Reservoir induced seismicity in the vicinity of Lake Bhatsa, Maharashtra, India

Physics of the Earth and Planetary Interiors, 44 (1986) 73-81

73

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Reservoir induced seismicity in the vicinity of Lake Bhatsa, Maharashtra, India D.N. Patil, V.N. Bhosale, S.K. Guha and K.B. Powar Department of Geology, Universityof Poona, Pune 411 007 (India) (Received June 4, 1985; revision accepted December 23, 1985)

Patil, D . N , Bhosale, V.N., Guha, S.K. and Powar, K.B., 1986. Reservoir induced seismicity in the vicinity of Lake Bhatsa, Maharashtra, India. Phys. Earth Planet. Inter., 44: 73-81. The township of Khardi, located 7 km northwest of the Bhatsa dam, in the western part of the Deccan Volcanic Province, was subjected to earthquakes of magnitudes 4.0 and 4.8 on August 17, 1983 and September 15, 1983, respectively. The Khardi-Bhatsa area is located to the east of the Panvel flexure axis and on a northwest-southeast trending belt, forming an extension of the Ghod lineament of the Deccan Plateau. The earth tremors at Khardi followed impoundment of water, during June-July 1983, behind the recently constructed Bhatsa dam, and the rise in lake-level from about 92 to 110 m above MSL. It is suggested that percolation of water during this period along fractures led to a build-up of pore-fluid pressure which was transmitted down to a stress zone with a potential to slip. Consequent reduction in effective stress (tectonic stress-pore fluid pressure) induced the earthquakes. Available seismic data for the Khardi-Bhatsa area suggest that b-values can also be used as a 'short term' precursor for detecting impending, strong reservoir-induced earth tremors in the area.

1. Introduction

2. Tectonic setting

The township of Khardi (19°35'1Y'N; 73o23 ' 02"E), located about 90 km northeast of Bombay, India, experienced earthquakes of magnitudes 4.0 and 4.8 on August 17, 1983 and September 15, 1983, respectively. These followed the first record of tremors in mid-May 1983 and initiated speculations regarding the possible relationship of tremors to the impoundment of water in the Bhatsa reservoir located about 7 km to the southeast of Khardi. Extensive field studies and analyses of satellite imageries, seismic data and lake-level data were undertaken to examine this possibility. The results, which tend to confirm the reservoir in. duced nature of seismicity, are presented in this paper.

The Khardi-Bhatsa area is located in the narrow coastal belt lying between the Arabian Sea and the Western Ghats scarp. In this coastal belt a thin cover of soil and alluvium overlies volcanics, dominantly basalts, of the Deccan Volcanic Province. These basalts are, at places, associated with acidic and intermediate volcanics (Mathur and Naidu, 1"932; Sukheswala and Poldervaart, 1958; Sukheswala and Sethna, 1962) and are intruded by a variety of hypabyssal rocks, the most dominant being dolerite dykes (Auden, 1949] Powar, 1981). Deep seismic soundings indicate that in this coastal belt the thickness of the basalts is between 1.1 and 1.5 km (Kaila et al., 1980, 1981). The best known

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75

the north to the Savitri river in the south (Powar et al., 1978). West of this axis, which lies about 50 km west of Khardi, the flows dip westward at angles ranging from 58 ° at Virar to about 3° at the Savitri river (Powar and PatH, 1982). Corresponding to this axis is a line of hot springs suggesting that the 'axis' is the surface expression of a deep fault. In the offshore region of the Arabian Sea, graben and horsts have been recognised. These include, from east to west, the West Indian depression, the Laccadivian system of uplifts, the West Laccadivian depression and the buried Fedynsky ridge (Babenko et al., 1981). It is, therefore, probable that the coastal belt lies on a cymatogenic arch. A study of satellite imageries suggests that the Ghod lineament (Powar and Patil, 1980), which delineates the Kurduwadi rift (Brahmam and Negi, 1973) of the Deccan Plateau, extends across the coastal belt and, as described later, is represented by a northwest-southeast trending belt of lineaments in the Khardi-Bhatsa area. The coastal belt under review is mildly to moderately seismic and largely falls within Seismic Zone 2 in the Seismic Zoning Map of India (Guha et al., 1980). A number of earthquakes have been recorded in this area during the past 300 years and TABLE I Earthquakes of magnitude 4.0 or greater recorded in the coastal belt of Maharashtra, India Serial

Date

no.

