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Continuous radon monitoring in soil gas towards earthquake precursory studies in basaltic region
Radiation Measurements 45 (2010) 935e942
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Continuous radon monitoring in soil gas towards earthquake precursory studies in basaltic region D.V. Reddy*, P. Nagabhushanam, B.S. Sukhija, G. Rajender Reddy National Geophysical Research Institute (Council of Scientific & Industrial Research), Uppal Road, Hyderabad-500 606, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2009 Received in revised form 1 May 2010 Accepted 3 May 2010
The Koyna-Warna region, near the west coast of India, is well known for reservoir-triggered seismicity. The seismic activity in this region greatly increased following the construction of an artificial reservoir across the Koyna River during the 1960s. A destructive earthquake of M 6.3 occurred on December 10, 1967, and further 19 earthquakes of M>5 have been recorded during the preceding 40 years until 2007. The soil gas radon (222Rn) has been studied as an earthquake precursor by continuous monitoring (hourly) at two sites around the Warna reservoir. One site has a multi-sensor probe (installed at three different depths), together with a rainfall recording facility, and another probe is mounted on a hillock at Nivle. During the study period (2005e2007), a total of 11 earthquakes (including 2 aftershocks) of M 4e4.8 were recorded. Most of these events had recorded precursory radon signals. For a given earthquake, the 222Rn precursory signatures were recorded at one of the two sites only. Even multiple depth probes showed discordant behaviour in recording temporal Rn variation. Causes of non-concurrence in Rn recording between sites and probes, including the combined effect of site heterogeneity, focal depth, epicentral distance, earthquake magnitude, faults responsible for the earthquake, etc, are discussed. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Radon Earquake precursor Basalts Koyna
1. Introduction Radon (219Rn, 220Rn and 222Rn), an inert radioactive gas that is produced during the decay of uranium and thorium, is present in the earth’s crust. The emanation of Rn is affected more by physical conditions in the environment than by chemical processes (Tanner, 1964; Reddy et al., 1996). Hence, Rn concentration levels in the subsurface are strongly controlled by geological conditions and atmospheric influences such as barometric pressure and rainfall. The migration of Rn from the soil or rock by diffusion is limited because of their short half-lives and it is 222Rn (here after called Rn) with the longest half life (3.82 d) that does migrate and is of particular interest in this study. Cracks and fissures play an important role in enhancing Rn flux (Holford et al., 1993) and a high concentration of Rn is often found in soils overlying highly fractured rocks such as geologic faults and active volcanoes (Banwell and Parizek, 1988; Reddy et al., 2006; Ciotoli et al., 1999; Berlo et al., 2004). In the case of an earthquake, the Rn emanation increases due to the creation of new fractures and fissures associated with the earthquake process. Rn is easily soluble in water and it diffuses into the groundwater and spring waters. The relationship between an * Corresponding author. Tel.: þ91 4023434602; fax: þ91 4023434651. E-mail address: [email protected] (D.V. Reddy). 1350-4487/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.05.010
earthquake and Rn emission was shown by Okabe (1956). Many investigators tried to correlate the Rn concentration in soil gas as well as in groundwater/spring water along with other hydrochemical parameters related to earthquake activity (Ulomov and Mavashev, 1971; Mogro-Campero et al., 1980; Virk and Singh, 1993; Igarashi et al., 1995; King et al., 1995). By continuous Rn monitoring, Chyi et al. (2002) reported a quick rise in Rn levels a couple of weeks before a noticeable earthquake. They have also observed a Rn peak 1e7 days before the occurrence of an earthquake on an active fault zone in Taiwan. Steinitz et al., (2003) monitored Rn concentration in 10 min interval in the Dead Sea rift valley for 8 years and found only a statistical correlation between Rn anomalies and earthquakes of magnitude 4.6 ML0. They showed a Rn correlation for 101 earthquakes and concluded that the earthquake occurs 3 days after a Rn anomaly. From continuous Rn measurements for five months at a depth of 1 m in fractured granitic area, Reddy et al., (2004) reported an enhanced Rn concentration that preceded the occurrence of a micro earthquake by 17 h. An enhanced Rn level (twice the normal concentration) was recorded 13 h before the occurrence of the event, and it decayed exponentially during the next 5 days to reach a normal level. However, negative anomalies were also found, by Planinic et al., (2000) during their year-long continuous monitoring of Rn together with pressure and temperature at Osijek, Croatia. Based on the long-term Rn measurements at the SKE site in IzuPeninsula,
