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. 2001, 2002; Brodsky et al. 2003; Wang et al. 2004). The pre-seismic changes are generally ex- plained 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 123 Nat Hazards (2007) 40:585–592 DOI 10.1007/s11069-006-9022-0 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 2005 / Accepted: 17 November 2005 / Published online: 15 November 2006 Ó Springer Science+Business Media B.V. 2006