Water level fluctuations due to earthquakes in Koyna-Warna region, India

Nat Hazards (27) 4:585 592 DOI 1.17/s1169-6-922- ORIGINAL PAPER Water level fluctuations due to earthquakes in Koyna-Warna region, India D. V. Ramana Æ R. K. Chadha Æ Chandrani Singh Æ M. Shekar Received: 28 June 25 / Accepted: 17 November 25 / Published online: 15 November 26 Ó Springer Science+Business Media B.V. 26 Abstract Earthquakes cause a variety of hydrological phenomena, including changes in the ground water levels in bore wells. The Koyna region in the peninsular shield of India, hitherto considered stable in terms of seismic activity, has been active since 1967. More recently, the earthquakes have been localized to the newly impounded Warna reservoir, which is located south of Koyna, where a burst of seismicity occurred in 1993. The region continues to remain seismically active even after four decades. Twenty-one bore wells were drilled around the seismic source volume in the region to observe water level changes resulting from earthquake phenomena. Our studies have shown coseismic anomalous water level changes to be associated with the moderate earthquakes of April 25, 1997 and February 11, 1998. Our results show that changes in the ground water level in bore wells are correlated with micro-earthquake activity, both preceding and following moderately sized earthquakes. The results have implications in enhancing our understanding of earthquake mechanisms. Keywords Koyna-Warna Æ Pore pressure Æ Water level fluctuations 1 Introduction Several research groups have reported anomalous changes in ground water levels in wells related to the pre-, co- and post-seismic phases of earthquakes (Wakita 1975; Igarashi et al. 1992; Roeloffs 1996, 1998; Grecksch et al. 1999; Chia et al. 21, 22; Brodsky et al. 23; Wang et al. 24). The pre-seismic changes are generally explained in terms of crustal volumetric strain prior to the occurrence of earthquakes and are detectable as a hydrological precursor. Changes in a confined aquifer within a few kilometers from the source of an earthquake can often be quantitatively D. V. Ramana (&) Æ R. K. Chadha Æ C. Singh Æ M. Shekar National Geophysical Research Institute, Hyderabad, India e-mail: dvr@ngri.res.in

586 Nat Hazards (27) 4:585 592 attributable to the poro-elastic response to the earthquake static strain field. The pre-seismic groundwater level changes observed in the Izu Peninsula were found to be associated with crustal deformation (Koisumi et al. 1999). Recent observations confirm that there is an additional class of water level response to large earthquakes that occurs at a distance of several hundreds of kilometers from the epicenter. The Shivajisagar reservoir behind Koyna Dam is situated in western India, about 2 km south of Bombay (Mumbai), in the state of Maharashtra. The dam was built in 1962 and was impounded in 1963. The largest triggered earthquake (M = 6.3) in the Koyna region occurred on December 1, 1967 and was followed by 2 earthquakes of M 5.; the region has been considered to be seismically active for the last four decades. The continued seismicity in Koyna and Warna continues to evoke interest amongst seismologists in the search for an understanding of seismogenesis (Gupta et al. 1972; Gupta 1992, 22). Increases in seismic activity have also been observed whenever the maximum water level in the reservoir level exceeds its previous high (Gupta and Rastogi 1976; Gupta 1983; Talwani et al. 1996; Talwani 1997). Prior to 1991 the majority of earthquakes were confined to a 2-km long seismic zone extending south from the Koyna dam. Since the impounding of the new Warna reservoir in 1993, south of Koyna, the earthquakes are mostly occurring near Warna, a shift in epicenter which may be related to the water level fluctuations in the reservoir. Prior to 1967, the region was believed to be seismically stable, with only two felt earthquakes reported in 1764 and 1913 (Verma 1985). The direct observation of pore pressure is aimed at gaining new insights into the role of fluids for the generation of seismicity. Changes in the pore pressure in rocks are now acknowledged to play an important role in earthquake processes. The measurement of the water level in wells provides a simple method for studying the variation of pore pressure in these rocks. To achieve this, since 1995 we have been monitoring fluctuations in water level that result from changes in pore pressure in the aquifers connected to deep bore wells in the region (Gupta et al. 2; Chadha et al. 23). The relation between the static deformation and the water level changes have also been studied in the Koyna region (Chadha et al. 24). In this paper, we correlate water level fluctuations in some wells and the seismicity in the region with respect to small-magnitude earthquakes. 2 Water level monitoring and data analysis A total of 21 bore wells were drilled during 1995 1998 in the Koyna-Warna region at depths ranging from 9 to 25 m (Fig. 1). The wells were equipped with pressure transducers and recording systems suitable for monitoring millimeter changes in the depth of water level in the wells. Other parameters, such as atmospheric pressure and rainfall, which also affect changes in the water level in bore wells are also being monitored in the region. Most of the well data exhibit tidal signals, thereby implying that the wells are sensitive to small strain changes in the order of nanometers and indicating that the aquifers are confined and can be used as strain meters. Fluctuations in the water level indicate changes in the pore pressure in the connected aquifers. In the present study, we have studied the water level response in different wells for two earthquakes of magnitudes 4.4 and 4.3. The earthquake of April 25, 1997 was located close to Koyna and the February 11, 1998 earthquake was close to the Warna reservoir.

