone and the main earthquake
magnitude as:
D = 100.43 M
(1)
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where M is magnitude of the earthquake [Richter] and D is earthquake’s effective
radius [km] called ‘strain radius’. This equation was developed for estimating the
deformation and tilts in earth’s surface as a function of the coming earthquake’s
magnitude and distance from the epicenter [13].
4. RESULTS
According to seismic data, the parameters of occurred earthquakes in Kerman
province during measurement period were accessible. Therefore, the distance
between epicenters of each earthquake to measurement station has been calculated.
Also, based on Eq.1 an impact factor value is achieved dividing Dobrovolsky value
(D) by distance between each earthquake’s epicenters to spa location (R) as D/R.
This factor indicates the range of ability and reliability of each earthquake to apply
as a precursor in a study area (Fig.3). Based on previous researches by other
scientists, the meaningful range of D/R is about 1.0 and more. But in this study, to
reach more accuracy, D/R larger than 0.4 has been selected. All seismic events
with D/R≥0.4 have been listed in Table 1.
In this research, temporal variation of water temperature as one of
environmental parameters in study area was examined to check their correlation
with radon anomaly (Fig. 4 - curve A). These anomalies in temperature could be
affected due to different reasons. The temperature has inconsistent fluctuations and
is hard to interpret.
Fig. 3 – Values for D/R factor in contrast with time as seismic event’s precursory ability.
The radon concentration of groundwater in Jooshan spa was not fairly stable
and followed an inconsistent fluctuation. But sometimes a sharp increasing in
radon concentration is distinguishable. For example 4.4 days before a seismic event
with M=3.3 on January 9, 2012 earthquake, radon concentration in Jooshan spa
started to decrease for about 13 hours. This peak reaches to 42 [kBq/m3] in
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minimum state and then started to increase while radon concentration reaches to
previous level of around 100 [kBq/m3] in average value before variations
(Fig. 4 - curve B).
Table 1
Selected earthquakes with D/R>0.4 and other seismic parameters
#
DATE
TIME
MAG
[Richter]
LAT
LONG
DEPTH
[km]
DISTANCE
to station [km]
D/R
1
2
3
4
5
6
7
8
9
10
11
01/05/2012
01/06/2012
01/09/2012
01/09/2012
01/09/2012
01/28/2012
02/10/2012
02/21/2012
02/23/2012
02/24/2012
02/27/2012
23:00
03:41
08:59
12:56
14:08
13:32
14:11
08:58
07:59
04:51
18:48
2.9
3.0
3.3
2.7
2.6
3.2
2.7
3.2
2.9
2.7
5.4
30.50
29.95
30.18
29.99
29.94
30.21
30.12
29.80
30.45
30.05
31.42
57.56
57.72
57.63
57.69
57.77
57.42
57.61
57.36
57.46
57.57
56.77
5.0
6.1
7.9
10.1
9.9
6.1
6.0
6.1
5.0
10.3
9.8
39.401
23.139
3.936
18.273
26.880
21.993
4.040
46.217
37.655
12.182
163.826
0.4482
0.8426
6.6661
0.7928
0.4881
1.0806
3.5857
0.5142
0.4690
1.189
1.281
Fig. 4 – Variation of radon concentration (B) and temperature (A) in contrast with time and other
parameters of selected events.
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5. DISCUSSION
This paper discusses variations in radon concentration and correlation with
earthquake parameters and also temporal variation of temperature. Moreover, it
would be difficult to explain such a large radon variation by mixing environmental
parameters, because based on researches by other scientists, the affect of whole
environmental parameters on radon concentration in less than 20%. So the
anomalies associated with these parameters could be ignored in some cases.
The epicenter distribution of earthquakes during the monitoring period in
study area indicate the significant relation between south and southeast of Kerman
province seismicity and the Sirch-Golbaf and Gowk faults. Radon monitoring
station in the Jooshan spa is located in ~2 [km] west of the Sirch-Golbaf fault
system and 10 [km] north of the Gowk fault. The anomalous declined in concentration
of radon observed at the Jooshan spa suggest that radon concentration in soil and
groundwater can be sensitive accurate tracer for fault strain variations in crust
associated with time, location and magnitude of posterior earthquakes.
The studied area is in a unique tectonic setting located at eastern extremity of
the Arabia-Eurasia collision zone [14]. The convergence of this collision zone is
mix of Africa-Eurasia and Arabia-Eurasia motions to be N-S in east and southeast
of Iran with rate of approximately 30 [mm/year] at 50°E and 40 [mm/year] at 60°E
[11]. All active deformations and tectonic movements are well documented in
eastern and southeastern part of Iran with the development of relatively rigid
blocks between major strike-slip faults and thrust faults [15]. Under focus on such
geological features of case study, this hypothesis that when regional stresses due to
tectonic movements increases, dilation of the rock mass occurs at a rate more faster
than the rate at which pore water migration into the newly pore is acceptable.
The main theory that governs earthquake mechanism is elastic rebound
hypothesis. In this theory, the elastic rebound energy and earthquake’s magnitude
determine with maximum deformation and modulus of elasticity of rocks which
constitute the crust. During the accumulation of elastic rebound energy, any
variation in radon concentration in soil and groundwater is happening and radon
level start to increase when stress exceeded one half on the rock strength and these
variations are directly related to porosity and permeability of soil in study region.
By increasing the elastic rebound energy cause to expand the volume of microfractures in the rocks and groundwater flows into them. If in a special region, these
fractures were interconnected, increase the region stress cause groundwater
penetrated into the other rocks and hence groundwater level was decreased. The
penetrating of the groundwater into the beside fracture rocks cause decrease the
level of groundwater and also increasing the level of radon concentration,
especially in spas and spas (due to higher temperature and therefore higher
solubility). So the radon flux starts to decrease while the fracture spaces in the rock
filled. Then the radon concentration starts to increase to reach the prior average.
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In this research the time interval between the radon anomaly and earthquake
occurring vary widely from 4 to 12 days. This time interval is depend on few
factors such as lithology and structology of study region, distance between
epicenter and monitoring station, mechanism of faulting and depth of hypocenter
which all can impact on migration of radon and its progeny toward the surface.
These factors are being investigated.
6. CONCLUSION
– Increasing the rock strain could be growing the micro-fractures and
decreasing the level of groundwater and radon concentration and correlated
with seismic events.
– Anomalous decreases of radon concentration are correlated with
earthquake’s occurring time and magnitude.
– It’s suitable that other geological setting studied and correlation between
them and radon concentration investigate.
– Notwithstanding D/R factor in a few regions is lower than 0.4 an event is
correlated with it. This indicates that the Dobrovolsky equation must
reconsider regionally.
Acknowledgements. The authors are extremely grateful to Dr. Majid Shahpasanszadeh and
Prof. Jamshid Shahabpour for their kind guidelines throughout the study period.
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