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Wakita (1996) concluded that the appearance of Rn signals due to earthquake activity depends on the state of stress accumulation in the region associated with opening and closing of micro fractures in the surrounding rocks. In this paper, we present the results pertaining to continuous Rn monitoring in soil gas using a multi-sensor Rn detector system at one location, and two single detector units at another site in the seismically active Warna region, India. The purpose of the experiment is to examine the precursory relationship between Rn signals and seismicity (M 4) in a seismically active basaltic region. 2. Study area and earthquake history The study area (Fig. 1) is covered by the Koyna and Warna reservoirs constructed across the two respective rivers at a distance of 33 km in the NS direction in Satara district, Maharashtra, India. The area forms part of Deccan Volcanic Province with extensive basaltic lava flows spread over a large area. Differential weathering is responsible for the formation of highly undulating topography associated with a series of hill ranges, ridges and valleys. The major lineaments in the area are trending NNE, NS, NE and less commonly east (Fig. 1). Extensive sheet joints (horizontal to sub-horizontal) cover the top of the individual lava flows. The Koyna-Warna region is one of the most significant sites of reservoir-triggered seismicity in the world (Gupta, 1992). Seismicity in this region started soon after the impoundment of Shivaji Sagar lake created by the Koyna Dam in 1962 (Gupta et al., 1969). Since the largest earthquake of magnitude 6.3 occurred near Koyna Dam on December 10, 1967 (17 39.00 N, 73 55.80 E), the seismicity is being continuously recorded by the National Geophysical Research Institute (NGRI). A total of 19 earthquakes of M >5 and more than 170 earthquakes of
M 4 occurred in the area, besides innumerable micro earthquakes until 2007 (Gupta et al., 2007). This study was initiated just after an earthquake of M 5.1 occurred on March 14, 2005 and continued to 2007, during which 11 earthquakes of M 4e4.8 occurred in the region. Continuous pore pressure changes are being monitored in this area by Chadha et al., (1997) in deep borewells drilled specially for precursor studies; and they observed significant water level changes for some of the earthquakes (Ramana et al., 2007; Chadha et al., 2008). To study the relationship between soil gas Rn and seismicity in the Koyna-Warna region, initially two sites Warna Inspection Bungalow (Warna IB) and Nivle in the Warna region (where present seismic activity is concentrated), were selected and Rn probes (Algade, France) installed (Sukhija et al., 2007). Warna IB site is located northeast of the Warna river valley (a partially plain area) and the Nivle site is over a hillock in a Reserved Forest, south of the Warna reservoir. Both the sites are 7.8 km apart in aerial view (Fig. 1). At Warna IB, a multi-sensor Rn unit consisting of one main unit and two Rn measuring heads were installed at three different depths i.e., 3.0, 1.7 and 0.6 m (Fig. 2) and the data collected from May 2005 to Dec. 2007. The main unit contains a data logger to record the data from the main unit and also from the two heads at a set time interval. The recording interval was set to 1 h. This unit also has a rain gauge attachment to measure the rainfall at the same site. The temperature and pressure are also monitored by the main probe which is at a depth of 3 m. Since the Rn concentration in the soil is affected by environmental factors (temperature, pressure and rainfall), it is vital to have a continuous measurement of these parameters. To reduce the direct effect of rainfall on the units, a small shed was erected to cover the units and an on-line data
Fig. 1. The study area of the Koyna-Warna region in Maharashtra, India. Earthquake epicenters of M 4 during Aug. 2005 to Dec. 2007, M 5.1 earthquake recorded on 14th Mar. 2005, a cluster of earthquakes M 4.2, 4.6 and 4.8 recorded on a single day (24th Nov. 2007,) in the hills, and prominent lineaments (Agrawal et al., 2004) are shown.
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transfer facility was arranged through the telephone line. These units are calibrated by the company every 2e3 years. However, primarily, we are looking for relative variation in Rn concentration with time rather than absolute values. At Nivle, a single Rn detector that measures Rn, temperature and pressure was installed at 60 cm depth and monitored continuously between May 2005 and Dec. 2007. Another single unit was also installed within 2 m distance of Unit I to cross check the Rn data. The second unit was operated for a shorter time, between Dec. 2005 and May 2006 (a dry period). 3. Results 3.1. Temporal variation in Rn concentration The hourly Rn recordings from five probes is expected to identify Rn anomalies pertaining to earthquakes. An “Rn anomaly” is considered if the Rn signal is one and half to two times more than the normal values recorded for a few days to few weeks prior to a Rn signal change. As the effect of atmospheric pressure and temperature on Rn concentration is only about 20% (Reddy et al., 1996; Reddy et al., 2004), it is not discussed here. However, we have considered the Rn changes due to rainfall in the present study.
Fig. 2. Experimental set-up for continuous recording of soil gas Rn measuring probes at three depths at Warna IB. This set-up also has the facility to measure the rainfall, and on-line data transmission.
3.1.1. Warna IB This site is covered with more than 3 m of lateritic soil. As expected, the Rn concentration in three probes installed at 3 different depths varied from 20 to 50 kBq m3. The lowest Rn concentration was recorded by the probe at 60 cm depth while the highest concentrations were at the 3 m depth probe. During the non-rainy period of 6 months (Nov. 05eApril 06) the Rn concentration at 60 cm depth varied between 20 and 30 kBq m3 (Fig. 3). For the same period, the 3 m depth probe recorded Rn concentration in the range 40e50 kBq m3. The probe at 1.7 m depth, recorded Rn concentrations between 30 and 40 kBq m3 for two months (Nov.eDec. 05) and later for 20 d the Rn concentration increased to 40e50 kBq m3. After 20 d of increase, the Rn concentration returned to normal level (30e40 kBq m3). Further, the Rn concentration at 1.7 m showed progressively increasing trend from Feb. 2006 and crossed the Rn level of the 3 m depth probe (50 kBq m3) before the rainy season (Jun.eAug.). Though the Rn concentration (20e30 kBq m3) at 60 cm depth remained stable during dry period, the concentration increased significantly from
Fig. 3. Continuous Rn measurement in the Warna IB and Nivle sites during the non-rainy period.
