A new case of reservoir triggered seismicity: Govind Ballav Pant reservoir (Rihand dam), central India

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1 Tectonophysics 439 (2007) A new case of reservoir triggered seismicity: Govind Ballav Pant reservoir (Rihand dam), central India Kalpna Gahalaut a, V.K. Gahalaut a,, M.R. Pandey b a National Geophysical Research Institute, Uppal Road, Hyderabad , India b Department of Mines and Geology, Lainchur, Kathmandu, Nepal Received 21 November 2006; received in revised form 27 March 2007; accepted 9 April 2007 Available online 14 April 2007 Abstract We report here that seismicity near Govind Ballav Pant reservoir is strongly influenced by the reservoir operations. It is the second largest reservoir in India, which is built on Rihand river in the failed rift region of central India. Most of the earthquakes occurred during the high water stand in the reservoir with a time lag of about 1 month. We use the concept of coulomb stress change and use Green's function based approach to estimate stresses and pore pressure due to the reservoir load. We find that the reservoir increases coulomb stress on the nearby faults of the region that are favourably oriented for failure in predominantly reverse slip manner under the NNE SSW compression and thus promotes failure. The above two factors make it an obvious, yet so far unreported case of reservoir triggered seismicity Elsevier B.V. All rights reserved. Keywords: Reserior triggered seismicity; Coulomb stress; Rihand reservoir; Peninsular India 1. Introduction Increases in the frequency of occurrence of earthquakes due to man's engineering activities have resulted from the reservoir impoundment, quarrying and mining, and fluid injection and extraction (McGarr and Simpson, 1997). However, earthquakes caused by reservoir impoundment are stronger than by any other engineering activities. So far globally about hundred sites of Reservoir Triggered Seismicity (RTS) have been reported which include at least eight sites from India, namely, Koyna, Warna, Bhatsa, Dhamni, Gandipet, Idukki, Mula and Sriramsagar (Gupta, 2002). According to the mechanism of RTS, reservoir load and/or induced pore pressure due to reservoir operations is Corresponding author. address: vkgahalaut@yahoo.com (V.K. Gahalaut). the cause of triggering of earthquakes on critically stressed pre-existing faults in the vicinity of the reservoirs (Simpson, 1986; Gupta, 1992; McGarr and Simpson, 1997). In this article, we report a new case of RTS associated with the Govind Ballav Pant reservoir, central India (Fig. 1). The reservoir is located on the Rihand river, a tributary of Son river, India. The 92 m high Rihand dam was built in 1962 and the reservoir is the second largest reservoir in India occupying an area of about km 2 having a maximum storage capacity of 10.6 km 3,anda capacity of electric power generation of 300 MW. In this article, we analyse the correlation between the times of high water levels in the reservoir and occurrence of the maximum number of earthquakes, and simulate the effect of the reservoir on the nearby earthquake causative faults to verify that these earthquakes were triggered by the reservoir /$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.tecto

2 172 K. Gahalaut et al. / Tectonophysics 439 (2007) Tectonics and earthquakes of the region The reservoir is located in a failed rift zone, which is known as the Narmada Son Tapti failed rift zone. The ENE WSW trending failed rift zone transects the Indian peninsular shield area into northern and southern blocks (Fig. 1). It is seismically the most active region of the peninsular India, which evolved during the Archean and Proterozoic period (Mahadevan and Subbarao, 1999). Many prominent faults have been mapped in the region but the faults bounding the failed rift zone to the south are suggested to be active and have witnessed reactivation till quaternary period. Most of the earthquake activity is also suggested to be associated with the southern faults (Rao et al., 2002). The reservoir lies close to and south of Son Narmada South fault. Numerous NW SE and NE SW trending small neotectonic faults have also been mapped in the vicinity of the reservoir (GSI, 2000), however, sense of motion on these faults is not known (Fig. 1). In past 100 years only one major earthquake, namely the June 2, 1927, Son Valley earthquake (M 6 1/2; Gutenberg and Richter, 1954, p.207), has occurred within 100 