Nat Hazards (27) 4:585 592 587 Fig. 1 The location of the Koyna and Warna reservoirs and bore wells (circles). Black dots the epicenters of earthquakes during 1996 1997, stars the main shocks 3 Case studies of water-level fluctuations resulting from moderately sized earthquakes 3.1 M = 4.4 earthquake of April 25, 1997 Figure 2 shows the monthly seismic activity (magnitude 2) in the Koyna-Warna region. It can be seen that micro-earthquake activity during the first 5 months is similar and can be compared to that of the last 7 months. The inset diagram in Fig. 2 is a frequency diagram of micro-earthquake activity (magnitude 2.) per month in the vicinity of the epicentral area of the April 25, 1997 earthquake. Analysis of this figure shows that the seismicity had increased in April prior to the main shock and that the enhanced micro-earthquake activity continued thereafter for a month, subsequently decreasing. The locations of the pre- and post-microearthquake activity with the main shock are shown in Fig. 3 together with the locations of nearby wells. 25 1997 Micro earthquakes in Koyna- Warna region No. of Events 2 15 1 5 M4.4 No. of events 14 12 1 8 6 4 2 March April May June Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Fig. 2 Histogram of the seismological events of 1997. Inset shows the events close to the epicenter of the main shock

588 Nat Hazards (27) 4:585 592 Fig. 3 The locations of the pre- and post-events of the main shock of the April 25, 1997 earthquake. Triangles Locations of wells, filled circles locations of micro-earthquakes (prior and following main shock), star main shock (M = 4.4) 17.38 17.37 17.36 1997 Micro earthquakes (March-June) RAS KON TAL KOY VAJ 17.35 GOV 17.34 CHA 17.33 73.65 73.69 73.73 73.77 73.81 73.85 We analyzed the water level time series at the nearest well, Taloshi (TAL), to determine if the April 25, 1997 earthquake had caused any fluctuations in the water level and observed that co-seismic water level fluctuations had occurred. The time series of the water levels at Taloshi for March 1 May 31 are shown in Fig. 4; this period spans the pre- and post-time period of the main shock. From the observed water levels shown in the Fig. 4a, it is clearly evident that this well responds very well to the tidal effects; in comparison, Fig. 4b shows the water level fluctuations following removal of the tidal effects. Analysis of this figure shows that changes in the water levels occurred prior to the earthquake of April 1997. Figure 4c shows the hourly differences in the water level fluctuations and the standard deviations of these differences. This exercise was carried out with a view to discern changes in the water levels due to smaller magnitude earthquakes. The most notable feature of these graphs is the preparatory stage of the earthquake just prior to the main shock, which is conspicuous in Fig. 4b. However, in the case of the differences in the water level fluctuations on a per-hour basis, the anomalous changesgreater than two standard deviations (SD =.13) could be correlated with micro-earthquakes whose magnitudes were between 2. and 4.4 and which occurred prior to and following the main shock, respectively. 3.2 M = 4.3 earthquake of February 11, 1998 The February 11, 1998 earthquake of M = 4.3 occurred close to the Warna reservoir. The main shock and the micro-earthquake s pre- and post-main shock are plotted in Fig. 5 together with the water level in the wells, which responded to this earthquake. The monthly frequency diagram of the 1998 earthquake activity is shown in Fig. 6. Analysis of this figure reveals enhanced micro-earthquake activity, especially following the main shock; the phenomenon was observed for a month, following which time the activity decayed.