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Day

Month

Year

area

1 2 3 4 5 6 7 8 9

? 09 05 29 20 22 04 25 16

? 12 01 05 03 08 10 12 02

1678 1751 1752 1792 1826 1828 1832 1856 1929

10 11 12 13

28 ? 17 15

05 09 08 09

1941 1962 1983 1983

Bassein Salsette Salsette Janjira Konkan Venguda Umate Dahanu South of Bombay Srivardhan Ratnagiri Khardi Khardi

Magnitude

5.0 4.3 4.3 4.3 4.3 4.5 5.0 5.7 4.2 4.3 4.3 4.0 4.8

Data from Bapat et al. (1983) updated from other sources, 1-11 on Richter magnitude scale. 12-13 on Richter local magnitude scale.

their epicentres are shown in Fig. 1. The earthquakes of magnitude 4.0 or greater, recorded during this period, are enumerated in Table I.

3. Structural framework of the Khardi-Bhatsa area

A study of LANDSAT-1 imageries on scale 1 : 250 000 was undertaken to delineate the structural fabric of the area around Khardi-Bhatsa. As shown in Fig. 2, the area is characterised by major lineaments trending northwest-southeast, and to a lesser extent approximately northeast-southwest, north-south and east-west. Significantly, most of the reservoirs are located on a northwest-southeast trending belt of closely spaced lineaments, margined by the Kalu-Surya lineament to the southwest and the Kengri lineament to the northeast. Field studies show that the lineaments in the Khardi-Bhatsa area are represented either by dykes or by shear zones; some of which have previously been recorded by Auden (1949). The dykes vary in thickness from 1.5 to 100 m and have evidently been emplaced along lines of tension. Axial depressions along the lengths of some of these dykes suggest possible re-opening due to tension. It is worth recording that the township of Khardi is located at the intersection of three lineaments; two of which represent dykes, and the third a northwest-southeast trending shear zone extending across the Bhatsa reservoir. The Kengri lineament constitutes the boundary between two contrasting geomorphological regimes. The area to the northeast of this lineament shows youthful topography characterised by deep gorges, narrow V-shaped valleys, valley-in-valley profiles, waterfalls and rapids. On the other hand, the area southwest of the lineament, exhibits a mature topography, marked by broad U- to Vshaped valleys and rolling water divides. This suggests (Patil et al., 1984) that the Kengri lineament could be a manifestation of a deep seated dislocation separating two structural blocks; the one lying to the southwest of it is undergoing subsidence with respect to the block to the northeast. Interestingly, the Bhatsa reservoir is located on the subsiding block like the ones at Kariba, Kremasta and Koyna (Gupta et al., 1973). o

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4. The Khardi earthquakes The township of Khardi is located about 90 km northeast of Bombay, in the Shahapur taluka of Thane district, Maharashtra. During the summer of 1983, the residents of this township reported the occurrence of feeble earth tremors, which were followed by earthquakes of magnitude 4.0 and 4.8 on August 17, 1983 and September 15, 1983, respectively. The damage to property due to these two earthquakes was slight and the effects were

largely observed after the second earthquake of September 15, 1983. The damage was mainly in the form of vertical to highly inclined cracks developed along door- and window-frames, and along wall intersections in houses. In a few cases plaster was dislodged from walls. Anticlockwise rotation of bricks through 10 ° to 15 ° was also recorded. Structured interviews, conducted after the second earthquake, and isoseismal maps (Figs. 3 and 4) constructed therefrom, indicate that both the earthquakes had their epicentres around Khardi

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with the affected area extending up to distances of about 50 and 130 krn in respective cases. At Khardi the maximum intensity (I0) of the earthquake of August 17, 1983, was V on the Modified Mercalli scale whereas that of the earthquake of September 15, 1983, was VI on the same scale. The isoseismal pattern is seen to be ellipsoidal (Figs. 3 and 4) with the elongation parallel to t h e northwest-southeast trending lineament belt (Fig. 2). Initial reaction related the earth tremors to impoundment of water behind the Bhatsa dam, which was under construction on the Bhatsai river, about 7 km SE of Khardi. It is a concrete dam,

938 m long, which at the end of the first phase of construction in 1983 had reached a height of 60 m above the river bed, i.e., 118 m above MSL. Impoundment of water behind this dam started in stages in June, 1977. The water-level which was at about 92.5 m above MSL (43.5 m above river bed-level) during July, 1983, rapidly increased to a little more than 110 m 1 month later. The two main shocks were felt in the subsequent months. The relationship between lake-level, rate of change of lake-level (dL/dT) and, earthquake frequency and magnitudes at Khardi-Bhatsa, during the period July to September 1983, is shown in Fig. 5.

