(The solid line indicates the mean values and +/− 1 standard deviation is represented by the dashed lines. Stars represent the predicted values and the observed PGA values are represented by circles)
In India, the first observation of Reservoir Triggered Seismicity (RTS) was noticed in the vicinity of Koyna dam just after its impoundment in 1962. This phenomenon successfully explains the observation of intra plate seismicity which corresponds to the impoundment of the reservoir. Another reservoir, Warna was created in 1985 and it also contributed to the RTS in the region. The seismicity occurs within a small region of 30 km X 20 km. In order to monitor the seismic activity, initially CSIR-NGRI took initiative to deploy digital seismic stations, however at present 15 surface broadband and 8 borehole short-period seismic stations are in operation. Previously, several observations have been made regarding the source processes and nature of seismicity in the region, such as the correlation between the water level and occurrence of seismicity, difference between the normal earthquake and RTS (Gupta et al., 1972a,b; Gupta and Rastogi, 1974; Gupta, 1992; Rastogi et al., 1997; Talwani, 1997). Recent analysis on variation in stress pattern and the segregation of focal mechanisms enabled to derive a tectonic model with alternating cycles of strike-slip and normal type (Rao and Shashidhar, 2016). Using the waveform inversion, a precise determination of focal depths has been attempted using local seismic waveforms (Shashidhar et al., 2011) identified the Donachiwada fault is the causative source for the 1967 Koyna earthquake (Mw6.3). A number of seismological studies have been carried out in this region to understand the source mechanism and structure, however, the triggering phenomenon of seismicity is poorly resolved. In this direction, to understand the role of fluid, pore pressure, loading and unloading of the reservoirs and source mechanism a major initiative were taken by the MoES to drill deep boreholes in and around the region. The main advantage of borehole observation is the increased sensitivity due to the rapid decrease in noise wave intensity with depth, since the interference consists mainly of surface waves. Scientific deep drilling in the region has revealed that the Deccan trap has 932.5m thick and underlain by the basement rock (Roy et al., 2013). The major science objectives and feasibility were discussed with the international community through ICDP workshops (Gupta and Nayak, 2011; Shashidhar et al., 2016). The deployment of borehole seismic sensors is first of its kind in India. The high signal to noise ratio waveforms has the potential not only to detect the sub M1.0 seismic events but also can provide structural information with unprecedented resolution. The results show that the absolute errors in locations of earthquake based on the borehole data ranges from 800 to 300m (Shashidhar et al., 2016; 2020). We aim for a detailed study of earthquake mechanism; to map the distribution of the faults based on the micro-seismicity and also to achieve the accurate velocity structure associated with the fault zones in this region.
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Senior Technical Officer(2)