Microearthquake activity near the Idukki Reservoir, south India: A rare example of renewed triggered seismicity

Microearthquake activity near the Idukki Reservoir, south India: A rare example of renewed triggered seismicity

Engineering Geology 153 (2013) 45–52 Contents lists available at SciVerse ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locat...

1MB Sizes 0 Downloads 23 Views

Engineering Geology 153 (2013) 45–52

Contents lists available at SciVerse ScienceDirect

Engineering Geology journal homepage: www.elsevier.com/locate/enggeo

Microearthquake activity near the Idukki Reservoir, south India: A rare example of renewed triggered seismicity Kusala Rajendran a,⁎, N. Thulasiraman a, K. Sreekumari b a b

Centre for Earth Sciences, Indian Institute of Science, Bangalore, 560012, India Peechi Seismic Observatory, Centre for Earth Science Studies, KFRI Campus, Thrissur 680 653, India

a r t i c l e

i n f o

Article history: Received 3 August 2012 Received in revised form 25 October 2012 Accepted 17 November 2012 Available online 27 November 2012 Keywords: Reservoir triggered earthquakes Idukki Reservoir Hydroseismicity Earthquake mechanisms

a b s t r a c t Earthquakes triggered by artificial reservoirs have been documented for more than seven decades and the processes leading to this phenomenon are fairly well understood. Larger among such earthquakes are known to occur within a few years of reservoir impoundment and usually the activity decreases with time. A documented example of Reservoir Triggered Seismicity (RTS), the Idukki Reservoir in Kerala, south India, impounded in 1975, is an exception wherein the triggered activity has been revived in 2011, nearly 35 years after the initial burst of activity in 1977, two years after the dam was filled. The magnitude of the largest shock in the 2011 sequence exceeded that of the previously documented largest microearthquake. Presence of faults that are close to failure and vulnerable to increase in pore pressure due to reservoir loading or increased rainfall, or a combination of both seems to trigger shocks in this area. The renewed burst of earthquakes after a prolonged period of reduced activity at the Idukki Reservoir is a rare example of RTS. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Causal relation between meteorological events (such as intense rainfall) and low-level seismicity has been discussed for nearly a century (Sayles, 1913, for example). That earthquakes can be triggered by artificial water reservoirs was observed for the first time at Lake Mead in the United States of America, leading to the concept of reservoir induced seismicity (Carder, 1970). The increase in seismicity observed at Koyna, following the filling of the Shivaji Sagar Lake and other cases reported globally led to further examine the characteristics and mechanisms of earthquakes near large reservoirs (e.g. Gupta and Rastogi, 1976; Gupta, 1992). While there are hundreds of reservoirs and river basins where hydrological cycles show no causal relationship with seismicity, increasing number of examples suggest a connection between the two where rocks are prone to failure through fluid–rock interactions. Today, over ninety reservoirs are known for triggering earthquakes and many of them are not necessarily very large (see Gupta, 2002 for a review). Only four among them are associated with earthquakes of M ≥ 6.0; ten have generated earthquakes of magnitude 5.0–5.9 and at twenty-eight sites, the maximum magnitude range from 4.0 to 4.9. At the remaining sites, the shocks are of M b 4.0. It has been fairly well understood that the stress changes introduced by the reservoir alone cannot lead to earthquakes, unless the geological environment is conducive to failure. Thus, most workers consider that the term, “triggered seismicity” is a more realistic representation of the process, than “induced ⁎ Corresponding author. Tel.: +91 8022932633. E-mail address: [email protected] (K. Rajendran). 0013-7952/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enggeo.2012.11.004

seismicity” (Simpson, 1986; Gupta, 2002), a terminology that we also follow in this paper, abbreviated as RTS (Reservoir Triggered Seismicity). In this paper we present the case of Idukki Reservoir, South India, a case of RTS where initial activity in 1977, including the largest microearthquake (M 3.5) and a subsequent phase of reduced activity was surpassed by a larger shock (M 3.9) in 2011, a rarity among global examples. Based on the delay between the initial impoundment and the onset of activity, RTS is classified as “rapid” and “delayed” (Simpson et al., 1988). While rapid response results from elastic effects and the undrained pore pressure changes, delayed response is attributed to the diffusion of pore pressure to seismogenic depths. The delay since impoundment is a function of the loading history, fracture connectivity and hydraulic diffusivity of the medium and in most cases it is of the order of a few years. Global studies corroborate with this view and it is generally noted that the increase in seismicity following impoundment usually occurs in the immediate vicinity of the reservoir and usually, within the first few years after filling (Gupta, 2002). The earthquake with the largest magnitude invariably forms part of the initial spurt of activity and the frequency and magnitude of shocks are known to reduce with time. A noted exception to this is the Koyna Reservoir (Maharashtra, India), where triggered sequences have continued, since 1967 (M 6.3). With a delay of five years since initial filling (1962–67), seismicity Koyna is considered as a typical example of delayed response, which Talwani (1997) refers to as “protracted seismicity” and Gupta (2002), as “continuing seismicity”. The distinctive hydrogeological environment at Koyna wherein the N–S vertical fractures within the overlying basalt layer recharges the narrow seismogenic fault zone is considered as a factor

