An attempt has been made to estimate the site amplification factors of different geomorphological units in Shillong city, northeastern India, based on the distribution of peak ground acceleration (PGA) due to maximum credible earthquakes... more
An attempt has been made to estimate the site amplification factors of different geomorphological units in Shillong city, northeastern India, based on the distribution of peak ground acceleration (PGA) due to maximum credible earthquakes that originated in nearby major faults. Empirical relations are used to determine the amount of shaking at base rock as a function of the magnitude of the earthquake (Mw), source depth (km), fault dimension (intraplate/interplate), velocity structure of medium and the distance from the source (km). The computed PGA values are convolved with the average shear wave velocity (Vs30) values at various geomorphological units of Shillong to ascertain the degree of amplification while travelling from base rock to the surface. The amplification for the highly dissected land in the city is found to be the maximum within a range of 2.77–2.92, while the high plateau segment is characterized by least values (2.01–2.16). Simultaneously, the effective ground motion mapped on the surface indicates the maximum value of 0.6–0.94 g for a probable earthquake of Mw 8.1 due to Dauki fault. Similarly, Dhubri and Kopili faults might produce a ground motion of 0.05–0.08 and 0.22–0.33 g for a maximum credible earthquake of Mw~7.0, respectively.
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Understanding the behaviour of soil and site in highly seismically active Assam valley, Shillong-Mikir Hills Plateau and Eastern Himalayan Syntaxis of Northeastern India is important for seismic hazard assessment. Constrained by this... more
Understanding the behaviour of soil and site in highly seismically active Assam valley, Shillong-Mikir Hills Plateau and Eastern Himalayan Syntaxis of Northeastern India is important for seismic hazard assessment. Constrained by this tectonic interaction, we estimate the ground motion parameters of these tectonic domains through the inversion of 129 three component strong motion records of earthquakes with magnitude Mw>4.0 & Mw<6.0. Remarkably, parameters estimated at 9 different sites help to classify the region into four different site class subjected to magnitude and epicentral distance of the earthquake. Higher spectral acceleration observed at low period in Diphu, Nongstoin, Tezpur and Guwahati infer embedded site condition as rock subjected to removal of source and path effect. On the other, low spectral acceleration at high period in Kokrajhar, Golaghat, Goalpara and Nagaon indicate distinctly thick alluvial basin. Fourier spectrum width in Guwahati, Diphu, Nongstoin an...
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We applied multiple linear regressions to scrutinize the maximum variability produced in soil gas radon (Rn-222) by pressure, temperature and rainfall. Statistical methodologies were applied to discriminate the effect of pressure,... more
We applied multiple linear regressions to scrutinize the maximum variability produced in soil gas radon (Rn-222) by pressure, temperature and rainfall. Statistical methodologies were applied to discriminate the effect of pressure, temperature and rainfall for precise identification of seismic induced anomalies. High Rn-222 anomalies (HRA) as well as Low Rn-222 anomalies (LRA) were apparent. There were nine earthquakes (Mw ≥ 3) which were discernible to be manifested by preseismic soil gas Rn-222 anomalies. Correspondingly, observation centered on radon data set considering the precursory time ‘T’ (days), epicentral distance ‘D’ (km) and magnitude ‘M’ (Mw) has been derived as Log (DT) = 0.79 M + b. The discernible linear equation was found to be substantial with value of coefficient ‘b’ as 0.18 and approximately equal to those obtained by various investigators. In general coefficient ‘b’ is assigned as 0.15 for gaseous geo-seismic precursor. The value of coefficient ‘a ~ 3.51’, ‘b ~ 0.18’ and correction factor ‘K ~ 0.49 (kBqm-3 d)−1/2 were estimated empirically for the first time in Tezpur (Eastern Himalaya), India region. The calculated empirical value of ‘a’ by us gives another form of precursory manifestation zone (D) equation as D ~ 100.58 M. The values of coefficient ‘a’, ‘b’ and correction factor ‘K’ estimated for Tezpur, Assam (Eastern Himalaya) region perhaps can be used for probable earthquake forecast in the region constrained by the peak of the radon anomaly. The number of earthquakes registered was meager and further long-term analysis can estimate more precise and robust values of these parameters for soil gas Rn-222. The investigation also reassures us in a physical sense that seismic induced soil gas Rn-222 perturbation do exist in nature. The investigation is a probable approach for identification of seismic induced anomalies in soil gas Rn-222 and their characteristics for possible earthquake forecasting.