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Fig. 4. Influence of rainfall on Rn migration from soils during the rainy season.
28 Dec. 2005 to 22 Jan. 2006 (>100 kBq m3, Fig. 3). During the rainy season, mainly from June to Aug. 2006, the rainfall influenced the Rn concentrations (Fig. 4). Due to the initial rains, when the top soil becomes saturated, the Rn concentration builds up below the soil zone, as there is no scope to escape to the atmosphere. Later, when the soil moisture levels increase around the probe, the Rn concentration decreases. For this reason, the Rn measurement in the soil gas during rainy season may not be reliable for the precursory evaluation.
Warna IB 60 cm depth probe. There is a great change in the Rn concentration at Nivle site (few hundreds to about 35 kBq m3). Nevertheless, the changes are quite systematic (Fig. 3), and are not related to weather parameters. The second Rn unit installed at Nivle, 2 m apart, did not show any comparable values, though both the units are from same company and have similar background Rn values. Non-concurrence of the Rn data between the two probes is attributed to subsurface heterogeneity. 4. Discussion
3.1.2. Nivle site The site is situated over a hill range of 900 m above mean sea level and adjacent to a deep valley (one branch of the Warna reservoir). The site is covered with a thin soil layer and has several outrops/boulders around it. The response of the Rn unit at Nivle site (installed at 60 cm depth), differs significantly from that of the
During the continuous Rn monitoring period (Aug. 2005eDec. 2007), the study area experienced 11 earthquakes (including 2 aftershocks) with magnitude ranging from 4 to 4.8. The Rn probes emplaced at two sites (Warna IB and Nivle) have registered a few Rn high anomalies independently but not in unison. The observed
Table 1 Rn response in different probes with respect to different M >4 earthquakes. S. no
Date
Earthquake M
Focal depth (km)
Rn-Site
Dist*-(km)
Probe Depth (m)
Rn Response
1 2 3
14/08/05 30/08/05 13/11/05
4.2 4.8 4.0
5 6.8 4.8
Nivle Nivle Warna-IB
4.5 4.0 10.4
0.60 0.60 0.60
4
20/11/05
4.0
5.9
Nivle Warna-IB
2.7 15.6
0.60 0.60
5
26/12/05
4.2
5.8
Warna-IB
10.3
0.60
6
17/04/06
4.7
3.9
Warna-IB
10.3
No prominent signal due to rainfall Rn high peak 6 days before and co-seismic peak up to 100 kBq m3 3 spikes were recorded against the normal level 30 kBq m3 1 spike,11 days before Rn increased by 60 times 2 and 3.1 day before Rn increased to 76 and 540 kBq m3 No prominent peaks Multiple spikes started 5 days before (45 kBq m3 to > 100 kBq/m3 with 20 kBq m3 background) Spike 1 (500 kBq m3),15 days before Spike 2 (35 kBq m3), 13 days before Spike 3 (>70 kBq m3), 8 days before Spike 4 (e35 kBq m3), 4 days before Step increase (30e40 kBq m3), 3 days before and continued 10 days before a Rn spike increased from 50 to 80 kBq m3 16 days before Rn spike increased from 45 to 75 kBq m3 5 days before Rn spike increased from 25 to 50 kBq m3 20e25% change in Rn conc. as pre-, co- and post-seismic from its normal levels 25 to 30 kBq m3 Rn decreased for 6 days until one day before the earthquake Rainfall effect in all the probes Rn started increase 12 days before
Nivle 7 8 9 10 11
22/05/06 20/08/07 24/11/07
4.2 4.3 4.8 4.6 4.2
4.7 3.6 2.7 4.6 2.1
Nivle Nivle Warna-IB
4.0 10.0 19.0
Dist* distance between the earthquake epicenter and the Rn measurment site.
1.7 3.0 1.7 0.60 0.60 0.60 0.60 1.7
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Fig. 5. Continuous Rn measurement at Nivle (60 cm depth) during August 2005 and two M >4 earthquakes during that period.