46 K. Rajendran et al. / Engineering Geology 153 (2013) 45–52 Fig. 1. a. Historical and recent seismicity in Kerala and vicinity (updated from Rajendran et al., 2009). Red filled circles: Historical earthquakes (since 1784; magnitude uncertain, mostly b4.5); yellow filled circles: Earthquakes from 1980 to August 2011; scaled to size; maximum magnitude, M 3–5.0. Rectangle identifies the area shown in Fig. 1b. PCH and TRD are broadband seismic observatories. b. Area around the Idukki Reservoir showing faults and lineations (modified from Rajendran et al., 2009). Seismic stations of the Idukki and Idamalayar nets are shown by triangles. Significant historic earthquakes and the recent earthquakes of M ≥ 4.5 are identified. The ML 3.5 (1977) earthquake that occurred very close to the Idukki Dam is the largest previously reported earthquake. Idukki and Mullaperiyar Reservoirs are also shown. Earthquakes until 2011 are plotted. Rectangle identifies area shown in Fig. 3. Background land cover is LANDSAT-7TM image acquired on January 14, 2001 (source: USGS).

K. Rajendran et al. / Engineering Geology 153 (2013) 45–52

47

that sustains the activity here (Rajendran and Harish, 2000). Although the activity has been sustained, the magnitude of the shocks has not exceeded that of the 1967 shock. Differing from all known examples of RTS, the 2011 sequence at Idukki Reservoir, South India (Figure 1a and b) include an earthquake whose magnitude exceeds that of the previous largest shock. Such renewal of microearthquake activity after a long phase of nominal activity makes Idukki an exceptional case of RTS, which provide further insight into the mechanism of triggered earthquakes associated with reservoirs that may be considered rather mature and stabilized. The example of Idukki points to the possibility that seismogenic reservoirs that are considered to have stabilized stands chance of being revived, and this exceptional behavior is what makes this case worth reporting.

in the region, which mostly control the river system, also follow a NE–SW trend (Figure 1b; GSI, 2000). Proximity to the ~ 110 year-old Mullaperiyar Reservoir (see Figure 2b for location), a contentious issue between the two neighboring states of India, Kerala and Tamil Nadu, is another important factor that has drawn recent attention to the seismogenic status of these aging reservoirs. Although the older dam has not triggered any earthquakes in its immediate vicinity during the documented history, the proposal to retain this dam and increase its loading capacity is being debated from the RTS point of view. In this paper we examine the recent microearthquakes near Idukki in the backdrop of its past activity and the context of reservoir-triggered seismicity and hydroseismicity models.

2. Background

2.2. Temporal and spatial association of RTS

2.1. Idukki Reservoir, South India

Global studies of RTS have led to some pertinent observations on the spatial association of earthquakes with the reservoir and causality with the impoundment and filling cycles (Gupta, 2002). For nearly 40% of the cases presented, the largest earthquake has occurred within the first two years of impoundment, suggesting a causal relationship with filling (Figure 2). Three of the larger earthquakes (M≥6) occurred 3–5 years after the initial filling and these are at Hsinfengkiang (China), Kariba (Zambia-Zimbabwe) and Koyna (India) reservoirs. At some sites, earthquakes were triggered 10 to 20 years after the reservoir impoundment, and there is just one case where the earthquake occurred 62 years after the initial impoundment. This earthquake occurred near the Gandipet/Osmansagar Reservoir (India), a 36 m-high-dam, filled in 1920 (Gupta, 2002). Based on the large distance (~20 km west of the reservoir) relative to other known cases of RTS, and the long gap since filling, Rastogi et al. (1986) concluded that the tremor was not triggered by the reservoir, and therefore, we have excluded this case from Fig. 2. Thus, based on the global review, it is reasonable to state that there are no known examples of RTS that have occurred beyond a period of two decades after initial impoundment.

