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Bora D.K.,Diphu Government College | Sokolov V.Y.,Karlsruhe Institute of Technology | Wenzel F.,Karlsruhe Institute of Technology
Geomatics, Natural Hazards and Risk | Year: 2014

We focused on validation of applicability of semi-empirical technique (spectral models and stochastic simulation) for the estimation of ground-motion characteristics in the northeastern region (NER) of India. In the present study, it is assumed that the point source approximation in far field is valid. The one-dimensional stochastic point source seismological model of Boore (1983) (Boore, DM. 1983. Stochastic simulation of high frequency ground motions based on seismological models of the radiated spectra. Bulletin of Seismological Society of America, 73, 1865–1894.) is used for modelling the acceleration time histories. Total ground-motion records of 30 earthquakes of magnitudes lying between MW 4.2 and 6.2 in NER India from March 2008 to April 2013 are used for this study. We considered peak ground acceleration (PGA) and pseudospectral acceleration (response spectrum amplitudes) with 5% damping ratio at three fundamental natural periods, namely: 0.3, 1.0, and 3.0 s. The spectral models, which work well for PGA, overestimate the pseudospectral acceleration. It seems that there is a strong influence of local site amplification and crustal attenuation (kappa), which control spectral amplitudes at different frequencies. The results would allow analysing regional peculiarities of ground-motion excitation and propagation and updating seismic hazard assessment, both the probabilistic and deterministic approaches. © 2014 Taylor & Francis

Bora D.K.,Diphu Government College | Bora D.K.,CSIR - Central Electrochemical Research Institute | Baruah S.,CSIR - Central Electrochemical Research Institute
Journal of Asian Earth Sciences | Year: 2012

In this study we have tried to detect and collect later phases associated with Moho discontinuity and used them to study the lateral variations of the crustal thickness in Shillong-Mikir Hills Plateau and its adjoining region of northeastern India. We use the inversion algorithm by Nakajima et al. (Nakajima, J., Matsuzawa, T., Hasegawa, A. 2002. Moho depth variation in the central part of northeastern Japan estimated from reflected and converted waves. Physics of the Earth and Planetary Interiors, 130, 31-47), having epicentral distance ranging from 60. km to 150. km. Taking the advantage of high quality broadband data now available in northeast India, we have detected 1607 Moho reflected phases (PmP and SmS) from 300 numbers of shallow earthquake events (depth ≤ 25 km) in Shillong-Mikir Hills Plateau and its adjoining region. Notably for PmP phase, this could be identified within 0.5-2.3. s after the first P-arrival. In case of SmS phase, the arrival times are observed within 1.0-4.2. s after the first S-arrival. We estimated the crustal thickness in the study area using travel time difference between the later phases (PmP and SmS) and the first P and S arrivals. The results shows that the Moho is thinner beneath the Shillong Plateau about 35-38. km and is the deepest beneath the Brahmaputra valley to the north about 39-41. km, deeper by 4-5. km compared to the Shillong Plateau with simultaneous observation of thinnest crust (~33. km) in the western part of the Shillong Plateau in the Garo Hills region. © 2011 Elsevier Ltd.

Bora D.K.,Diphu Government College | Baruah S.,CSIR - Central Electrochemical Research Institute
Geomatics, Natural Hazards and Risk | Year: 2012

In this study, an attempt is made to estimate the depth of mid-crustal discontinuity beneath the Shillong Plateau in northeast India region using broadband seismogram of local earthquakes. Principle of the technique is to relate the seismic travel times of the reflected phases (SxS) with the crustal thickness above the discontinuity. Though mid-crustal discontinuity (or Conrad discontinuity) is reported in some parts of the world, no such study was undertaken in the present study area due to complexity in analogue seismograms recorded before 2001. The digital waveforms of the local seismic events recorded by broadband digital network in the study area, however, make it possible precise detection of the seismic phases that are reflected at this discontinuity. The results show that the mid-crustal discontinuity exists at a depth 18±0.5 km beneath the Shillong Plateau, which provide a better understanding of the crustal velocity structure of the region for future study. © 2012 Copyright Taylor and Francis Group, LLC.