km of the reservoir (Chandra, 1977). Three estimates of epicentre are available (Fig. 1) and suggest variation in the estimate of up to 100 km. The earthquake is assumed to have occurred in the lower crust of failed rift regions of Narmada Son and Tapti, similar to the March 14, 1938 Satpura (M 6 1/4 Gutenberg and Richter, 1954) and May 21, 1997 Jabalpur earthquakes (Singh et al., 1999; Rao et al., 2002; Gahalaut et al., 2004). The rupture of the recent 1997 Jabalpur earthquake (Mw 5.8), which occurred about 300 km WSW of the reservoir, is suggested to lie in the downdip parts of the Son Narmada South fault. Focal mechanisms of this (Singh et al., 1999) and 1970 Broach Fig. 1. Broad tectonic features of the part of the failed rift region of Narmada Son and Tapti over a smooth topographic map. Three estimates of epicentre of 1927 Son Valley earthquake (Ms 6.5), by Gutenberg and Richter (1954), ISC and USGS, are also shown. Stars denote earthquakes reported in the ISC and IMD catalogues during (M 3) while small circles denote epicentres by DMG during Jan Dec (M 3). Bold arrows show the direction of maximum compression due to plate movement (Gowd et al., 1992). Dark brown colour faults, marked as F 1, F 2 and F 3 and the Son Narmada South fault are the neotectonic faults (GSI, 2000). The 1970 Broach and 1997 Jabalpur earthquakes are shown in the inset. Inset also shows the Indian and Nepalese national seismological networks.

3 K. Gahalaut et al. / Tectonophysics 439 (2007) earthquake, Mw 5.4, (Chung, 1993), which occurred at the western extreme of the failed rift zone (Fig. 1), suggest predominance of the compressive regime. Since 1984, about sixty small magnitude (M 3) earthquakes have been reported in the vicinity of the reservoir by the national network (Fig. 1) of India Meteorological Department (IMD) and International Seismological Centre (ISC). Prior to 1984, there is no documentary evidence of earthquake occurrence in the region. However, it does not imply that such earthquakes did not occur prior to 1984 and after the impoundment of the reservoir in Probably they were missed due to absence of nearby seismic stations. Incidentally, the year 1984 approximately coincides with the period of worldwide strengthening of seismological network. However, from earthquake catalogues, it appears certain that no strong or even moderate magnitude earthquake has occurred in the region since the reservoir impoundment. Department of Mines and Geology, Kathmandu, Nepal (DMG), which operates a 17 station network across Nepal (Fig. 1), about 400 km north of the reservoir, has reported numerous small magnitude (M 3) earthquakes from the study region. These earthquakes cluster close to the reservoir (Fig. 1). The data from January 1997 to December 1999 only are available to us. Epicentres of these earthquakes as reported by IMD, ISC and DMG, coincide within km in a tight cluster around the reservoir, though their focal depths are unreliable. We agree that the location of these earthquakes contains error of about km, but it does not change our results and interpretation in any significant way. Occurrence of earthquakes near the reservoir makes it imperative to test the hypothesis of RTS. 3. Temporal variation of seismicity Temporal variation of the earthquakes reported by DMG that occurred within 30 km from the reservoir, and the maximum water levels in the reservoir are shown in Fig. 2. Frequency of earthquake occurrence is the lowest during June July in each year and then it increases. The water level in the reservoir is the minimum during late May and then it increases with the onset of monsoon each year. We discretized the two time series at equal interval of 1 month and calculated the cross-correlation. The cross correlation coefficient between the two time series is 0.71 at an average lag of about a month between reservoir water level and frequency of earthquake occurrence. The correlation decreases to 0.67 if we consider earthquakes within 50 km from the centre of Fig. 2. Monthly frequency of earthquakes reported by DMG (M 3) that occurred within 30 km from the reservoir, the reservoir water levels and the Crosscorrelation function between the two time series. The correlation is the maximum (0.71) at a time lag of about 1 month. Graph with dashed line shows the temporal variation of change in stress in wet state, ΔS w, at a point located at a depth of 8 km under the reservoir and the prominent earthquake cluster.