Idukki Reservoir (south India) formed by the Idukki Dam, one of the highest (169 m) arch dams in Asia is a documented example of RTS (Gupta, 2002). The region started experiencing low magnitude tremors soon after the dam was impounded in 1975, and the activity peaked around 1977 and the epicenter of the largest shock (M 3.5) was very close to the dam (Figure 1b). Due to the generally low background seismicity in the reservoir area, which was being monitored since 1972, the M 3.5 shock in the close proximity of the dam was considered as an indication that it was triggered by the reservoir. Smaller shocks followed, mostly of magnitude b 2.0, but their numbers reduced substantially until 1983, when the downstream Idamalayar Dam, also of 169 m depth at full capacity, was commissioned. The filling of the new reservoir led to increase in microearthquakes and the activity peaked around 1985, as observed from the stations closer to the Idamalayar Reservoir. The Idukki Reservoir is located in a region that is quite active, for a site within the stable peninsular India. Although there are no records of any M > 4 earthquakes in the immediate vicinity of the reservoir (within 20 km radius), its outlying regions have experienced low magnitude earthquakes in the historic period, notable among them are the twin earthquakes (M 4.8) in 1953 (Figure 1b). Two earthquakes of magnitudes 5.0 and 4.8 (2000 and 2001) and a third one of M 4.5 (1988) within 40 km of this reservoir are the largest instrumentally recorded regional earthquakes (Singh et al., 1989; Bhattacharya and Dattatrayam, 2002). These earthquakes are considered as part of the shield seismicity associated with the NW–SE oriented structures, which are being reactivated, in the current stress regime (Rajendran et al., 2009). The mapped faults and lineaments

2.3. Proposed mechanisms Intraplate earthquakes are known to occur in response to plate driving forces, as the stresses tend to accumulate in relatively weaker regions of the seismogenic crust (Johnston, 1989). The seismogenic crust in such regions is assumed to be in a self-organized critical state (Grasso and Sornette, 1998), and earthquakes occur in response to minor changes that affect this state of criticality. In other words, the earth behaves as if the brittle crust in many regions is near failure and small, localized changes in stresses are sufficient to trigger

Fig. 2. Number of RTS cases and the delay between initial impoundment and the onset of largest triggered earthquake based on global data from Gupta (2002).

48

K. Rajendran et al. / Engineering Geology 153 (2013) 45–52

earthquakes. Observations that stress increase of ~ 0.05 MPa provides sufficient conditions to trigger earthquakes on critically stressed faults add credence to this argument (Harris, 1998). It has been observed that rapid changes in water level as low as 20 cm have lead to increase in seismicity at Nurek Reservoir, a tall dam (317 m) in the former USSR (Simpson and Negmatullaev, 1981). The seismogenic fault zone beneath Lake Jocassee, USA (107 m) is found to be sensitive to weekly lake level changes of the order of l–l.5 m (Rajendran, 1995). These observations lend support to the argument that a fractional increase in stress caused by reservoir impoundment or through hydrological cycles such as increase in rainfall can locally destabilize faults that would otherwise remain stable. This is the basic premise behind the proposed mechanism of a broad class of earthquakes triggered through increased pore-fluid pressure, explained by the “hydroseismicity model”. The hydroseismicity model proposed by Costain et al. (1987) suggests that, hydrologic cycle plays an important role in the generation of intraplate earthquakes. It may be understood that the sources of such earthquakes are not necessarily in the proximity of reservoirs, and they may of them are in the catchment areas of rivers. Artificial as well as natural changes in crustal pore-fluid pressure is known to trigger earthquakes (Saar and Manga, 2003), broadening the class of earthquakes triggered by fluid-induced weakening. Hydrologic changes occur from excessive rainfall, reservoir loading and unloading, fluid injection and withdrawal from aquifers (oil reservoirs), and changes induced by earthquakes themselves. These observations essentially support the idea that a change in the hydraulic head over the earth's surface, when transmitted to the substratum can trigger small or moderate earthquakes, where the conditions for failure are favorable. Implicit in this statement is the idea that not every region is prone to hydroseismicity and the failure depends on the geological and hydrological conditions. Therefore, it has to be understood that favorable hydrogeological conditions appear to be a necessary, and not a sufficient condition for triggering earthquakes either by man-made reservoirs or through natural changes in the hydrological cycle. Just what are these conditions for failure induced by the filling of a reservoir or through critical changes in hydrologic cycles? Here we provide a brief background. Conditions for brittle failure may be defined using Coulomb failure criterion expressed as: τ ¼ C þ μ σn

