DIPOK BORA | Diphu Govt. College, Assam University. (original) (raw)

Papers by DIPOK BORA

Research paper thumbnail of Simultaneous monitoring of soil 222Rn in the Eastern Himalayas and the geothermal region of eastern India: an earthquake precursor

Research paper thumbnail of Crustal shear-wave velocity structure beneath Sumatra from receiver function modeling

Journal of Asian Earth Sciences, 2016

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband sei... more We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong-Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto34-38 km thick crust beneath the Shillong Plateau which increases to 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto37-38 km beneath the Brahmaputra valley and 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin46-48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2-8 km along with a low velocity zone within 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin8-13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range 10−18kmandlowvelocityzonewithin10-18 km and low velocity zone within 10−18kmandlowvelocityzonewithin18-36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 30 0 toward northwest.

Research paper thumbnail of Estimation of crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau, Northeast India

International Journal of Earth Sciences, 2011

In this study, an attempt is made to determine seismic velocity structure of the crust and upper ... more In this study, an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the Shillong-Mikir Hills Plateau in northeast India region. The principle of the technique is to relate seismic travel times with crustal thickness above the Conrad and Moho discontinuities. Broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities. The results show that the Conrad discontinuity is at 18-20 (±0.5) km beneath the Shillong-Mikir Hills Plateau and the Moho discontinuity is at 30 ± 1.0 km beneath the Shillong Plateau and at 35 ± 1.0 km beneath the Mikir Hills.

Research paper thumbnail of Moho Depth Variation in Shillong-Mikir Hills Plateau and Its Adjoining Region of Northeastern India Estimated From Reflected and Converted Waves

In this study an attempt is made to estimate the depth distribution of Mohorovičić (Moho) discont... more In this study an attempt is made to estimate the depth distribution of Mohorovičić (Moho) discontinuity beneath the Shillong-Mikir plateau and its adjoining region of northeast India. We have used 195 earthquakes that are recorded by the permanent broadband network during 2001-2009 in this region. The first P and S wave arrivals reflected P (PmP) and S (SmS), converted P to S (PS) and S to P (SP) waves at the Moho discontinuity are precisely identified and analyzed. A total of 956 reflected (PmP and SmS) phase arrival times of 172 shallow (depth <25 km) earthquakes and 70 converted (PS and SP) phases from 23 intermediate depth (with depth > 38 km) earthquakes are used. The results show that the Moho depth beneath the Shillong plateaus is 33.5 -37.0 km, and it is deeper, about 39.0 -41.0 km beneath the Brahmaputra valley to the north.

Research paper thumbnail of The 10th General Assembly of Asian Seismological Commission (ASC), 2014

Research paper thumbnail of Author's personal copy Mapping the crustal thickness in Shillong–Mikir Hills Plateau and its adjoining region of northeastern India using Moho reflected waves

Journal of Asian Earth Sciences

a b s t r a c t In this study we have tried to detect and collect later phases associated with Mo... more a b s t r a c t 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 adjoin-ing region of northeastern India. We use the inversion algorithm by Nakajima et al. (Nakajima, J., Matsuz-awa, 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 epi-central 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 6 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 obser...

Research paper thumbnail of Estimation of crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau, Northeast India

International Journal of Earth Sciences, 2010

In this study, an attempt is made to determine seismic velocity structure of the crust and upper ... more In this study, an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the Shillong-Mikir Hills Plateau in northeast India region. The principle of the technique is to relate seismic travel times with crustal thickness above the Conrad and Moho discontinuities. Broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities. The results show that the Conrad discontinuity is at 18-20 (±0.5) km beneath the Shillong-Mikir Hills Plateau and the Moho discontinuity is at 30 ± 1.0 km beneath the Shillong Plateau and at 35 ± 1.0 km beneath the Mikir Hills.

Research paper thumbnail of Validation of strong-motion stochastic model using observed ground motion records in north-east India

Geomatics, Natural Hazards and Risk, 2014

We focused on validation of applicability of semi-empirical technique (spectral models and stocha... more 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 M W 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.

Research paper thumbnail of The 9th General Assembly of Asian Seismological Commission (ASC), 2012

Journal of the Geological Society of India, 2013

Research paper thumbnail of The effects of attenuation and site on the spectra of microearthquakes in the Jubilee Hills region of Hyderabad, India

Journal of Earth System Science, 2007

Microearthquake spectra from Jubilee Hills, Hyderabad are analyzed to observe the effect of atten... more Microearthquake spectra from Jubilee Hills, Hyderabad are analyzed to observe the effect of attenuation and site on these spectra. The ratios of spectral amplitudes at lower and higher frequency 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. Average estimates of Qp and Qs are 235 and 278 respectively. Value of Qs/Qp larger than 1.0 suggests dry crust for most of the Jubilee Hills region. The near-surface low Qp and Qs for 0 km to 0.9 km depth coincide with the soil layer, top and semi-weathered and highly fractured zone. In contrast, at a shallower depth beneath the Jubilee Hills area, Hyderabad, we obtain high Qp and Qs zone, which corresponds to the dense and high velocity rocks of the region. The varying corner frequencies for these spectra are inferred to be characteristics of site. Comparisons of disparity in spectral content with reference to hard rock site conclude that lithology of the northwest part of Jubilee Hills area amplify about twice the incoming seismic signal, as compared to the southern part best outlined at 8 to 10 Hz only.