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Research Interests: Geology and Seismology
Abstract A systematic study towards understanding the correlation between polarization direction of crustal anisotropy with seismogenic stress field at different locations of the Shillong-Mikir Plateau and its vicinity in North East India... more
Abstract A systematic study towards understanding the correlation between polarization direction of crustal anisotropy with seismogenic stress field at different locations of the Shillong-Mikir Plateau and its vicinity in North East India is attempted. We used data from a 17-station broadband seismic network. In our earlier work , crustal anisotropic parameters were determined using ANISOMAT + for the 17 seismic stations. In this study, we have estimated stress field around the stations using focal mechanism solutions (FMS). Some 215 FMS are obtained by waveform inversion. These solutions are used for stress tensor inversion to estimate stress field around each location. It is observed that polarization direction of crustal anisotropy is consistent with that of the maximum horizontal stress (Ơmax) as well as the minimum horizontal stress (Ơmin). In addition to this, two orthogonal fast polarizations in some locations are also noted. The bivariate nature of correlations helps us to understand that the major mechanisms of seismic crustal anisotropy are not only due to the regional stress, but active faults and other geological conditions play a significant role in contemporary orientation of seismic crustal anisotropy and seismogenic stress field.
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Monthly mean foF2 measured at Tiruchirapally, Hyderabad, Ahmedabad and Delhi during over two solar cycles had been analyzed. Applying a set of diurnal and seasonal harmonic coefficients, foF2 predictions can be generated for every season... more
Monthly mean foF2 measured at Tiruchirapally, Hyderabad, Ahmedabad and Delhi during over two solar cycles had been analyzed. Applying a set of diurnal and seasonal harmonic coefficients, foF2 predictions can be generated for every season and any level of solar activity (F10.7≦200), at any location along the 75°E meridian from equatorial to lower mid-latitudes. F10.7 was used as the indicator
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Saurabh Baruah Geoscience Division, North-East Institute of Science and Technology (CSIR), Jorhat-785006, Assam, India. ... Keywords: Great Assam earthquake, Isoseismal Map ABSTRACT: The study estimates the extent of seismic hazard in... more
Saurabh Baruah Geoscience Division, North-East Institute of Science and Technology (CSIR), Jorhat-785006, Assam, India. ... Keywords: Great Assam earthquake, Isoseismal Map ABSTRACT: The study estimates the extent of seismic hazard in North East India in ...
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Research Interests: Geology and Seismology
Chedrang valley and its vicinity of Shillong Plateau was the rupture area of Great Assam earthquake of 1897(M~8.7), the region had ever experienced. This earthquake caused considerable damage and resulted highest ever ground motion in the... more
Chedrang valley and its vicinity of Shillong Plateau was the rupture area of Great Assam earthquake of 1897(M~8.7), the region had ever experienced. This earthquake caused considerable damage and resulted highest ever ground motion in the valley. The important ground ruptures mapped by Oldham1 were the Chedrang and Samin. Oldham1 observed 11m of co-seismic slip down to the west of location of Chedrang Fault. The Chedrang fault ran for a distance about 20 km NNW direction from the headwater of Chedrang river through Dalbot (25.83oN ; 90.73oE), Dilma (25.01oN ; 90.71oE) and Jira (25.91oN ; 90.68oE) with a vertical throw varying from 0.60-10.60 meters, up through always being east2. In this study, we have prepared an earthquake catalogue based on relocated events since 1982 the inception of real time seismic monitoring network established in northeastern region of India. This forms an input to the present seismotectonic study for the region. The relocated seismic database has SEISMOTEC...