4 174 K. Gahalaut et al. / Tectonophysics 439 (2007) Fig. 3. Average monthly frequency of earthquakes (M 3) in ISC and IMD catalogues during the period along with the average reservoir water levels. the reservoir. The diffused earthquake cluster north of the reservoir may not be correlated with the reservoir water level variations as the correlation decreases to 0.60 once we include it. The only limitation here is that the catalogue is of shorter duration. The IMD and ISC catalogues cannot be used for such quantitative correlation exercise, as there are not many events in these catalogues and their completeness may also be doubtful. However, we qualitatively analysed the temporal variation in the occurrence of these earthquakes reported by ISC and IMD since As reported earlier, in the period from , 59 earthquakes of MN3 occurred in the region. Maximum number of earthquakes occurred in the months of December and January, followed by occurrences in the months of February, March and April (Fig. 3). Earthquake occurrences are the least in remaining months of the year. Even this correlation, though qualitative, indicates that majority of the earthquakes occurred in the periods after high water level was attained in the reservoir. Thus the good correlation between the water level and earthquakes (Figs. 2 and 3) suggests that the frequency of earthquake occurrence is influenced by the annual changes in the reservoir water levels. 4. Effect of reservoir operation on the earthquake causative faults We consider stress changes due to reservoir water load and the pore pressure. Since most of the earthquakes appear to have occurred during the periods of high water level in the reservoir, first we considered the effect of reservoir water load alone in earthquake triggering. In the subsequent analysis we included the effect of pore pressure also Effect of reservoir water load Following Chander and Kalpna (2000), we simulated the load of Rihand reservoir through a series of rectangular loads. The load was uniform in each rectangle but its magnitude decreased with distance of the rectangle upstream from the dam. The maximum water depth was considered as 90 m at the dam. We used formulas based on 3d Boussineq solutions (Jaeger and Cook, 1969, p.281) to compute cumulative values of six stress tensors. These stress tensors were used to calculate change in normal (Δσ) and shear (Δτ) stress on a given fault plane, which were then used to calculate change in stability in dry state, i.e., without considering pore pressure, ΔS d,(=δτ μδσ, where μ is the friction coefficient) due to reservoir load (Jaeger and Cook, 1969; Gough and Gough, 1970; Bock, 1980) on the considered faults. Positive values of ΔS d suggest destabilization and promote failure on the considered planes and vice-versa. Unfortunately, focal mechanisms of the earthquakes that occurred close to the reservoir are not available. We assumed that these earthquakes accompanied similar motion as was involved during 1997 Jabalpur and 1970

5 K. Gahalaut et al. / Tectonophysics 439 (2007) Broach earthquakes. It has been suggested that during these earthquakes reverse motion occurred on the south dipping plane (Singh et al., 1999; Gahalaut et al., 2004) which is consistent with the NE to NNE directed compressional regime of central and peninsular India (Gowd et al., 1992). Thus we calculated ΔS d on the south dipping fault planes of 1997 Jabalpur (strike, ϕ 61, dip, δ 64 and rake, λ 74 ) and 1970 Broach earthquake (ϕ 35, δ 49 and λ 44 ) at a depth of 10 km assuming that all the earthquakes near the reservoir occurred at shallow depth in the upper crust, as at deeper depth, reservoir effects will not be very significant. This is in contrast with the focal depth of 36 km for the 1997 Jabalpur earthquake (Singh et al., 1999), but consistent with 11 km focal depth for 1970 Broach earthquake (Chung, 1993). Focal depths of both the earthquakes were determined from the waveform modeling (Chung, 1993; Singh et al., 1999) and hence appear to be reliable. Region of increased ΔS d in both cases lie southeast of the reservoir, whereas the region of earthquake occurrence lie west of the reservoir. Thus the two did not correlate in any significant way. Computations of ΔS d even on the north dipping planes did not produce favourable results. Thus we suggest that the reservoir load does not destabilize the faults of 1997 Jabalpur or 1970 Broach earthquake type. We even changed the depth at which ΔS d is calculated but the results and interpretation do not change in any significant way. Unfavourable results of the above section prompted us to search for the faults on which the reservoir effects were favourable. We considered all the geologically mapped faults in the vicinity of the reservoir for our Fig. 4. Stress changes ΔS (in kpa) on the SSW dipping planes (ϕ 120, δ 70 and λ 120, corresponding to the plane a of the derived fault plane solution, shown at the top right) are shown. Geologically mapped neotectonic faults having similar orientation (F 1, F 2 and F 3 ) are also shown. Red colour contours indicate the region of destabilization. (a) ΔS d at 8 km depth with contour interval of 10 kpa. (b) ΔS d in a vertical section along A A, contour interval is 5 kpa. (c) ΔS w at 8 km depth (d) ΔS w in a vertical section along B B.