Biswas R.,Tezpur University | Baruah S.,CSIR - Central Electrochemical Research Institute | Bora D.K.,Diphu Government College | Kalita A.,CSIR - Central Electrochemical Research Institute
Pure and Applied Geophysics | Year: 2013

Microearthquake spectra from the Shillong region are analyzed to observe the effect of attenuation and site on these spectra. The spectral ratio method is utilized to estimate the Q values for both P- and S-waves in the subsurface layer, wherein the ratio of spectral amplitudes at lower and higher frequencies are taken into consideration for three stations at varying epicentral distances. Average estimates of Q P and Q S are 178 and 195. The ratio of Q S to Q P is estimated to be greater than 1 in major parts of the Shillong area, which can be related to the dry crust prevailing in the Shillong region. Typically, the variation in corner frequencies for these spectra is inferred to be characteristic of the site. Simultaneously, observations from spectral content of local earthquakes recorded at two different stations with respect to the reference site yield greater amplification of incoming seismic signals in the frequency range of 2-5 Hz, which is found to be well supported by the existing local lithology pertinent to that region. © 2013 Springer Basel.

Bora D.K.,Diphu Government College | Baruah S.,CSIR - Central Electrochemical Research Institute | Biswas R.,Tezpur University | Gogoi N.K.,CSIR - Central Electrochemical Research Institute
Bulletin of the Seismological Society of America | Year: 2013

We estimate the source parameters (seismic moment, source radius, stress drop, and source displacement) and scaling laws for local earthquakes that occurred in the Shillong-Mikir plateau, Assam Valley, and Arunachal Himalaya in northeast India during 2001-2008. The source parameters were determined using the spectral analysis of P waves from the vertical component seismograms, after cor-rection for attenuation. Seismic moments are observed within the range from 9:51 × 1012 to 2:74 × 1015 N·m; stress drop ranges from 4 × 105 to 9 × 107 Pa for the Brune model and 7 × 105 to 1 × 108 Pa for the Madariaga model. Seismic events in this study are prominent with an average stress drop of 0.1-10 MPa. The effect of site geology may be a contributing factor for such a variation in stress drop. Source dimensions are, however, found to be smaller within the major part of the plateau. It is suggested that local earthquakes in the region are associated with a brittle shear-failure mechanism on fault segments and/or the presence of weakened zones, and earth-quakes are triggered by low deviatoric stress. Empirical relations between Mw-ML and M0-ML are developed leading to the future prediction of calibration coefficients for the local earthquakes in the Shillong-Mikir plateau and its adjoining region.

Biswas R.,Tezpur University | Baruah S.,CSIR - Central Electrochemical Research Institute | Bora D.K.,Diphu Government College
Acta Geophysica | Year: 2013

We examine the influence of attenuation and site on the spectra of microearthquakes having origin within the Shillong region. The ratios of spectral amplitudes at lower and higher frequencies are measured for three different stations at varying epicentral distances to estimate Q value for both P- and S-wave in near and sub-surface layer. The average estimates of QP and QS are found to be 178 and 195. The ratio of QS to QP emerges to be greater than unity in major parts of the Shillong area, suggesting dominance of dry crust prevailing in Shillong region. The variation in corner frequencies for these spectra is inferred to be characteristics of the site. Besides, the disparity in spectral content with reference to hard rock site yields the inference that the incoming seismic signals get amplified considerably while traversing from southern part to northeastern part of Shillong, best outlined at 2 to 5 Hz, which is well corroborated by the existing lithology. © 2013 Versita Warsaw and Springer-Verlag Wien.

Kakati R.K.,Diphu Government College | Mehra R.,National Institute of Technology Jalandhar | Bhattacharjee B.,Gauhati University
International Journal of Applied Environmental Sciences | Year: 2011

In this paper we report our results on seasonal variation of indoor radon and thoron concentration of various places of Karbi Anglong District of Assam. The global position of Karbi Anglong district is-latitudinal extension 25°32' North-26°33' North and longitudinal extension 92°09' East-93°52' East. The technique of passive time integrated method has been adopted using LR-115 (type-II) detectors in plastic twin chamber dosimeter cup. Significant variations of indoor radon concentrations have been observed in various places of the studied locations for one complete session. The annual average indoor radon concentration varies from 62.81 Bqm-3 at Dokmoka to 251.19 Bqm-3 at Mongjang. For Thoron concentration the minimum and maximum average values as found from this study are 19.75 Bqm-3 at Taradubi and 43.47 Bqm-3 at Hojaipur. © Research India Publications.

