• K S Prakasam

      Articles written in Journal of Earth System Science

    • Shear wave splitting observations from the Indian shield

      D S Ramesh K S Prakasam

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      Measurements of shear wave splitting of the waveforms of SKS, SKKS phases recorded at all WWSSN stations (1977–1988) in the Indian shield located on diverse geotectonic units are used to retrieve the anisotropic properties of the sub-Continental lithosphere beneath these regions. The azimuth of fast polarization direction (FPD) ‘α’ and delay time ‘δt’ of the split shear waves with their uncertainties are estimated. Events well distributed in azimuth yield tightly constrained average splitting parameters of α, δt that are roughly:KOD (ENE. 0.50s); HYB (NNE, 145s); POO (N-S, 0.9s); NDI (NE, 0.95s). No consistent anisotropic direction was found at SHL, though the phenomenon of shear wave splitting was clearly observed. In order to test the utility of analog data to document such secondary effects and to authenticate our digitizing procedures, results from GEOSCOPE digital data at HYB were compared with analog data results from the same location. Presence of detectable anisotropy at all the stations is explained either in terms of past and present deformations by tectonic episodes or by plate motion related strain which forms the two end member models in interpreting the observed azimuthal anisotropy. Knowledge of surface geology and maximum horizontal compressive stress (MHS) orientations are invoked to constrain the most plausible hypothesis that explains the observed anisotropic signatures at each of these locations.

    • Seismic evidence for thick and underplated late Archaean crust of eastern Dharwar craton

      S S Rai P V S S Rajagopala Sarma K S Prakasam V K Rao

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      The deep crustal structure of eastern Dharwar craton has been investigated through τ-p extremal inversion of P-wave travel times from a network of seismographs recording quarry blasts. Travel times have been observed in the distance range 30–250 km in a laterally homogeneous lithospheric segment Main features of the inferred velocity-depth relationship include: (a) 29 km thick combined upper and middle crust velocity varying from 6 km/s to 7 km/s, with no observable velocity discontinuity in this depth range; (b) a lower crust (∼ 29–41 km) with velocity increasing from 7.0 to 7.3 km/s; (c) an average upper mantle velocity of 8.1 km/s; and (d) presence of a 12 km thick high velocity crustal layer (7.4 – 7.8 km/s) in the depth range 41–53 km, with a distinct velocity gradient marking a velocity increase of 0.4 km/s. The anomalous 53 km thick crust is viewed as a consequence of magmatic underplating at the base of the crust in the process of cratonization of the eastern Dharwar craton during late Archaean. The underplated material reflects here with the velocity of 7–3 to 7–8 km/s below the depth of 40 km. Our proposition of magmatic underplating is also supported by the presence of large scale I-granitoid, a product of partial melting of the upper mantle material.

    • What triggers Koyna region earthquakes? Preliminary results from seismic tomography digital array

      S S Rai Sunil K Singh P V S S Rajagopal Sarma D Srinagesh K N S Reddy K S Prakasam Y Satyanarayana

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      The cause for prolific seismicity in the Koyna region is a geological enigma. Attempts have been made to link occurrence of these earthquakes with tectonic strain as well as the nearby reservoirs. With a view to providing reliable seismological database for studying the earth structure and the earthquake process in the Koyna region, a state of the art digital seismic network was deployed for twenty months during 1996–97. We present preliminary results from this experiment covering an area of 60 × 80 km2 with twenty seismic stations. Hypocentral locations of more than 400 earthquakes confined to 11×25 km2 reveal fragmentation in the seismicity pattern — a NE — SW segment has a dip towards NW at approximately 45°, whilst the other two segments show a near vertical trend. These seismic segments have a close linkage with the Western Ghat escarpment and the Warna fault. Ninety per cent of the seismicity is confined within the depth range of 3–10 km. The depth distribution of earthquakes delimits the seismogenic zone with its base at 10 km indicating a transition from an unstable to stable frictional sliding regime. The lack of shallow seismicity between 0 and 3 km indicates a mature fault system with well-developed gouge zones, which inhibit shallow earthquake nucleation. Local earthquake travel time inversion for P- and S-waves show ≈ 2% higher velocity in the seismogenic crust (0–10 km) beneath the epicentral tract relative to a lower velocity (2–3%) in the adjoining region. The high P- and S-wave velocity in the seismogenic crust argues against the presence of high pressure fluid zones and suggests its possible linkage with denser lithology. The zone of high velocity has been traced to deeper depths (≈ 70 km) through teleseismic tomography. The results reveal segmented and matured seismogenic fault systems in the Koyna region where seismicity is possibly controlled by strain build up due to competent lithology in the seismic zone with a deep crustal root.

    • Pn wave velocity and Moho geometry in north eastern India

      S S Rai K S Prakasam N Agrawal

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      Pn velocity has been computed across the NE India and Moho geometry constrained, using regional earthquake travel times recorded by a network of 30 seismological stations operated during February-May 1993. Using an appropriate velocity model and the arrival times at the network stations, preliminary hypocentres of 16 regional earthquakes provided by NEIC were also improved. The average Pn wave velocity in NE India has been found to be 8.5 ±0.2 km/s. It varies from 8.3 to 8.5 km/s beneath the Shillong Plateau, Mikhir hills and Assam valley, which is significantly higher than those in other parts of India. The crustal thickness in NE India is also high, varying from 45–49 km under the Shillong plateau and the adjoining region to 55–65 km in the convergence zone. The presence of a thick crust and high Pn velocity suggests that the lithosphere in NE India is colder, as also indicated by the observed deeper level (45-51 km) seismicity of the region.

    • The South India Precambrian crust and shallow lithospheric mantle: Initial results from the India Deep Earth Imaging Experiment (INDEX)

      S S Rai Kajaljyoti Borah Ritima Das Sandeep Gupta Shalivahan Srivastava K S Prakasam K Sivaram Sudesh Kumar Rishikesh Meena

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      We present here the most comprehensive study of the thickness and composition ($Vp/Vs$ ratio) of the South India Precambrian crust and the nature of shallower mantle inferred from analysis of teleseismic receiver functions from 70 broad-band seismic stations operated as a part of the India Deep Earth Imaging Experiment (INDEX). South India could be broadly divided into regions with thin crust (32–38 km) and thick crust (38–54 km). Thin crust domains include the East Dharwar Craton (EDC), Cuddapah basin and Madurai/Kerala Khondalite Block. The thicker crust domain includes the Western Dharwar Craton (WDC) and northern part of Southern Granulite Terrain. The WDC shows progressive increase in thickness from 38 km in north to 46–54 km in south, compared to an almost flat Moho beneath the EDC. Compositionally, most of the crustal domains are felsic to intermediate ($Vp/Vs$ ∼ 1.69–1.75) except the mid Archean block in the southern WDC where it is mafic ($Vp/Vs$ < 1.81). Considering erosion depth in the WDC, we argue for Himalaya like ∼70 km thick crust beneath it during the Archean. Variation in crustal thickness does not have a first-order influence on regional topography in South India and suggests significant role for the crustal composition. We also present evidence of mid-lithospheric low velocity at ∼85–100 km beneath South India.

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