ð1Þ

where τ and σn are respectively the shear and normal stress on a plane in question; μ is the coefficient of friction and C is the cohesive strength of the rock (Scholtz, 2002). The mechanical effect of pore pressure is to reduce the confining pressure and thereby modify the failure condition as:   τ ¼ C þ μ σ n– pp

ð2Þ

where pp is the pore fluid pressure. Changes in the hydraulic head and or static or dynamic stress changes caused by other processes including earthquakes can change the failure condition defined by Eq. (2). Where the reservoir loading leads to instability of a fault, the following expression given by Bell and Nur (1978) best explains the conditions of failure. Thus, the incremental strength Δs across a fault plane is expressed as:   Δs ¼ μ Δσ−ΔPp  ΔT

ð3Þ

where Δσ and ΔT are the incremental normal and shear stresses on the plane due to the water load, μ is the coefficient of friction and ΔPp is the incremental pore pressure. Changes in each of these parameters occur through elastic, undrained, and drained responses of the media which depend on

the filling history as well as the geologic and hydrogeologic characteristics of the region (Simpson et al., 1988; Rajendran and Talwani, 1992). What range of stress changes can induce failure has been an issue of debate and the evidence currently available suggests that certain fault zones are sensitive to lake level changes in the range of l– l.5 m/week, which may lead to stress change in the order of 0.1 bar or less at the shallow seismogenic depths (2–5 km) applicable to most reservoir triggered earthquakes (Rajendran, 1995). These examples suggest that the magnitude of change required for triggering earthquakes is quite small, where faults are close to failure stressed. Our studies of the regional seismicity suggest that the central mid-land region comprising central Kerala (latitude 9–11°N; Figure 2) is more prone to low–moderate earthquakes and the NW– SE oriented faults are most vulnerable (Rajendran et al., 2009). Further, it is noted that mircoseismic activity observed in different parts of Kerala shows a seasonal dependence on the post-monsoon periods, suggesting that the hydroseismicity model might apply to these cases (Rajendran and Rajendran, 1996). Such seasonality of earthquakes elsewhere is interpreted as a consequence of downward propagation of pore fluid pressure thereby changing the failure conditions dictated by Eq. (3). Seasonal variations in water storage and the resulting hydrological changes also influences the failure conditions (Costain and Bollinger, 2010). 3. Previous studies on Idukki seismicity A seismological network was operational since 1971, making the Idukki Reservoir one among the few examples of RTS that has a record of pre-impoundment seismicity. As the reservoir is located in the stable continental region of India with a general low-level seismicity, there was no local network prior to 1972 and the nearest station at that time was at Trivandrum (TRD), about 150 km south of Idukki (Figure 1a). After the local network was established, a large number of mild shocks were recorded, but only a few of these were locally felt and no significant seismicity followed the initial impoundment. The largest microearthquake in the vicinity of the reservoir occurred in 1977 (M 3.5; Guha and Patil, 1990) and small tremors continued to occur in the periphery of the reservoir. Based on the proximity of their sources to the reservoir, onset of activity following the initial impoundment and the temporal correlation with the monsoonal filling, the low-level seismic activity at Idukki was linked to the reservoir (Guha et al., 1981). A second peak in the activity occurred in 1985, subsequent to the filling of the Idamalayar Reservoir, located ~ 80 km NW of Idukki Dam. This activity also reduced during the subsequent years. However, a few other earthquakes in the range on M 4.5 to 5.0 occurred in the region during 1988, 2000 and 2001, but they were 25–40 km away from the Idukki Reservoir. They were attributed to reactivation of preexisting NW–SE trending faults (Harendranath et al., 2005; Rajendran et al., 2009). Although the outlying areas have generated some notable earthquakes, the area just around the Idukki Reservoir experienced only very low-level microearthquake activity (Figure 1b). 4. Renewal of microseismicity near Idukki Reservoir 4.1. Data and methods The analysis presented here is based on the microearthquake data from the network of seismic stations, maintained by the Kerala State Electricity Board (KSEB). Some of the stations of the Idukki network were operational since 1972, and more stations have been added subsequently, especially after the Idamalayar Dam was commissioned. Instruments including Portacorders, Wood-Anderson, EM and strong motion recorders form part of the Idukki and Idamalayar networks (see Figure 1b for location of stations). Due to problems related to maintenance, time keeping and other logistical issues during the