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5. Discussion

In recent years there has been growing awareness of the fact that rapid impoundment of water behind high dams, in moderately seismic areas, can induce earth tremors. Such a connection between artificial reservoirs and earthquakes was first recorded in Greece, where water impoundment behind the Marathon dam was initiated in 1929, reached its highest level in 1931, and in-

duced earth tremors. The strongest seismicity during the period 1931-1966 was always associated with rapid rise in water-level (Coates, 1981, p. 299). Since then over 70 cases of reservoir induced seismicity have been recorded (Guha, 1982) including those at Hsinfengchiang, China, in 1962 (Ding et al., 1982); Kariba, Zambia-Zimbabwe border, in 1963 (Gough and Gough, 1970); Kremasta, Greece, in 1966 (Comninakis et al., 1968) and Koyna, India, in 1967 (Guha et al.,

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1970; G u p t a et al., 1972), which were of magnitude greater than 6.0 on the Richter scale. There is a more than causal relationship between seismicity and height of water column. Coates (1981) recorded that only 0.63% of the world's largest 11 000 dams, of height greater than 10 m, have induced seismicity. However, 10% of reservoirs greater than 90 m in depth have related seismic events whereas 21% of reservoirs deeper than 140 m have significant earthquakes associated with them. Snow (1972) pointed out that in the case of narrow valleys, where normal faulting or strike-slip faulting environments prevail, the filling up of a reservoir causes a decrease in the m a x i m u m shearing stress (ol) and minimum shearing stress (o3), shifting the Mohr Circle towards the failure envelope, and at critical values causing failure. In the case of the Bhatsa there is a clear relationship between impoundment of water and earth

tremors. It is seen from Fig. 5 that there was an increase of lake-level of the order of 14 m within 1 month corresponding to a m a x i m u m dL/dT of 6.5 m per week. The largest earthquake of September 15, 1983, ( M = 4.8), took place after a period of 7 weeks after the peak in dL/dT was attained. The reservoir itself, as indicated earlier, is located on a sinking block. We, therefore, believe that the rapid impoundment of water in a structurally unstable zone, where shearing has been responsible for high permeability, is one of the major factors causing the Khardi earthquakes. It is possible that percolation of water along permeable zones led to build-up of pore-fluid pressure which was then transmitted down to a stress zone with a potential to slip. Consequent reduction in the effective stress (tectonic stress-pore fluid pressure) induced the earthquakes. The time required for build-up of pore-fluid pressure and its transmission to the stress zone accounts for the time lag between the water load and the earthquake. In view of the possibility of earthquake recurfence in the K h a r d i - B h a t s a area it is necessary to establish some premonitory index for predicting impending seismic events. A premonitory index, that has been found to be effective for earthquake

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Fig. 6. Plots of b-values against time for Khardi-Bhatsa area.

80 prediction, is b-values in the G u t e n b e r g - R i c h t e r e q u a t i o n log N = a - b M (Padale et al., 1979; G u h a , 1982). It has b e e n f o u n d that, o n micro- to global scales, lower b-values associated with higher stress, precede earthquakes (Guha, 1983). I n Fig. 6 b-values have b e e n plotted against time for the K h a r d i - B h a t s a region. These b-values were comp u t e d b y a least square m e t h o d for g r o u p s of 100 consecutive shocks recorded o n a S p r e n g n e t h e r M E Q 800 micro-seismograph installed n e a r the dam. It is seen that there was a significant fall in b-values before b o t h the A u g u s t 17, 1983 a n d September 15, 1983 earthquakes. The fall started o n l y a b o u t 1 week prior to the earthquakes. This suggests, that in the case of the Bhatsa reservoir, b-values could be used as a n effective ' s h o r t term' precursor for predicting i m p e n d i n g , strong reservoir-induced tremors in the K h a r d i - B h a t s a area.

Acknowledgements The authors t h a n k f u l l y acknowledge the receipt of financial assistance for field studies from the School of E n v i r o n m e n t a l Sciences, U n i v e r s i t y of Poona, P u n e 411 007.

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