6 176 K. Gahalaut et al. / Tectonophysics 439 (2007) analysis. Throughout our analysis, the sense of motion on geologically mapped faults is chosen in such a way so as to be consistent with NNE SSW compression due to plate tectonic forces. This is also a condition in the analysis related to the RTS, as earthquakes will be triggered only on those faults which are critically stressed for failure under the ambient stresses (Simpson, 1986; McGarr and Simpson, 1997; Talwani, 1997; Gupta, 2002). The reservoir acts only as trigger. Thus under the compressive environment of central and peninsular India, all the faults present in the region will either be reverse or strike slip faults, depending upon the orientation of the faults with respect to the NNE to NE directed maximum compression (Gowd et al., 1992). We considered the orientation of the geologically mapped faults and performed a grid search to estimate the dip direction, dip amount and slip direction of these faults in such a way that the reservoir causes destabilization on these faults in the region of earthquake occurrences. The computations of ΔS d were done at several depths but here we show ΔS d at 8 km depth (Fig. 4a), where the effect of the reservoir load is most pronounced (Fig. 4b). We find that WNW ESE oriented geologically mapped neotectonic faults (GSI, 2000), shown as F 1, F 2 and F 3 in Figs. 1 and 4, withδ of 70 ±10 and λ of 120 ±10, having strike of about N120 are destabilized by the reservoir load in the region of prominent earthquake cluster reported by DMG and ISC (Fig. 4). Specifically, the east west trending south dipping Son Narmada South fault, the largest fault in the region, did not produce destabilization in the region of earthquake occurrence. It caused destabilization north of it. This was expected, as a reservoir located in the hanging wall will stabilize a thrust fault, rather than destabilizing it (Roeloffs, 1988). The faults F 1, F 2 and F 3 have been mapped, before reservoir impoundment in 1962, by the Geological Survey of India (GSI), in their district level map series (e.g., see geological and tectonic maps of Sarguja, Koriya, Sidhi and Mirzapur districts published by GSI) and are shown in Figs. 1 and 4. The derived fault plane solution corresponding to the above plane is also shown in Fig. 4. In an effort to further strengthen our analysis, we performed the grid search to identify the strike along with the dip and rake of the fault and found that the above estimated planes (ϕ 120, δ 70 and λ 120 ) are the only fault planes on which the effect of reservoir is favourable in the region of intense earthquake activity. We also show an east west vertical cross section of ΔS d across the reservoir, in which it is seen that at all depths, the region west of the reservoir experiences destabilization due to reservoir load. A nominal value of 0.65 for μ is assumed in these calculations. Change in μ by ±0.2 does not alter the pattern of destabilization in any significant way Effect of pore pressure We calculate stress changes in wet state, i.e., considering the pore pressure, ΔS w (=Δτ μ(δσ ΔP), where ΔP is the pore pressure due to reservoir operations since its impoundment started in Change in normal (Δσ) andshear(δτ) stresses due the reservoir are calculated in the same manner as in the previous section. ΔP is calculated by solving the following inhomogeneous diffusion equation, Cj 2 P ¼ A AT P B 3 h ; where c is the hydraulic diffusivity, B is the Skempton's coefficient and θ /3 is the mean stress. The value of c, hydraulic diffusivity constant was considered to be 10 m 2 /s,bytrialanderrormethod,soastogiveatime lag of about 1 month at a point beneath the maximum seismicity at an assumed depth of 8 km. The adopted value of c is consistent also with Talwani and Acree's (1984) estimates derived from seismicity observations near various reservoirs worldwide. Higher value of c decreases the time lag and increases the magnitude of pore pressure, and vice-versa. B variesfrom0to1,here it is considered as 0.7 (Talwani et al., 1999). The solution of above equation is given by Kalpna and Chander (2000). Pore pressure due to cyclic loading and unloading develops gradually in the initial periods (till 1970), but later on, pore pressure, and hence ΔS w, mimics the pattern of annual cycle of water level changes in the reservoir with a time lag (Fig. 2). Considering the lag of 1 month between the water level and monthly frequency of earthquakes, we calculated ΔS w on a plane at depth of 8 km, after 1 month of maximum water level in the reservoir. We repeated the exercise of the previous section to compute ΔS w and found that the pattern in the zone of stability did not change significantly from the previous corresponding cases. Thus even after considering the pore pressure the region of increased ΔS w corresponding to the 1997 Jabalpur and 1970 Broach earthquake faults does not coincide with the region of earthquake occurrence. However, the results of grid search method suggested that once again faults with ϕ 120 ± 10, δ 70 ±10 and λ 120 ±10 are destabilized by the reservoir load and induced pore in the region of prominent earthquake cluster reported by DMG and ISC (Fig. 4).