anomalies are mainly in the form of short-lived spikes. Sometimes, the Rn levels are continuously low (less than normal level) and vice versa. The seismic activity was quite high during 2005 and the first half of 2006. Starting from the second half of 2006 to Dec. 2007 the seismicity has been at low ebb. Here, we tried to correlate the observed Rn anomalies with the earthquakes of M >4 that occurred within the radius of 20 km from the Rn measuring sites, and which are shallow in origin (<10 km). The prominent anomalies observed at Nivle and Warna IB sites are summarized below and shown in Table 1. 4.1. Nivle site Two earthquakes of M >4 were recorded, one on 14th August 2005 (M 4.2) and the other on 31st August 2005 (M 4.8), at a distance of about 4 km north of the Rn measurement site with a focal depth of about 5 km. In the first week of August 2005, the Rn levels remained quite low (few hundred Bq m3) and a spike of
10 kBq m3 occurred on 8th August about 5 days before the M 4.2 earthquake (Fig. 5). During the next 10 days, the Rn levels show high fluctuation and after 24th August the levels increased to 30 kBq m3. A Rn spike of 100 kBq m3 was observed on August 28, 3 days before the M 4.8 earthquake. The Rn anomaly for the M 4.8 earthquake is 7e10 times more than that of corresponding M 4.2 earthquake on August 14 (Fig. 5). Though there were two earthquakes during November and one in December 2005, all of magnitude >4, Rn changes were not detected at this site. However, the 17th April 2006, M 4.7 event generated pre-, co- and post-seismic Rn changes in one of the two units, of 20e25% greater than the normal values (20e30 kBq m3, Fig. 6). The M 4.2 earthquake on May 22, 2006, indicated decreased Rn concentration for 7 days before its occurrence (Fig. 7), but the Rn levels reached to normal levels one day before the earthquake. A drop in Rn levels was also observed by Wakita (1996) more than 3 months before the M 7.0 Izu-Oshima-Kinkar earthquake on January
Fig. 6. Continuous Rn measurements at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (60 cm depth) during April 2006 and an earthquake of M 4.7 that occurred on April, 17.
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Fig. 7. Continuous Rn measurements at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (2 probes within a distance of 2 m, both at 60 cm depth,) during May 2006 and one M >4 earthquake occurred during that period. A sudden increase in Rn levels in the IB-0.6 m probe on 31 May 2006 is attributed to rainfall.
14, 1978. In the present study at Nivle, there is no prominent change in the Rn levels observed for three earthquakes (M 4.8, 4.6 and 4.1) on 24 Nov. 2007. 4.2. Warna IB The multi-sensor Rn probe (Warna IB) response during the study period for different earthquakes has differed to the Nivle site probe. The Rn concentrations were relatively stable during the nonrainy period. Out of three probes at this site at different depths, the 60 cm depth probe recorded the most frequent number of high Rn signals. This may be due to a lower moisture content at shallow depth mainly during dry period. We have correlated these Rn spikes/anomalies with respective earthquakes. After 15 days of initiation of Rn monitoring at this site, an earthquake (M 4.0) occurred on 13/11/05 and another earthquake of the same magnitude on 20/11/05. There is no change in the Rn concentration at the two deeper probes (1.7 and 3 m), but the probe at 0.6 m showed a few peaks before the earthquake (Fig. 8). Eleven days before the earthquake (on 13/11/05), the Rn concentration increased to 60
times the normal level for about 15 h and about 1 day before the same earthquake, two Rn spikes were recorded, out of which the first spike is about 2.5 times and the second one is about 20 times of normal value (Fig. 8). However, in response to the 20 Nov. 2005 earthquake, the Rn concentration showed multiple spikes (10e50 times the normal value) about 5 days earlier than the earthquake and continued until one day before the earthquake (Fig. 8). Similarly, Rn spikes (2e10 times higher than the normal value) were observed 15, 13, 8 and 4 days before the Dec. 26 2005 M 4.2 earthquake (Fig. 9). For this event, the Rn concentration increased from 35 to 45 kBq m3 at the 1.7 m depth probe 3 days before the earthquake and continued following it (Fig. 9). Significantly, for the earthquake on April 17, 2006, the three probes at different depths showed high Rn spikes with almost 100% increase at different times: for the 3 m depth probe 10 days before; for the 1.7 m depth probe 16 days before and for the 60 cm probe 5 days before the earthquake (Fig. 6). The 1.7 m depth probe responded to three successive earthquakes of M >4 on 24 Nov. 2007 (Fig. 10) by indicating the increase in Rn from its normal levels (33 kBq m3) 12 days before the
Fig. 8. Continuous Rn measurement at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (60 cm depth) during Nov. 2005 and two earthquakes of M 4 during that period.
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Fig. 9. Continuous Rn measurement at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (2 units, both at 60 cm depth,) during Dec. 2005 and an earthquake M 4.2 during that period.
Fig. 10. Continuous Rn measurement at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (60 cm depth) during Nov. 2007 and 3 earthquakes (M >4) on a single day (24 Nov.).The probe at 1.7 m depth recorded Rn anomaly 12 days before (pre-seismic) the earthquake.
earthquake and reaching very high levels (>100 kBq m3) 7 days before the earthquake, and continuing for several days after the earthquake. Both Nivle and Warna IB sites responded by registering high Rn levels for the earthquake on April 17, 06 (Fig. 6).
heterogeneity, focal depth of earthquake, epicentral distance from the probe, preferential paths in and around the Rn probe provided by the network of fractures, etc.
5. Conclusions
The authors are grateful to Director, NGRI, and Dr. H.K Gupta for their support and encouragement, and the NGRI Koyna Seismology Team for the earthquake data, DST for financing the Project. BSS thanks the CSIR for the Emeritus Scientist Scheme.
