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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 Source

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. Source

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. Source

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. Source

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. Source

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