Bora D.K.,Diphu Government College | Borah K.,Indian Institute of Science | Goyal A.,Indian Institute of Science
Journal of Asian Earth Sciences | Year: 2016

We estimated the shear-wave velocity structure and Vp/Vs ratio of the crust beneath the Sumatra region by inverting stacked receiver functions from five three-component broadband seismic stations, located in diverse geologic setting, using a well known non-linear direct search approach, Neighborhood Algorithm (NA). Inversion results show significant variation of sediment layer thicknesses from 1 km beneath the backarc basin (station BKNI and PMBI) to 3-7 km beneath the coastal part of Sumatra region (station LHMI and MNAI) and Nias island (station GSI). Average sediment layer shear velocity (Vss) beneath all the stations is observed to be less (~1.35 km/s) and their corresponding Vp/Vs ratio is very high (~2.2-3.0). Crustal thickness beneath Sumatra region varies between 27 and 35 km, with exception of 19 km beneath Nias island, with average crustal Vs ~3.1-3.4 km/s (Vp/Vs ~1.8). It is well known that thick sediments with low Vs (and high Vp/Vs) amplify seismic waves even from a small-magnitude earthquake, which can cause huge damage in the zone. This study can provide the useful information of the crust for the Sumatra region. Since, Sumatra is an earthquake prone zone, which suffered the strong shaking of Great Andaman-Sumatra earthquake; this study can also be helpful for seismic hazard assessment. © 2016 Elsevier Ltd.

In this study, we aim to improve the scaling between the moment magnitude (MW), local magnitude (ML), and the duration magnitude (MD) for 162 earthquakes in Shillong-Mikir plateau and its adjoining region of northeast India by extending the MW estimates to lower magnitude earthquakes using spectral analysis of P-waves from vertical component seismograms. The MW-ML and MW-MD relationships are determined by linear regression analysis. It is found that, MW values can be considered consistent with ML and MD, within 0.1 and 0.2 magnitude units respectively, in 90 % of the cases. The scaling relationships investigated comply well with similar relationships in other regions in the world and in other seismogenic areas in the northeast India region. © 2016 The Author(s)

Borah K.,Indian Institute of Science | Bora D.K.,Diphu Government College | Goyal A.,Indian Institute of Science | Kumar R.,Indian Institute of Science
Physics of the Earth and Planetary Interiors | Year: 2016

We estimated crustal shear velocity structure beneath ten broadband seismic stations of northeast India, by using H-Vp/Vs stacking method and a non-linear direct search approach, Neighbourhood Algorithm (NA) technique followed by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data. Results show significant variations of thickness, shear velocities (Vs) and Vp/Vs ratio in the crust of the study region. The inverted shear wave velocity models show crustal thickness variations of 32–36 km in Shillong Plateau (North), 36–40 in Assam Valley and ∼44 km in Lesser Himalaya (South). Average Vp/Vs ratio in Shillong Plateau is less (1.73–1.77) compared to Assam Valley and Lesser Himalaya (∼1.80). Average crustal shear velocity beneath the study region varies from 3.4 to 3.5 km/s. Sediment structure beneath Shillong Plateau and Assam Valley shows 1–2 km thick sediment layer with low Vs (2.5–2.9 km/s) and high Vp/Vs ratio (1.8–2.1), while it is observed to be of greater thickness (4 km) with similar Vs and high Vp/Vs (∼2.5) in RUP (Lesser Himalaya). Both Shillong Plateau and Assam Valley show thick upper and middle crust (10–20 km), and thin (4–9 km) lower crust. Average Vp/Vs ratio in Assam Valley and Shillong Plateau suggest that the crust is felsic-to-intermediate and intermediate-to-mafic beneath Shillong Plateau and Assam Valley, respectively. Results show that lower crust rocks beneath the Shillong Plateau and Assam Valley lies between mafic granulite and mafic garnet granulite. © 2016 Elsevier B.V.

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