During our experimental period (2005e2007), a number of Rn anomalies which are almost one to two magnitude differences have been observed in different probes installed at two stations close to the seismically active Warna area. Many Rn anomalies were observed as minimum less than one day to a maximum 16 days before the earthquakes. A few co-seismic and post-seismic anomalies were also identified. However, it was observed that some seismic events are preferentially picked up by some probes at one time or the other. This could be due to the factors like site
Acknowledgements
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Continuous radon monitoring in soil gas towards earthquake precursory studies in basaltic region D.V. Reddy*, P. Nagabhushanam, B.S. Sukhija, G. Rajender Reddy National Geophysical Research Institute (Council of Scientific & Industrial Research), Uppal Road, Hyderabad-500 606, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2009 Received in revised form 1 May 2010 Accepted 3 May 2010
The Koyna-Warna region, near the west coast of India, is well known for reservoir-triggered seismicity. The seismic activity in this region greatly increased following the construction of an artificial reservoir across the Koyna River during the 1960s. A destructive earthquake of M 6.3 occurred on December 10, 1967, and further 19 earthquakes of M>5 have been recorded during the preceding 40 years until 2007. The soil gas radon (222Rn) has been studied as an earthquake precursor by continuous monitoring (hourly) at two sites around the Warna reservoir. One site has a multi-sensor probe (installed at three different depths), together with a rainfall recording facility, and another probe is mounted on a hillock at Nivle. During the study period (2005e2007), a total of 11 earthquakes (including 2 aftershocks) of M 4e4.8 were recorded. Most of these events had recorded precursory radon signals. For a given earthquake, the 222Rn precursory signatures were recorded at one of the two sites only. Even multiple depth probes showed discordant behaviour in recording temporal Rn variation. Causes of non-concurrence in Rn recording between sites and probes, including the combined effect of site heterogeneity, focal depth, epicentral distance, earthquake magnitude, faults responsible for the earthquake, etc, are discussed. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Radon Earquake precursor Basalts Koyna
1. Introduction Radon (219Rn, 220Rn and 222Rn), an inert radioactive gas that is produced during the decay of uranium and thorium, is present in the earth’s crust. The emanation of Rn is affected more by physical conditions in the environment than by chemical processes (Tanner, 1964; Reddy et al., 1996). Hence, Rn concentration levels in the subsurface are strongly controlled by geological conditions and atmospheric influences such as barometric pressure and rainfall. The migration of Rn from the soil or rock by diffusion is limited because of their short half-lives and it is 222Rn (here after called Rn) with the longest half life (3.82 d) that does migrate and is of particular interest in this study. Cracks and fissures play an important role in enhancing Rn flux (Holford et al., 1993) and a high concentration of Rn is often found in soils overlying highly fractured rocks such as geologic faults and active volcanoes (Banwell and Parizek, 1988; Reddy et al., 2006; Ciotoli et al., 1999; Berlo et al., 2004). In the case of an earthquake, the Rn emanation increases due to the creation of new fractures and fissures associated with the earthquake process. Rn is easily soluble in water and it diffuses into the groundwater and spring waters. The relationship between an * Corresponding author. Tel.: þ91 4023434602; fax: þ91 4023434651. E-mail address: [email protected] (D.V. Reddy). 1350-4487/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.05.010
earthquake and Rn emission was shown by Okabe (1956). Many investigators tried to correlate the Rn concentration in soil gas as well as in groundwater/spring water along with other hydrochemical parameters related to earthquake activity (Ulomov and Mavashev, 1971; Mogro-Campero et al., 1980; Virk and Singh, 1993; Igarashi et al., 1995; King et al., 1995). By continuous Rn monitoring, Chyi et al. (2002) reported a quick rise in Rn levels a couple of weeks before a noticeable earthquake. They have also observed a Rn peak 1e7 days before the occurrence of an earthquake on an active fault zone in Taiwan. Steinitz et al., (2003) monitored Rn concentration in 10 min interval in the Dead Sea rift valley for 8 years and found only a statistical correlation between Rn anomalies and earthquakes of magnitude 4.6 ML0. They showed a Rn correlation for 101 earthquakes and concluded that the earthquake occurs 3 days after a Rn anomaly. From continuous Rn measurements for five months at a depth of 1 m in fractured granitic area, Reddy et al., (2004) reported an enhanced Rn concentration that preceded the occurrence of a micro earthquake by 17 h. An enhanced Rn level (twice the normal concentration) was recorded 13 h before the occurrence of the event, and it decayed exponentially during the next 5 days to reach a normal level. However, negative anomalies were also found, by Planinic et al., (2000) during their year-long continuous monitoring of Rn together with pressure and temperature at Osijek, Croatia. Based on the long-term Rn measurements at the SKE site in IzuPeninsula,
