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Earthquakes continue to occur in the vicinity of Shivaji Sagar Lake since its creation by the Koyna Dam in 1962. The seismicity peaked in 1967 with a M 6.3 earthquake which claimed over 200 human lives and destroyed the Koyna township. Earthquakes of M`4 occur every year following an increase of water level in the reservoir. During 1973During , 1980During and 1993 earthquakes exceeding magnitude 5 occurred. Most earthquakes of M`4 are associated with pronounced foreshocks and aftershocks. Starting Sepember 1993, seismic monitoring was vastly improved with the deployment of additional close-by stations (analog and digital). The focal parameters now available have enabled delineation of the active faults and deciphering of the earthquake nucleation process. During 1995 -96, 13 boreholes were drilled to depths of 130 to 250 m and measurement of water levels in these wells was initiated. A preliminary analysis of one year's data from a borehole 1 km south of Koyna reveals tidal signatures, indicating connection of the well to a confined aquifer which is favorable for detection of pore pressure anomalies induced by crustal strain. We hope to improve our understanding of the genesis of reservoir-induced earthquakes at Koyna with these new measurements.

Predictive relations are developed for peak ground acceleration (PGA) from the engineering seismoscope (SRR) records of the 2001 M w 7.7 Bhuj earthquake and 239 strong-motion records of 32 significant aftershocks of 3.1 B M w B 5.6 at epicentral distances of 1 B R B 288 km. We have taken advantage of the recent increase in strong-motion data at close distances to derive new attenuation relation for peak horizontal acceleration in the Kachchh seismic zone, Gujarat. This new analysis uses the Joyner-Boore's method for a magnitude-independent shape, based on geometrical spreading and anelastic attenuation, for the attenuation curve. The resulting attenuation equation is, lnðYÞ ¼ À7:9527 þ 1:4043 M W À ln r 2 jb þ 19:82 2 1=2 À0:0682 S for 3:1 \ M W 7:7 std: dev: r ð Þ : AE0:8243; where, Y is peak horizontal acceleration in g, M w is moment magnitude, r jb is the closest distance to the surface projection of the fault rupture in kilometers, and S is a variable taking the values of 0 and 1 according to the local site geology. S is 0 for a rock site, and, S is 1 for a soil site. The relation differs from previous work in the improved reliability of input parameters and large numbers of strong-motion PGA data recorded at short distances (0-50 km) from the source. The relation is in demonstrable agreement with the recorded strongground motion data from earthquakes of M w 3.5, 4.1, 4.5, 5.6, and 7.7. There are insufficient data from the Kachchh region to adequately judge the relation for the magnitude range 5.7 B M w B 7.7. But, our groundmotion prediction model shows a reasonable correlation with the PGA data of the 29 March, 1999 Chamoli main shock (M w 6.5), validating our ground-motion attenuation model for an M w 6.5 event. However, our groundmotion prediction shows no correlation with the PGA data of the 10 December, 1967 Koyna main shock (M w 6.3). Our ground-motion predictions show more scatter in estimated residual for the distance range (0-30 km), which could be due to the amplification/noise at near stations situated in the Kachchh sedimentary basin. We also noticed smaller residuals for the distance range (30-300 km), which could be due to less amplification/noise at sites distant from the Kachchh basin. However, the observed less residuals for the longer distance range (100-300 km) are less reliable due to the lack of available PGA values in the same distance range.Pure and Applied Geophysics studied intraplate region in the world. However, no strong-motion records from large earthquakes (M w > 7) are available for ENA. Thus, the ground-motion attenuation relations for ENA have been estimated based on stochastic modeling using constraints from the strong-motion records of smaller earthquakes (M B

The Koyna Dam site in western India has been seismically active since its impounding in 1962. The seismicity peaked in 1967 when an earthquake of magnitude 6.3 claimed 200 human lives and caused some damage to the dam itself. Subsequently, bursts of seismicity with M ≧ 5 earthquakes have occurred during 1973, 1980, and 1993 to 1995. The epicenters during the latest burst are concentrated around the newly impounded Warna reservoir, 25 km south of Koyna. Several shocks of smaller magnitude have occurred in this area since 1967 under the influence of the Koyna (Shivajisagar) reservoir, but the impoundment of the Warna reservoir to over 60 m has led to the enhanced seismic activity in the region. The space-time pattern of epicenters located for the period 28 August 1993 to 31 December 1995 clearly delineates two parallel trends in the NNE-SSW direction. The correlation between seismic activity and water levels in both the reservoirs suggests that the present activity was influenced more by the Warna than the Koyna reservoir. The discriminatory characteristics of reservoir-induced seismicity based on b-value, foreshock-aftershock pattern, and decay rate of aftershocks continue to be valid.