S U M M A R YThe Koyna-Warna region in western India is the best example of reservoir triggered seismicity. The world's largest triggered earthquake of magnitude 6.3 occurred in Koyna in 1967, followed by several moderate to small earthquakes ever since. A digital seismograph network deployed for seismic monitoring during 2005 August to 2008 December has indicated a shift in concentration of seismicity towards south in the Warna region including a new zone of seismic activity to the southwest. During the observation period 13 earthquakes of magnitude 4 and greater have occurred of which 11 occurred near the Warna region while only two occurred in Koyna. In this study we modelled broad-band waveform data of six of these earthquakes near Warna using waveform inversion approach. Initially a new velocity model was determined using a joint hypocentral determination approach that simultaneously solves for the velocity structure as well as the hypocentral parameters. A trap thickness of 1.2 km with a P-wave velocity of 4.40 km s −1 and an upper crustal layer down to 10 km with a velocity of 5.96 km s −1 are obtained. The new model not only provides the minimum residual error for the traveltimes, but consistently provides the least mismatch error in the waveform inversion of all the events, performing better than any of the previously determined velocity models. In general, focal mechanisms of normal type with NS to NNW-SSE oriented fault planes are obtained for all these events that are correlated with probable faults inferred from satellite images and aeromagnetic anomalies. Focal depths in the range of 5-6 km are obtained for earthquakes in the Warna region based on the sensitivity of whole waveform inversion at local distances. It is felt that joint inversion of waveform data of several earthquakes with a wider spatial distribution, along with the velocity structure would help in precisely characterizing the faulting mechanism and seismogenic depth range for the Koyna-Warna region in future.
high-quality magnetotelluric data at 100 stations, provide both regional information about the thickness of the Deccan Traps and the occurrence of localized density heterogeneities and anomalous conductive zones in the vicinity of the hypocentral zone. Acquisition of airborne LiDAR data to obtain a high-resolution topographic model of the region has been completed over an area of 1,064 km 2 centred on the Koyna seismic zone. Seismometers have been deployed in the granitic basement inside two boreholes and are planned in another set of six boreholes to obtain accurate hypocentral locations and constrain the disposition of fault zones.
Koyna, located in the Deccan Volcanic Province in western India, is the most significant site of reservoir triggered seismicity (RTS) globally. The largest RTS event of M 6.3 occurred here on December 10, 1967. RTS at Koyna has continued. This includes 22 M≥5.0 and thousands of smaller events over the past 50 years. The annual loading and unloading cycles of the Koyna Reservoir and the nearby Warna Reservoir influence RTS. Koyna provides an excellent natural laboratory to comprehend the mechanism of RTS because earthquakes here occur in a small area, mostly at depths of 2–7 km, which are accessible for monitoring. A deep borehole laboratory is therefore planned to study earthquakes in the near-field to understand their genesis, especially in an RTS environment. Initially, several geophysical investigations were carried out to characterize the seismic zone, including 5000 line kilometres of airborne gravity gradiometry and magnetic surveys, high-quality magnetotelluric data from 100 stations, airborne LiDAR surveys over 1064 km2, drilling of 8 boreholes of approximately 1500 m depth and geophysical logging. To improve the earthquake locations a unique network of borehole seismometers was installed in six of these boreholes. These results, along with a pilot borehole drilling plan, are presented here.
Earthquake activity is monitored in real time at the Koyna reservoir in western India, beginning from August 2005 and successful short term forecasts have been made of M ~ 4 earthquakes. The basis of these forecasts is the observation of nucleation that precedes such earthquakes. Here we report that a total of 29 earthquakes in the magnitude range of 3.5 to 5.1 occurred in the region during the period of August 2005 through May 2010. These earthquakes could broadly be put in three zones. Zone-A has been most active accounting for 18 earthquakes, while 5 earthquakes in Zone-B and 6 in Zone-C have occurred. Earthquakes in Zone-A are preceded by well defined nucleation, while it is not the case with zones B and C. This indicates the complexity of the earthquakes processes and the fact that even in a small seismically active area of only 20 km x 30 km earthquake forecast is difficult.
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