K. Rajendran et al. / Engineering Geology 153 (2013) 45–52

early days of monitoring, the data from this network is not uniformly of good quality. However, the daily reports maintained by the KSEB are meticulous on the number of tremors recorded by each instrument and the epicentral distances are based on the S-P estimates as recorded at the Idukki station. Many of the tremors were too small to be recorded on more than one station and only a few of the larger ones were located. Through an assignment to evaluate the quality of earthquake data, we had examined the original records during 1996–97 and found that the tremors were invariably recorded by one or two stations and they occurred within 2–6 km of the Idukki station and our observations were consistent with that of KSEB (Rajendran et al., 1999). Thus, for the sake of uniformity and continuity, in this study we used the record of microearthquakes (M > 2.0) maintained by KSEB for the period between 1972 and 2000. The broadband observatory at Peechi (PCH) ~ 110 km NW of Idukki, operational since December 1999 (Figure 1a for location), also maintains a record of local and regional events. Thus, from the year 2000 onwards, there is an additional record of microearthquakes originating from the Idukki region. After the December 12, 2000, M 5.0 earthquake ~ 25 km southwest of the Idukki Dam, followed by another one of M 4.8 on 7 January 2001, some of the stations were upgraded and there is an improvement in data quality and epicentral locations are also available. As we have been keeping a continuous record at the Peechi station since January 2000, we are using the data from Peechi, since that time. We have reproduced the history of microtremors and the filling history at Idukki Reservoir, based on the KSEB records. For the period starting

49

2007, we have used the daily reservoir levels and the rain gauge data from Idukki. 4.2. Observations and analysis The Idukki Reservoir does not undergo severe fluctuations due to monsoonal filling and it has not undergone any drastic changes in its filling history (Figure 3a). The first spurt of activity occurred in 1977, two years after filling and it consisted of an event of M 3.5 (Figure 3a). Most of the activity during the initial years was confined to ~ 7 km of the Idukki station (Figure 3b and c). There was notable decrease in activity in the subsequent years, but it picked up again, by 1985. This phase temporally corresponds to the filling of the Idamalayar Reservoir, which commenced in 1983 and was completed by August 1985. There was an immediate increase in the earthquake frequency following the filling of the reservoir and the epicentral distances grew to ~ 10 km away from Idukki station (Figure 3c). Most epicenters located by KSEB are close to the eastern limb of the reservoir, a geomorphological feature that shows remarkable parallelism with the nearly NW–SE oriented lineament. After this second pulse, which lasted only for a year, the seismicity around the reservoir was insignificant and the region was considered to have reached equilibrium in terms of its sensitivity to the hydrogeological changes. Starting July 2011, there was a revival in the microearthquake activity and there were at least three earthquakes of magnitude ML ≥ 3.0. Seventeen tremors were recorded until November 2011 by the KSEB network; three of these were of ML ≥ 3.0 and the tremor of

Fig. 3. a) Reservoir filling history and corresponding seismicity at Idukki during 1972–1986. Flipped numbers above bars represent number of events above M 2.0 (data source: Kerala State Electricity Board); b) The expansion of epicentral growth during 1975–1986 based on epicentral distances from Idukki station (IDK) and c). Spatial distribution of epicenters around Idukki Reservoir (1975–1986).

50

K. Rajendran et al. / Engineering Geology 153 (2013) 45–52

Fig. 4. Epicentral distribution of microtremors during 2011. Data source: Peechi Observatory.

maximum magnitude (ML 3.8) occurred on 16 July, 2011. The broadband seismic station at Peechi also recorded these tremors, but the magnitude estimate is slightly different in that there are four earthquakes of ML ≥ 3.0. The largest event was assigned magnitudes 3.8 and 3.9, respectively, by KSEB and Peechi BB observatory (Figure 4). The 2011 epicenters are located near the western limb of the reservoir, a region that has only been marginally active, previously. The

focal depth estimates of these earthquakes are likely to be in error due to the lack of a well-constrained velocity model and the large spread of stations. However, based on the review of seismicity, Rastogi et al. (1995) suggested that the hypocenters are less than 5 km. This depth of range seems to be generally true for this region as observed also for the aftershocks of the 2000–2001 earthquakes (Bhattacharya and Dattatrayam, 2002). This is the depth range

Fig. 5. a. Average lake level in the Idukki Reservoir (scale on the left), weekly change in lake level (scale on the right) during 2007. b. Daily rainfall (mm) recorded at Idukki during 2007. c. Same as the left panel for the year 2011. Earthquakes are identified as dots, corresponding with the time of their occurrence. d. Same as the left panel for the year 2011.