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Wakita (1996) concluded that the appearance of Rn signals due to earthquake activity depends on the state of stress accumulation in the region associated with opening and closing of micro fractures in the surrounding rocks. In this paper, we present the results pertaining to continuous Rn monitoring in soil gas using a multi-sensor Rn detector system at one location, and two single detector units at another site in the seismically active Warna region, India. The purpose of the experiment is to examine the precursory relationship between Rn signals and seismicity (M 4) in a seismically active basaltic region. 2. Study area and earthquake history The study area (Fig. 1) is covered by the Koyna and Warna reservoirs constructed across the two respective rivers at a distance of 33 km in the NS direction in Satara district, Maharashtra, India. The area forms part of Deccan Volcanic Province with extensive basaltic lava flows spread over a large area. Differential weathering is responsible for the formation of highly undulating topography associated with a series of hill ranges, ridges and valleys. The major lineaments in the area are trending NNE, NS, NE and less commonly east (Fig. 1). Extensive sheet joints (horizontal to sub-horizontal) cover the top of the individual lava flows. The Koyna-Warna region is one of the most significant sites of reservoir-triggered seismicity in the world (Gupta, 1992). Seismicity in this region started soon after the impoundment of Shivaji Sagar lake created by the Koyna Dam in 1962 (Gupta et al., 1969). Since the largest earthquake of magnitude 6.3 occurred near Koyna Dam on December 10, 1967 (17 39.00 N, 73 55.80 E), the seismicity is being continuously recorded by the National Geophysical Research Institute (NGRI). A total of 19 earthquakes of M >5 and more than 170 earthquakes of
M 4 occurred in the area, besides innumerable micro earthquakes until 2007 (Gupta et al., 2007). This study was initiated just after an earthquake of M 5.1 occurred on March 14, 2005 and continued to 2007, during which 11 earthquakes of M 4e4.8 occurred in the region. Continuous pore pressure changes are being monitored in this area by Chadha et al., (1997) in deep borewells drilled specially for precursor studies; and they observed significant water level changes for some of the earthquakes (Ramana et al., 2007; Chadha et al., 2008). To study the relationship between soil gas Rn and seismicity in the Koyna-Warna region, initially two sites Warna Inspection Bungalow (Warna IB) and Nivle in the Warna region (where present seismic activity is concentrated), were selected and Rn probes (Algade, France) installed (Sukhija et al., 2007). Warna IB site is located northeast of the Warna river valley (a partially plain area) and the Nivle site is over a hillock in a Reserved Forest, south of the Warna reservoir. Both the sites are 7.8 km apart in aerial view (Fig. 1). At Warna IB, a multi-sensor Rn unit consisting of one main unit and two Rn measuring heads were installed at three different depths i.e., 3.0, 1.7 and 0.6 m (Fig. 2) and the data collected from May 2005 to Dec. 2007. The main unit contains a data logger to record the data from the main unit and also from the two heads at a set time interval. The recording interval was set to 1 h. This unit also has a rain gauge attachment to measure the rainfall at the same site. The temperature and pressure are also monitored by the main probe which is at a depth of 3 m. Since the Rn concentration in the soil is affected by environmental factors (temperature, pressure and rainfall), it is vital to have a continuous measurement of these parameters. To reduce the direct effect of rainfall on the units, a small shed was erected to cover the units and an on-line data
Fig. 1. The study area of the Koyna-Warna region in Maharashtra, India. Earthquake epicenters of M 4 during Aug. 2005 to Dec. 2007, M 5.1 earthquake recorded on 14th Mar. 2005, a cluster of earthquakes M 4.2, 4.6 and 4.8 recorded on a single day (24th Nov. 2007,) in the hills, and prominent lineaments (Agrawal et al., 2004) are shown.
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transfer facility was arranged through the telephone line. These units are calibrated by the company every 2e3 years. However, primarily, we are looking for relative variation in Rn concentration with time rather than absolute values. At Nivle, a single Rn detector that measures Rn, temperature and pressure was installed at 60 cm depth and monitored continuously between May 2005 and Dec. 2007. Another single unit was also installed within 2 m distance of Unit I to cross check the Rn data. The second unit was operated for a shorter time, between Dec. 2005 and May 2006 (a dry period). 3. Results 3.1. Temporal variation in Rn concentration The hourly Rn recordings from five probes is expected to identify Rn anomalies pertaining to earthquakes. An “Rn anomaly” is considered if the Rn signal is one and half to two times more than the normal values recorded for a few days to few weeks prior to a Rn signal change. As the effect of atmospheric pressure and temperature on Rn concentration is only about 20% (Reddy et al., 1996; Reddy et al., 2004), it is not discussed here. However, we have considered the Rn changes due to rainfall in the present study.
Fig. 2. Experimental set-up for continuous recording of soil gas Rn measuring probes at three depths at Warna IB. This set-up also has the facility to measure the rainfall, and on-line data transmission.
3.1.1. Warna IB This site is covered with more than 3 m of lateritic soil. As expected, the Rn concentration in three probes installed at 3 different depths varied from 20 to 50 kBq m3. The lowest Rn concentration was recorded by the probe at 60 cm depth while the highest concentrations were at the 3 m depth probe. During the non-rainy period of 6 months (Nov. 05eApril 06) the Rn concentration at 60 cm depth varied between 20 and 30 kBq m3 (Fig. 3). For the same period, the 3 m depth probe recorded Rn concentration in the range 40e50 kBq m3. The probe at 1.7 m depth, recorded Rn concentrations between 30 and 40 kBq m3 for two months (Nov.eDec. 05) and later for 20 d the Rn concentration increased to 40e50 kBq m3. After 20 d of increase, the Rn concentration returned to normal level (30e40 kBq m3). Further, the Rn concentration at 1.7 m showed progressively increasing trend from Feb. 2006 and crossed the Rn level of the 3 m depth probe (50 kBq m3) before the rainy season (Jun.eAug.). Though the Rn concentration (20e30 kBq m3) at 60 cm depth remained stable during dry period, the concentration increased significantly from
Fig. 3. Continuous Rn measurement in the Warna IB and Nivle sites during the non-rainy period.