Research paper thumbnail of Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

Journal of Asian Earth Sciences, 2014

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband sei... more We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong-Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto34-38 km thick crust beneath the Shillong Plateau which increases to 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto37-38 km beneath the Brahmaputra valley and 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin46-48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2-8 km along with a low velocity zone within 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin8-13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range 10−18kmandlowvelocityzonewithin10-18 km and low velocity zone within 10−18kmandlowvelocityzonewithin18-36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 30 0 toward northwest.

Research paper thumbnail of Mapping the crustal thickness in Shillong–Mikir Hills Plateau and its adjoining region of northeastern India using Moho reflected waves

Journal of Asian Earth Sciences, 2012

In this study we have tried to detect and collect later phases associated with Moho discontinuity... more 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. . 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 6 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.

Research paper thumbnail of Depth of mid-crustal discontinuity from reflected seismic waves on local earthquake seismograms recorded at Shillong Plateau, Northeast India

Geomatics, Natural Hazards and Risk, 2012

Research paper thumbnail of Estimation of Source Parameters of Local Earthquakes Originated in Shillong-Mikir Plateau and its Adjoining Region of Northeastern India

Bulletin of the Seismological Society of America, 2013

We estimate the source parameters (seismic moment, source radius, stress drop, and source displac... more 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 correction for attenuation. Seismic moments are observed within the range from 9:51 × 10 12 to 2:74 × 10 15 N·m; stress drop ranges from 4 × 10 5 to 9 × 10 7 Pa for the Brune model and 7 × 10 5 to 1 × 10 8 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 earthquakes are triggered by low deviatoric stress. Empirical relations between M w -M L and M 0 -M L are developed leading to the future prediction of calibration coefficients for the local earthquakes in the Shillong-Mikir plateau and its adjoining region.

Research paper thumbnail of Influence of attenuation and site on microearthquakes’ spectra in Shillong region, of Northeast, India: A case study

Acta Geophysica, 2013

We examine the influence of attenuation and site on the spectra of microearthquakes having origin... more 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 Q P and Q S are found to be 178 and 195. The ratio of Q S to Q P 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.

Research paper thumbnail of Simultaneous monitoring of soil 222Rn in the Eastern Himalayas and the geothermal region of eastern India: an earthquake precursor

Research paper thumbnail of Crustal shear-wave velocity structure beneath Sumatra from receiver function modeling

Journal of Asian Earth Sciences, 2016

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband sei... more We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong-Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto34-38 km thick crust beneath the Shillong Plateau which increases to 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto37-38 km beneath the Brahmaputra valley and 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin46-48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2-8 km along with a low velocity zone within 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin8-13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range 10−18kmandlowvelocityzonewithin10-18 km and low velocity zone within 10−18kmandlowvelocityzonewithin18-36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 30 0 toward northwest.

Research paper thumbnail of Estimation of crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau, Northeast India

International Journal of Earth Sciences, 2011

In this study, an attempt is made to determine seismic velocity structure of the crust and upper ... more In this study, an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the Shillong-Mikir Hills Plateau in northeast India region. The principle of the technique is to relate seismic travel times with crustal thickness above the Conrad and Moho discontinuities. Broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities. The results show that the Conrad discontinuity is at 18-20 (±0.5) km beneath the Shillong-Mikir Hills Plateau and the Moho discontinuity is at 30 ± 1.0 km beneath the Shillong Plateau and at 35 ± 1.0 km beneath the Mikir Hills.

Research paper thumbnail of Moho Depth Variation in Shillong-Mikir Hills Plateau and Its Adjoining Region of Northeastern India Estimated From Reflected and Converted Waves

In this study an attempt is made to estimate the depth distribution of Mohorovičić (Moho) discont... more In this study an attempt is made to estimate the depth distribution of Mohorovičić (Moho) discontinuity beneath the Shillong-Mikir plateau and its adjoining region of northeast India. We have used 195 earthquakes that are recorded by the permanent broadband network during 2001-2009 in this region. The first P and S wave arrivals reflected P (PmP) and S (SmS), converted P to S (PS) and S to P (SP) waves at the Moho discontinuity are precisely identified and analyzed. A total of 956 reflected (PmP and SmS) phase arrival times of 172 shallow (depth <25 km) earthquakes and 70 converted (PS and SP) phases from 23 intermediate depth (with depth > 38 km) earthquakes are used. The results show that the Moho depth beneath the Shillong plateaus is 33.5 -37.0 km, and it is deeper, about 39.0 -41.0 km beneath the Brahmaputra valley to the north.