K. Rajendran et al. / Engineering Geology 153 (2013) 45–52

commonly observed for earthquakes in this part of southern India (Rajendran et al., 2009). We examined the relation between the reservoir filling, weekly change in the lake level and the frequency of tremors since 2007 (Rajendran et al., 2012). There was no significant change in any of these parameters during these five years, and there was nothing unusual about the rate of reservoir filling or rainfall intensity. Here we present data for 2007, a year when a higher reservoir level was reached. The rainfall intensity was also high; so was the weekly rate of loading, but no earthquakes occurred in the vicinity of the reservoir (Figure 5a,b). The microearthquake activity during this period was of very low, with two mild tremors during 2008 (M 3.4); one in 2009 (M 3.5) and one in 2010 (M 2.9). However, there was an increase in the level of activity during 2011, which followed the monsoon when the weekly refilling increased to about 3–4 ft/week (Figure 5c and d). The 2011 epicenters are located close to the western limb of the reservoir, which had generated fewer shocks during its early history (Figure 4). The increase in microearthquake activity does not appear to be associated with any notable change in filling rate or increase in rainfall intensity, but it did follow the seasonal monsoonal recharge. The frequency of earthquakes has reduced since August and there was no significant activity till December 2011. 5. Discussion and conclusions Previous studies have noted spatial and temporal correlations of microearthquake activity with the reservoir filling and the tremors were generally located around the Idukki Reservoir (Rastogi et al., 1995). There was an increase in the frequency of shocks after the Idamalayar Reservoir was filled during 1983–85. The growth in the epicenter area, as observed in 1985 is considered as an indication of the expanding spatial influence of the reservoir, a phenomenon also noted at many other reservoirs (Talwani and Acree, 1984). The activity around the reservoir decreased since 1986 and during the two and a half decades that followed, the reservoir did not experience any notable activity. However, this was a period when the outlying areas to the west and east of the reservoir experienced earthquakes in the magnitude range of 4.5 to 5.0, which are in fact among the most significant regional earthquakes. It has been suggested that the NNW– SSE oriented faults in Kerala may host potentially active sources capable of generating moderate-sized earthquakes (Rajendran et al., 2009). It is intriguing that despite the filling of the reservoir and the existence of NW–SE oriented faults/lineaments in its immediate vicinity, the maximum magnitude of the triggered earthquake is not more than 4.0. Sustaining the aging Mullaperiyar Reservoir, located upstream of the Idukki Reservoir and perhaps further increase its capacity, is an issue to be reviewed in the context of the seismogenic history of the Idukki Reservoir. The observations from Idukki suggest that the reservoir and its environments remain sensitive to hydrogeological conditions and the regions close to failure may be vulnerable to small changes induced either by reservoir filling or by rainfall or both. However, this change alone does not constitute a sufficient condition for triggering earthquakes, because such phases during the past have not created any notable microearthquake activity. The recent spurt of post-monsoonal microearthquake activity near the Idukki Reservoir suggests that faults in the vicinity of the reservoir may generate low-magnitude tremors following the reservoir refilling or a post-monsoonal recharge. Most of the tremors in the past have occurred in the periphery of the reservoir and the recent activity is close to the western limb of the reservoir, a region that has experienced very few tremors in the past. As more global examples of earthquakes triggered by reservoirs are being reported, cause of some earthquakes in the proximity of large reservoirs, remain debated. For example, the question whether the 12 May, 2008 Mw 7.9 Wenchuan (China) was triggered by the Three Gorges Dam was ruled considering its large distance from the