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Fig. 4. Influence of rainfall on Rn migration from soils during the rainy season.
28 Dec. 2005 to 22 Jan. 2006 (>100 kBq m3, Fig. 3). During the rainy season, mainly from June to Aug. 2006, the rainfall influenced the Rn concentrations (Fig. 4). Due to the initial rains, when the top soil becomes saturated, the Rn concentration builds up below the soil zone, as there is no scope to escape to the atmosphere. Later, when the soil moisture levels increase around the probe, the Rn concentration decreases. For this reason, the Rn measurement in the soil gas during rainy season may not be reliable for the precursory evaluation.
Warna IB 60 cm depth probe. There is a great change in the Rn concentration at Nivle site (few hundreds to about 35 kBq m3). Nevertheless, the changes are quite systematic (Fig. 3), and are not related to weather parameters. The second Rn unit installed at Nivle, 2 m apart, did not show any comparable values, though both the units are from same company and have similar background Rn values. Non-concurrence of the Rn data between the two probes is attributed to subsurface heterogeneity. 4. Discussion
3.1.2. Nivle site The site is situated over a hill range of 900 m above mean sea level and adjacent to a deep valley (one branch of the Warna reservoir). The site is covered with a thin soil layer and has several outrops/boulders around it. The response of the Rn unit at Nivle site (installed at 60 cm depth), differs significantly from that of the
During the continuous Rn monitoring period (Aug. 2005eDec. 2007), the study area experienced 11 earthquakes (including 2 aftershocks) with magnitude ranging from 4 to 4.8. The Rn probes emplaced at two sites (Warna IB and Nivle) have registered a few Rn high anomalies independently but not in unison. The observed
Table 1 Rn response in different probes with respect to different M >4 earthquakes. S. no
Date
Earthquake M
Focal depth (km)
Rn-Site
Dist*-(km)
Probe Depth (m)
Rn Response
1 2 3
14/08/05 30/08/05 13/11/05
4.2 4.8 4.0
5 6.8 4.8
Nivle Nivle Warna-IB
4.5 4.0 10.4
0.60 0.60 0.60
4
20/11/05
4.0
5.9
Nivle Warna-IB
2.7 15.6
0.60 0.60
5
26/12/05
4.2
5.8
Warna-IB
10.3
0.60
6
17/04/06
4.7
3.9
Warna-IB
10.3
No prominent signal due to rainfall Rn high peak 6 days before and co-seismic peak up to 100 kBq m3 3 spikes were recorded against the normal level 30 kBq m3 1 spike,11 days before Rn increased by 60 times 2 and 3.1 day before Rn increased to 76 and 540 kBq m3 No prominent peaks Multiple spikes started 5 days before (45 kBq m3 to > 100 kBq/m3 with 20 kBq m3 background) Spike 1 (500 kBq m3),15 days before Spike 2 (35 kBq m3), 13 days before Spike 3 (>70 kBq m3), 8 days before Spike 4 (e35 kBq m3), 4 days before Step increase (30e40 kBq m3), 3 days before and continued 10 days before a Rn spike increased from 50 to 80 kBq m3 16 days before Rn spike increased from 45 to 75 kBq m3 5 days before Rn spike increased from 25 to 50 kBq m3 20e25% change in Rn conc. as pre-, co- and post-seismic from its normal levels 25 to 30 kBq m3 Rn decreased for 6 days until one day before the earthquake Rainfall effect in all the probes Rn started increase 12 days before
Nivle 7 8 9 10 11
22/05/06 20/08/07 24/11/07
4.2 4.3 4.8 4.6 4.2
4.7 3.6 2.7 4.6 2.1
Nivle Nivle Warna-IB
4.0 10.0 19.0
Dist* distance between the earthquake epicenter and the Rn measurment site.
1.7 3.0 1.7 0.60 0.60 0.60 0.60 1.7
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Fig. 5. Continuous Rn measurement at Nivle (60 cm depth) during August 2005 and two M >4 earthquakes during that period.
anomalies are mainly in the form of short-lived spikes. Sometimes, the Rn levels are continuously low (less than normal level) and vice versa. The seismic activity was quite high during 2005 and the first half of 2006. Starting from the second half of 2006 to Dec. 2007 the seismicity has been at low ebb. Here, we tried to correlate the observed Rn anomalies with the earthquakes of M >4 that occurred within the radius of 20 km from the Rn measuring sites, and which are shallow in origin (<10 km). The prominent anomalies observed at Nivle and Warna IB sites are summarized below and shown in Table 1. 4.1. Nivle site Two earthquakes of M >4 were recorded, one on 14th August 2005 (M 4.2) and the other on 31st August 2005 (M 4.8), at a distance of about 4 km north of the Rn measurement site with a focal depth of about 5 km. In the first week of August 2005, the Rn levels remained quite low (few hundred Bq m3) and a spike of
10 kBq m3 occurred on 8th August about 5 days before the M 4.2 earthquake (Fig. 5). During the next 10 days, the Rn levels show high fluctuation and after 24th August the levels increased to 30 kBq m3. A Rn spike of 100 kBq m3 was observed on August 28, 3 days before the M 4.8 earthquake. The Rn anomaly for the M 4.8 earthquake is 7e10 times more than that of corresponding M 4.2 earthquake on August 14 (Fig. 5). Though there were two earthquakes during November and one in December 2005, all of magnitude >4, Rn changes were not detected at this site. However, the 17th April 2006, M 4.7 event generated pre-, co- and post-seismic Rn changes in one of the two units, of 20e25% greater than the normal values (20e30 kBq m3, Fig. 6). The M 4.2 earthquake on May 22, 2006, indicated decreased Rn concentration for 7 days before its occurrence (Fig. 7), but the Rn levels reached to normal levels one day before the earthquake. A drop in Rn levels was also observed by Wakita (1996) more than 3 months before the M 7.0 Izu-Oshima-Kinkar earthquake on January
Fig. 6. Continuous Rn measurements at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (60 cm depth) during April 2006 and an earthquake of M 4.7 that occurred on April, 17.