Research paper thumbnail of The 10th General Assembly of Asian Seismological Commission (ASC), 2014

Research paper thumbnail of Author's personal copy Mapping the crustal thickness in Shillong–Mikir Hills Plateau and its adjoining region of northeastern India using Moho reflected waves

Journal of Asian Earth Sciences

a b s t r a c t In this study we have tried to detect and collect later phases associated with Mo... more a b s t r a c t 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 adjoin-ing region of northeastern India. We use the inversion algorithm by Nakajima et al. (Nakajima, J., Matsuz-awa, 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 epi-central 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 6 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 obser...

Research paper thumbnail of Estimation of crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau, Northeast India

International Journal of Earth Sciences, 2010

In this study, an attempt is made to determine seismic velocity structure of the crust and upper ... more In this study, an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the Shillong-Mikir Hills Plateau in northeast India region. The principle of the technique is to relate seismic travel times with crustal thickness above the Conrad and Moho discontinuities. Broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities. The results show that the Conrad discontinuity is at 18-20 (±0.5) km beneath the Shillong-Mikir Hills Plateau and the Moho discontinuity is at 30 ± 1.0 km beneath the Shillong Plateau and at 35 ± 1.0 km beneath the Mikir Hills.

Research paper thumbnail of Validation of strong-motion stochastic model using observed ground motion records in north-east India

Geomatics, Natural Hazards and Risk, 2014

We focused on validation of applicability of semi-empirical technique (spectral models and stocha... more 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 M W 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.

Research paper thumbnail of The 9th General Assembly of Asian Seismological Commission (ASC), 2012

Journal of the Geological Society of India, 2013

Research paper thumbnail of The effects of attenuation and site on the spectra of microearthquakes in the Jubilee Hills region of Hyderabad, India

Journal of Earth System Science, 2007

Microearthquake spectra from Jubilee Hills, Hyderabad are analyzed to observe the effect of atten... more Microearthquake spectra from Jubilee Hills, Hyderabad are analyzed to observe the effect of attenuation and site on these spectra. The ratios of spectral amplitudes at lower and higher frequency 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. Average estimates of Qp and Qs are 235 and 278 respectively. Value of Qs/Qp larger than 1.0 suggests dry crust for most of the Jubilee Hills region. The near-surface low Qp and Qs for 0 km to 0.9 km depth coincide with the soil layer, top and semi-weathered and highly fractured zone. In contrast, at a shallower depth beneath the Jubilee Hills area, Hyderabad, we obtain high Qp and Qs zone, which corresponds to the dense and high velocity rocks of the region. The varying corner frequencies for these spectra are inferred to be characteristics of site. Comparisons of disparity in spectral content with reference to hard rock site conclude that lithology of the northwest part of Jubilee Hills area amplify about twice the incoming seismic signal, as compared to the southern part best outlined at 8 to 10 Hz only.

Research paper thumbnail of Crustal shear-wave velocity structure beneath northeast India from teleseismic receiver function analysis

Journal of Asian Earth Sciences, 2014

We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband sei... more We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong-Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto34-38 km thick crust beneath the Shillong Plateau which increases to 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto37-38 km beneath the Brahmaputra valley and 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin46-48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2-8 km along with a low velocity zone within 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin8-13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range 10−18kmandlowvelocityzonewithin10-18 km and low velocity zone within 10−18kmandlowvelocityzonewithin18-36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 30 0 toward northwest.

Research paper thumbnail of Mapping the crustal thickness in Shillong–Mikir Hills Plateau and its adjoining region of northeastern India using Moho reflected waves

Journal of Asian Earth Sciences, 2012

In this study we have tried to detect and collect later phases associated with Moho discontinuity... more 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. . 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 6 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.

Research paper thumbnail of Depth of mid-crustal discontinuity from reflected seismic waves on local earthquake seismograms recorded at Shillong Plateau, Northeast India

Geomatics, Natural Hazards and Risk, 2012

Research paper thumbnail of Estimation of Source Parameters of Local Earthquakes Originated in Shillong-Mikir Plateau and its Adjoining Region of Northeastern India

Bulletin of the Seismological Society of America, 2013

We estimate the source parameters (seismic moment, source radius, stress drop, and source displac... more 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 correction for attenuation. Seismic moments are observed within the range from 9:51 × 10 12 to 2:74 × 10 15 N·m; stress drop ranges from 4 × 10 5 to 9 × 10 7 Pa for the Brune model and 7 × 10 5 to 1 × 10 8 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 earthquakes are triggered by low deviatoric stress. Empirical relations between M w -M L and M 0 -M L are developed leading to the future prediction of calibration coefficients for the local earthquakes in the Shillong-Mikir plateau and its adjoining region.

Research paper thumbnail of Influence of attenuation and site on microearthquakes’ spectra in Shillong region, of Northeast, India: A case study

Acta Geophysica, 2013

We examine the influence of attenuation and site on the spectra of microearthquakes having origin... more 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 Q P and Q S are found to be 178 and 195. The ratio of Q S to Q P 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.