51

epicenter, but the potential role of the Zipingpu Dam, within 5.5 km could not be ruled out. Models incorporating Coulomb stress changes introduced by the filling of this reservoir however do not support the nucleation of the earthquake at ~ 14 km depth (Deng et al., 2010). Further, these authors argue that the filling of the Zipingpu Reservoir could only result in an increase in the rate of shallow earthquakes with hypocenter depth of ~ 5 km, around a distance of ~ 5 km around the reservoir region. The example of the Zipingpu Reservoir is cited here to suggest that the region of influence of a large reservoir is usually in the range of ~ 10 km. Thus, we consider that the low moderate earthquakes that occurred during 1988–2001 in the outlying areas of the Idukki reservoir are due to the tectonic adjustments, as has been interpreted earlier. We conclude that the recent increased activity in the vicinity of the Idukki Reservoir is unusual, considering the long temporal gap since its filling and initial onset of microerathquakes. The current activity could have resulted from localized changes in the hydrologic conditions, caused by the monsoonal recharge and/or reservoir filling. Small changes in pore pressure introduced due to reservoir filling or through monsoonal recharge may destabilize the reservoir as predicted by the conditions of failure discussed earlier. As an aging reservoir that seems to have attained hydrogeologic equilibrium, Idukki Reservoir has not been the subject of extensive study. But for the seismic station network and other routine monitoring, there is not enough data to assess the pore pressure conditions in relation to the reservoir impoundment. Thus, the inference of the state of criticality of stress or readiness of fault for failure is a conjecture, based on the fact that the region has a history triggered microearthquakes following reservoir filling. Further, the region host faults that are being reactivated and at least a few of the microtemors are attributed to hydroseismicity, as discussed earlier. In summary, Idukki Reservoir appears to be conducive to low-level microearthquake activity in response to small changes in hydrologic conditions. In a region where the regional faults have a history of low-moderate earthquakes, the region in the immediate vicinity of the Idukki reservoir has generated only shocks of lower magnitude. Occurrence of a larger shock more than 35 years after the onset of initial activity makes Idukki Reservoir a special case of RTS. Careful monitoring of this region through strengthening the seismic network and gathering relevant hydrologic data would help understand conditions that favor failure. The example of Idukki Reservoir suggests that even the mature reservoirs, considered to have attained stability might hold potential for revived activity, in favorable circumstances. Future studies on the mechanism of RTS should not ignore such possibilities. Acknowledgments The authors thank the two anonymous reviewers for their comments and suggestions, which have greatly improved this manuscript. The authors from IISc acknowledge funding from the Ministry of Earth Sciences, Government of India (MoES/ATMOS/PP-IX/09), for the infrastructure development project that supports the seismology Lab at the Centre for Earth Sciences. The earthquake and reservoir level data are from KSEB, and the data since 1999 are from the Broadband observatory at Peechi, operated by the Centre for Earth Science Studies, Trivandrum. References Bell, M.L., Nur, A., 1978. Strength changes due to reservoir-induced pore pressure and application to Lake Oroville. Journal of Geophysical Research 83, 4469–4483. Bhattacharya, S.N., Dattatrayam, R.S., 2002. Earthquake sequence in Kerala during December 2000 and January 2001. Current Science, India 82, 1275–1278. Carder, D.S., 1970. Reservoir loading and local earthquakes. In: Adams, W. (Ed.), Engineering Geology Case Histories, 8. Geological Society of America, pp. 51–61. Costain, J.K., Bollinger, G.A., 2010. Review: research results in hydroseismicity from 1987 to 2009. Bulletin of the Seismological Society of America 100 (5A), 1841–1858.