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Fig. 7. Continuous Rn measurements at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (2 probes within a distance of 2 m, both at 60 cm depth,) during May 2006 and one M >4 earthquake occurred during that period. A sudden increase in Rn levels in the IB-0.6 m probe on 31 May 2006 is attributed to rainfall.
14, 1978. In the present study at Nivle, there is no prominent change in the Rn levels observed for three earthquakes (M 4.8, 4.6 and 4.1) on 24 Nov. 2007. 4.2. Warna IB The multi-sensor Rn probe (Warna IB) response during the study period for different earthquakes has differed to the Nivle site probe. The Rn concentrations were relatively stable during the nonrainy period. Out of three probes at this site at different depths, the 60 cm depth probe recorded the most frequent number of high Rn signals. This may be due to a lower moisture content at shallow depth mainly during dry period. We have correlated these Rn spikes/anomalies with respective earthquakes. After 15 days of initiation of Rn monitoring at this site, an earthquake (M 4.0) occurred on 13/11/05 and another earthquake of the same magnitude on 20/11/05. There is no change in the Rn concentration at the two deeper probes (1.7 and 3 m), but the probe at 0.6 m showed a few peaks before the earthquake (Fig. 8). Eleven days before the earthquake (on 13/11/05), the Rn concentration increased to 60
times the normal level for about 15 h and about 1 day before the same earthquake, two Rn spikes were recorded, out of which the first spike is about 2.5 times and the second one is about 20 times of normal value (Fig. 8). However, in response to the 20 Nov. 2005 earthquake, the Rn concentration showed multiple spikes (10e50 times the normal value) about 5 days earlier than the earthquake and continued until one day before the earthquake (Fig. 8). Similarly, Rn spikes (2e10 times higher than the normal value) were observed 15, 13, 8 and 4 days before the Dec. 26 2005 M 4.2 earthquake (Fig. 9). For this event, the Rn concentration increased from 35 to 45 kBq m3 at the 1.7 m depth probe 3 days before the earthquake and continued following it (Fig. 9). Significantly, for the earthquake on April 17, 2006, the three probes at different depths showed high Rn spikes with almost 100% increase at different times: for the 3 m depth probe 10 days before; for the 1.7 m depth probe 16 days before and for the 60 cm probe 5 days before the earthquake (Fig. 6). The 1.7 m depth probe responded to three successive earthquakes of M >4 on 24 Nov. 2007 (Fig. 10) by indicating the increase in Rn from its normal levels (33 kBq m3) 12 days before the
Fig. 8. Continuous Rn measurement at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (60 cm depth) during Nov. 2005 and two earthquakes of M 4 during that period.
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Fig. 9. Continuous Rn measurement at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (2 units, both at 60 cm depth,) during Dec. 2005 and an earthquake M 4.2 during that period.
Fig. 10. Continuous Rn measurement at Warna IB (at 3 depths: 3 m, 1.7 m and 0.6 m) and Nivle (60 cm depth) during Nov. 2007 and 3 earthquakes (M >4) on a single day (24 Nov.).The probe at 1.7 m depth recorded Rn anomaly 12 days before (pre-seismic) the earthquake.
earthquake and reaching very high levels (>100 kBq m3) 7 days before the earthquake, and continuing for several days after the earthquake. Both Nivle and Warna IB sites responded by registering high Rn levels for the earthquake on April 17, 06 (Fig. 6).
heterogeneity, focal depth of earthquake, epicentral distance from the probe, preferential paths in and around the Rn probe provided by the network of fractures, etc.
5. Conclusions
The authors are grateful to Director, NGRI, and Dr. H.K Gupta for their support and encouragement, and the NGRI Koyna Seismology Team for the earthquake data, DST for financing the Project. BSS thanks the CSIR for the Emeritus Scientist Scheme.
During our experimental period (2005e2007), a number of Rn anomalies which are almost one to two magnitude differences have been observed in different probes installed at two stations close to the seismically active Warna area. Many Rn anomalies were observed as minimum less than one day to a maximum 16 days before the earthquakes. A few co-seismic and post-seismic anomalies were also identified. However, it was observed that some seismic events are preferentially picked up by some probes at one time or the other. This could be due to the factors like site
Acknowledgements
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