Nat Hazards (27) 4:585 592 589 Taloshi 1997 Changes in water level (cm).6.4.2. -.2 -.4 S.D. =.13 M4.4 (c) Water level fluctuations (cm) 7 5 3 1-1 -3-5 (b) Water level (cm) 43 415 4 (a) 385 1997/3/1 1997/4/1 1997/5/1 1997/6/1 Fig. 4 (a) Observed water levels in Taloshi (TAL), (b) water level fluctuations after removing the tidal effects, (c) hourly fluctuations in water level. Filled circles in (c) represent the micro-earthquake events of magnitudes 2 4.4 Fig. 5 The locations of the pre- and post-events of the main shock of February 11, 1998. Stars Main shock with M = 4.3 (February 11) and M = 4.2 (February 14), triangles locations of the wells, filled circles locations of the micro-earthquakes 17.25 17.21 17.17 1998 Micro earthquakes (Jan. - March) SHR NAY 17.13 MAN 17.9 UKA 17.5 73.6 73.7 73.8 73.9

59 Nat Hazards (27) 4:585 592 6 5 1998 Micro earthquakes in Koyna - Warna region 3 25 No. of Events 4 3 2 4.3 No. of events 2 15 1 5 Jan Feb March 1 Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Fig. 6 The histogram of the seismic events of 1998. Inset shows the events close to the epicenter of the main shock Figure 7a shows water level fluctuations together with tidal effects at Ukalu, Fig. 7b depicts water level fluctuations following the remval of the tidal effect, and Fig. 7c shows the hourly differences in the water levels with standard deviation (SD =.3). Further analysis of Fig. 7b shows a conspicuously long wavelength Changes in water level (cm) 3 2 1-1 -2 S.D.=.3 Ukalu 1998 M4.3 (c) Water level fluctuations (cm) Water level (cm) 8 6 4 2-2 -4-6 24 22 2 18 16 14 12 1 1998/1/1 1998/2/1 1998/3/1 (b) (a) 1998/4/1 Fig. 7 (a) Observed water levels in Ukalu, (b) water level fluctuations after removing the tidal effects, (c) hourly changes in the water level fluctuations