52

K. Rajendran et al. / Engineering Geology 153 (2013) 45–52

Costain, J.K., Bollinger, G.A., Speer, J.A., 1987. Hydroseismicity: a hypothesis for the role of water in the generation of intraplate seismicity. Seismological Research Letters 58, 41–64. Deng, K., Zhou, S., Wang, R., Robinson, R., Zhao, C., Cheng, W., 2010. Evidence that the 2008 Mw 7.9 Wenchuan Earthquake could not have been induced by the Zipingpu Reservoir. Bulletin of the Seismological Society of America 100 (5B), 2805–2814. Grasso, J.-R., Sornette, D., 1998. Testing self-organized criticality by induced seismicity. Journal of Geophysical Research 103, 29965–29987. GSI, 2000. Seismotectonic Atlas of India and Its Environs. Geol. Surv., Kolkata, India. (209 pp.). Guha, S.K., Patil, D., 1990. Large water reservoir related induced seismicity. Gerlands Beitrage zur Geophysik, Leipzig 99, 265–288. Guha, S.K., Padale, J.G., Gosavi, P.D., 1981. Probable risk estimation due to reservoirinduced seismicity. Dams and Earthquakes. Institution of Civil Engineers, London, pp. 297–305. Gupta, H.K., 1992. Reservoir-Induced Earthquakes. Elsevier, Amsterdam . (364 pp.). Gupta, H.K., 2002. A review of recent studies of triggered earthquakes by artificial water reservoirs with special emphasis on earthquakes in Koyna, India. EarthScience Reviews 58, 279–310. Gupta, H.K., Rastogi, B.K., 1976. Dams and Earthquakes. Elsevier, Amsterdam. (229 pp.). Harendranath, L., Rao, K.C.B., Balachandran, V., Rajagopal, G., 2005. Recent significant earthquakes in quick succession in Kottayam Idukki region, Kerala — a macroseismic study. Engineering Geology 32, 31–35. Harris, R., 1998. Introduction to special section: stress triggers, stress shadows, and implications for seismic hazard. Journal of Geophysical Research 103, 24347–24358. Johnston, A.C., 1989. The seismicity of stable continental interiors. In: Gregersen, S., Basham, P.W. (Eds.), Earthquakes at North-Atlantic Passive Margins: Neotectonics and Postglacial Rebound. Kluwer Academic Publishers, Dordrecht, pp. 299–327. Rajendran, K., 1995. Sensitivity of a seismically active reservoir to low-amplitude fluctuations: observations from Lake Jocassee, South Carolina. Pure and Applied Geophysics 145, 87–95. Rajendran, K., Harish, C.M., 2000. Mechanism of triggered seismicity at Koyna: an assessment based on relocated earthquake during 1983–1993. Current Science, India 79 (3), 358–363. Rajendran, K., Rajendran, C.P., 1996. Mechanism of microseismic activity in Kerala — a suggestion. Journal of the Geological Society of India 47, 467–476. Rajendran, K., Talwani, P., 1992. The role of elastic, undrained and drained responses in triggering earthquakes at Monticello Reservoir, South Carolina. Bulletin of the Seismological Society of America 82, 1867–1888.

Rajendran, K., John, B., Rajendran, C.P., 1999. Report on the microseismic activity around Idukki and Idamalayar reservoirs, Centre for Earth Science Studies, project report, 11 pp. Rajendran, C.P., John, B., Sreekumari, K., Rajendran, K., 2009. Re-assessing the earthquake hazard in Kerala based on the historical and current seismicity. Journal of the Geological Society of India 73, 785–802. Rajendran, K., Rajendran, C.P., Sreekumari, K., Naveen, R., 2012. Recent microtremors near the Idukki Reservoir, Kerala, South India. Current Science, India 102, 1446–1451. Rastogi, B.K., Rao, C.V.R.K., Chadha, R.K., Gupta, H.K., 1986. Microearthquakes near Osmansagar reservoir, Hyderabad, India. Physics of the Earth and Planetary Interiors 44, 134–141. Rastogi, B.K., Chadha, R.K., Sarma, C.S.P., 1995. Investigations of June 7, 1988 earthquake of magnitude 4.5 near Idukki Dam in southern India. Pure and Applied Geophysics 145 (1), 109–122. Saar, M., Manga, M., 2003. Seismicity induced by seasonal groundwater recharge at Mt. Hood, Oregon. Earth planet. Earth and Planetary Science Letters 214, 605–618. Sayles, R., 1913. Earthquakes and rainfall. Bulletin of the Seismological Society of America 3, 51–56. Scholtz, C., 2002. The Mechanics of Earthquake Faulting, Second ed. Cambridge University Press, p. 496. Simpson, D.W., 1986. Triggered earthquakes. Annual Review of Earth and Planetary Sciences 14, 21–42. Simpson, D.W., Negmatullaev, S.K., 1981. Induced seismicity at Nurek Reservoir, Tadjikistan, USSR. Bulletin of the Seismological Society of America 71 (5), 1561–1586. Simpson, D.W., Leith, W.S., Scholz, C.H., 1988. Two types of reservoir-induced seismicity. Bulletin of the Seismological Society of America 78, 2025–2040. Singh, H.N., Raghavan, V., Varma, A.K., 1989. Investigation of Idukki earthquake sequence of 7th–8th June 1988. Journal of the Geological Society of India 34, 133–146. Talwani, P., 1997. On the nature of reservoir-induced seismicity. Pure and Applied Geophysics 150, 473–492. Talwani, P., Acree, S., 1984. Pore pressure diffusion and the mechanism of reservoirinduced seismicity. Pure and Applied Geophysics 122, 947–965.