7 K. Gahalaut et al. / Tectonophysics 439 (2007) The east west vertical cross section of ΔS w across the reservoir (Fig. 4d) suggests that the region west of the reservoir experiences destabilization due to reservoir operations at all depths. However, the effect is most pronounced at about 8 km. The analysis suggests that consideration of pore pressure not only leads to increase in the magnitude of stress changes (ΔS w NΔS d ) on the considered fault, it also leads to an increase in the region of destabilization and now the region of destabilization encompasses the entire region of earthquake activity near the reservoir. We computed temporal variation of ΔS w (Fig. 2)ata point that lies at 8 km depth under the reservoir and where the increased ΔS w is the maximum (Fig. 4c). It can be seen that earthquakes do not appear to have occurred during the low water stand, even though the effect of water load was to destabilize the fault even during that period (Fig. 2). Thus the analysis implies that the faults in the region are critically stressed for failure under compressive regime and a small increase in stress change due to reservoir operation, by about 25 kpa only, corresponding to an annual increase of water load by m (Fig. 2), triggers earthquakes on these faults. Presence of critically stressed faults is also supported by the observation that this region is the seismically most active region of peninsular India. Thus the above analyses pertaining to the quantitative effect of reservoir operation on the seismogenic faults suggest that the reservoir operations destabilize the nearby neotectonic faults. 5. Concluding remarks Several factors control earthquake triggering by the reservoir, important of them are, reservoir dimensions; annual changes in the reservoir water level; ambient stresses and presence of faults and their orientation with respect to the ambient stresses, etc. (Simpson, 1986). In this case the Govind Ballav Pant reservoir on Rihand river is very large, in fact the second largest in India, having annual water level changes of the order of about 12±2 m, it is situated in a failed rift region of Narmada Son Tapti, which is seismically the most active region in the peninsular India (Gahalaut et al., 2004) and where several faults have been mapped (Fig. 1). These factors make it an ideal site where reservoir may trigger earthquakes on the nearby faults. A good correlation between temporal variation of seismicity and reservoir water levels (Fig. 2) suggest a possible case of reservoir triggering. Our simulation using a Green's functionbased approach related to stability change on the earthquake causative neotectonic faults due to reservoir operation (Fig. 4) supports the view that water level changes in the reservoir have caused pore pressure changes at hypocentral depths on the pre-existing faults, which are favourably oriented, to trigger the earthquakes. This makes it a persuasive, yet so far unreported, case of triggered seismicity due to the reservoir loading. In the region of interest, we have no knowledge about the earthquake occurrence immediately after the reservoir impoundment in 1962 and before 1984, the later year marks the year of beginning of reliable earthquake catalogues. But good correlation between earthquakes during (for M N 3 from ISC and IMD catalogues) and during (for Mb3 from DMG catalogue) with reservoir water level possibly suggests that the reservoir triggered earthquakes occurred in this region after the impoundment and the reservoir continued to trigger earthquakes even after 40 years of impoundment. 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