Nat Hazards (27) 4:585 592 591 feature, especially following the main shock. This probably signifies the post-volumetric strain relaxation. An M = 4.2 earthquake also followed the main shock. The qualitative analysis of the hourly differences in the water level fluctuations reveals conspicuous anomalous excursions in the rise and fall of water levels that are considered to be significant above the standard deviation (SD =.3). These changes can be correlated with the micro-earthquakes that occurred prior and following the main shock. 4 Conclusions The water level fluctuations due to the M = 4.4 earthquake of April 25, 1997 and the M = 4.3 earthquake of February 11, 1998 in the Koyna-Warna region have been studied in detail. The micro-earthquake activity close to the source region was found to have increased before the occurrence of the main shocks and to have continued for a month before once again reaching background level. The hourly changes in the water levels of more than two standard deviation are considered to be significant as the water level fluctuations could be correlated with the observed micro-earthquake activity. These water level fluctuations were observed at Taloshi for the M = 4.4 earthquake of April 25, 1997 and at Ukalu for M = 4.3 earthquake of February 11, 1998, respectively. Based on the water level fluctuations and aftershock activity we infer that the relaxation of volumetric strain changes is observed for over a month. We suggest that the monitoring of water level fluctuations as a means to deciphering anomalous changes has a great potential in providing information on earthquake mechanisms in the Koyna-Warna region and can be extended to other seismically active reservoirs as a means to develop suitable mathematical models to quantify the phenomenon. Acknowledgements The authors are grateful to the Director, National Geophysical Research Institute for permission to publish this work. References Brodsky EE, Roeloffs W, Woodcock D, Gall I (23) A mechanism for sustained groundwater pressure changes induced by distant earthquakes. J Geophys Res 18:239 Chadha RK, Pandey AP, Kuempel HJ (23) Search for earthquake precursors in well water levels in a localized seismically active area of Reservoir Triggered Earthquakes in India. Geophys Res Lett 3:69-1 69-4 Chadha RK, Srivastava K, Kumpel H-J (24) Earthquake-related changes in well water level and their relation to a static deformation model for the seismically active Koyna-Warna region, India. In: Rummel F (ed) Rock mechanics with emphasis on stress. Oxford & IBH Publ., New Delhi, pp 135 15 Chia Y, Wang YS, Chiu JJ, Liu CW (21) Changes of Groundwater level due to the 1999 Chi-Chi earthquake in the Choshui River Alluvial Fan in Taiwan. B.S.S.A. 91:162 168 Chia Y, Wang YS, Huang CC, Chen JS, Wu HP (22) Coseismic changes of groundwater level in response to the 1999 Chi-Chi earthquake. West Pac Earth Sci 2:261 272 Grecksch G, Roth F, Kumpel HJ (1999) Coseismic well level changes due to the 1992 Roermond earthquake compared with static deformation of half space solutions. Geophys J Int 138:47 478 Gupta HK (1983) Induced seismicity hazard mitigation through water level manipulation: a suggestion. Bull Seismol Soc Am 73:679 682 Gupta HK (1992) Reservoir induced earthquakes. Elsevier, Amsterdam Gupta HK (22) A review of recent studies of triggered earthquakes by artificial water reservoirs with special emphasis on earthquakes in Koyna, India. Earth Sci Rev 58:279 31

592 Nat Hazards (27) 4:585 592 Gupta HK, Rastogi BK (1976) Dams and earthquake. Elsevier, Amsterdam Gupta HK, Rastogi BK, Narain H (1972) Common features of the reservoir associated seismic activities. Bull Seismol Soc Am 62:481 492 Gupta HK, Radhakrishna I, Chadha RK, Kumpel HJ, Grecksch G (2) Pore pressure studied initiated in area of reservoir induced earthquakes in India, EOS. Trans Am Geophys Union 81:145 151 Igarashi G, Wakita H, Sato T (1992) Precursory and coseismic anomalies in well water levels observed for the February 2, 1992, Tokyo. Bay earthquake. Geophys Res Lett 19:1583 1586 Koisumi N, Tsukuda E, Kamigaichi O (1999) Preseismic changes in groundwater level and volumetric strain associated with earthquake swarms off the east coast of the Izu Peninsula, Japan. Geophys Res Lett 26:359 3512 Roeloffs EA (1996) Poroelastic techniques in the study of earthquake-related hydrologic phenomena. In: Dmoeska R (ed) Advances in geophysics, vol 37. Academic Press, San Diego, pp 135 195 Roeloffs EA (1998) Persistent water level changes in a well near Parlfield, California, due to local and distant earthquakes. J Geophys Res 13:869 889 Talwani P (1997) Seismotectonics of the Koyna Warna Area, India. Pure Appl Geophys 15:511 55 Talwani P, Kumar Swamy SV, Sawalwade CB (1996) The revaluation of seismicity data in the Koyna-Warna area, 1963 1995. Univ S C Tech Rep Verma RK (1985) Gravity field, seismicity and tectonics of the Indian Peninsula and the Himalaya. Allied Publ, India Wakita H (1975) Water wells as possible indicators of tectonic strain. Science 189:553 555 Wang CY, Wang CH, Kuo CH (24) Temporal changes in groundwater level following the 1999 (Mw = 7.5) Chi-Chi earthquake. In: Geofluids, vol 4. Taiwan