Earthquake Hazard

Seismological studies at Koyna:

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.

A network map of the borehole and broadband seismic stations in the Koyna-Warna region. In the inset, India map indicates the study region.
Earthquakes of M≥4.0 in the Koyna-Warna region since 1967. The star (in red) denotes the largest earthquake of M 6.3 of 10 December 1967. The open circles in white are the locations of events without focal mechanism solutions, while the yellow ones are the recent ones with mechanisms. The lines are the faults / lineaments inferred from LANDSAT images (Langston 1981). Also seen are the Koyna and Warna reservoirs situated about 25 km apart, just east of the Western Ghat escarpment. (Rao and Shashidhar 2016)
The comprehensive set of 50 earthquake focal mechanism solutions in the Koyna-Warna region compiled from previous studies (grey) and from moment tenor inversion of waveform data by the authors in their previous work and the present work (black). (Rao and Shashidhar 2016).
Tectonics of the Koyna region as inferred jointly from the 1967 (M 6.3) and the 2012 (Mw 4.8) earthquakes are plotted with the topography of the study region as indicated in figure1. Focal mechanism solution for the 1967 earthquake is by Chandra (1976)-b while the solution for the 2012 earthquake is from the present study. Circles represent the aftershocks including the largest aftershock of Mw 3.7 located very close to the Mw 4.8 earthquake. KRFZ is the Koyna River Fault Zone and D is the Donachiwada fault. (Shashidhar et al. 2013)
Focal mechanism solutions in the Koyna–Warna region plotted as a function of time from 1967 to 2017 show a periodic variation of predominantly strike-slip (SS), normal type(N) mechanisms over the years. The dotted line indicates the lower latitude limit of the M ≥ 4 earthquakes so far except for the earthquake of Mw 4.0 of 2017. (Shashidhar et al. 2019).
Comparison of event locations obtained by combining surface and borehole networks using the ‘HYPOCENTRE’ location algorithm (squares), event locations retrieved using borehole network data and a grid search algorithm (triangles) and event locations attained employing borehole network data and a cross-correlation relative location algorithm (circles). Western Ghats escarpment as well as water reservoirs close to Kandavan and Paraleninai marked by arrows. Dashed line: deep-reaching lineament taken from Arora et al. (2018). UDG: Udgiri surface station. (Shashidhar et al. 2020).
Individual splitting measurements (lines) made using the local S phases plotted at the stations (circles). Rose diagram of the fast polarization azimuths (grey shaded) is shown for the surface stations within the rectangles and at the respective stations (circle). At the respective stations, the stereographic projection of the fast polarization azimuths and delay times (lines) are plotted as a function of backazimuth and incidence angle. In each stereograph, the circles represent the incidence angle from 10° to 50° with a 10° increment. The orientation and length of the lines correspond to the fast polarization azimuth and delay time, respectively. The blue and magenta lines correspond to the surface and borehole stations. The grey arrow represents the absolute plate motion direction in a no-net rotation frame. (Roy and Shashidhar 2023).

 

Head of the group

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Group Members


Dr. Vijaya Raghavan R

Dr.Vijayaraghavan R

Chief Scientist

Dr. Shekar M

Dr. Shekar M

Principal Scientist

Dr. Shashidhar D

Dr. Shashidhar D

Principal Scientist

Dr. Naba Kanta Borah

Dr.Naba Kanta Borah

Senior Scientist

Dr. Pavan Kumar Vengala

Dr. Pavan Kumar Vengala

Senior Scientist

Dr. Nitin Sharma

Dr. Nitin Sharma

Scientist

Srinivas Dakuri

Dr. Srinivas Dakuri

Senior Technical Officer(1)

Suresh Gudapati

Dr. Suresh Gudapati

Senior Technical Officer(1)

Sanjeeb Ghosh

Sanjeeb Ghosh

Annapurna B

Annapurna B