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We endeavor to assess site response exploiting ambient noise measurements which we carried out at 70 sites in different parts of Shillong City, one of the seismically active regions. We estimate the spectral ratio from these recordings.... more
We endeavor to assess site response exploiting ambient noise measurements which we carried out at 70 sites in different parts of Shillong City, one of the seismically active regions. We estimate the spectral ratio from these recordings. The spectral ratios reveal the resonant frequency for Shillong City in the range of 3 to 8 Hz. Short scale spatial variation in the resonant frequencies suggests that there is prominent lateral heterogeneity in the underlying layers. Besides, a non-linear earthquake site response analysis is also attempted using available geotechnical data. A good correlation is observed between site response analysis and HVSR results constrained by wide variation of resonant frequency at short distances. The results are found to be tallying well with the geological data.
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Abstract Mechanical deformations from within the earthquake preparation zones are believed to cause seismo-electromagnetic (SEM) emission in ultra-low frequency (ULF) band, i.e. between 0.001 and 10 Hz, and these are studied as a... more
Abstract Mechanical deformations from within the earthquake preparation zones are believed to cause seismo-electromagnetic (SEM) emission in ultra-low frequency (ULF) band, i.e. between 0.001 and 10 Hz, and these are studied as a precursor for short range earthquake forecast. Data from highly noise-resilient, 3-component ULF induction coil magnetometers, installed at the Multi-parametric Geophysical Observatory (MPGO), Tezpur, which is situated at a very close proximity with the northern end of the Kopili Fault and the Bomdila Fault, the Main Boundary Thrust of the Eastern Himalaya, the Naga and Disang Thrust, the Assam Syntaxis Zone as well as the two tectonically active Precambrian shields – Shillong and Mikir, was used for the first time to study SEM emissions employing both polarization ratio analysis and fractal analysis in the background of an intense phase of seismicity during the campaign period of April 20 – September 3, 2019. Fifty-one events were filtered via strain radius and index of seismicity calculations to nominate credible events which were studied vis-a-vis polarization ratio and fractal dimension parameters of 3-h night time data, centered around local midnight contemporaneous with the 18–21 UT window of the KP index (global geomagnetic activity index) in the 0.03–0.1 Hz frequency band. The findings show candidate SEM emissions, in the form of enhancements in SZ/SH, associated with all the seven credible events, even as nine enhancements could not be attributed to immediately adjacent credible events. The fractal dimensions, calculated with the spectral density method, ambiguously depicted a gradual increase prior to the intense phase of seismicity.
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Research Interests: Geology and Seismology
We made an attempt to assess the shear wave velocity values
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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... 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.
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... 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...
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Modified form of Nakamura method,H/Vratio, is used to assess the site response through estimation of fundamental resonant frequency at 70 sites using three component digital seismographs in Shillong city, capital of Meghalaya in northeast... more
Modified form of Nakamura method,H/Vratio, is used to assess the site response through estimation of fundamental resonant frequency at 70 sites using three component digital seismographs in Shillong city, capital of Meghalaya in northeast India. With available borehole information, an attempt is made to develop an empirical relationship between sediment thickness and resonant frequency estimated fromH/Vratio technique. Simultaneously, shear wave velocities are computed entailing resonant frequency and sediment thickness for these boreholes. We also endeavored building another empirical relation between sediment thickness andVS. With the help of this, the probableVSvalues for other sites were also evaluated. It is observed that shear wave velocities range from 200 to 550 m/s while sediment thickness ranges from 10 to 80 m, implicating the heterogeneity prevailing in the soil layers of the Shillong city.
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ABSTRACT 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... more
ABSTRACT 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 observed within 1.0–4.2 s after the first S-arrival. We estimated the crus-tal 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.
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ABSTRACT The 18 September 2011 Sikkim Himalaya earthquake of Mw 6.9 (focal depth 50 km, NEIC report) with maximum intensity of VII on MM scale (www.usgs.gov) occurred in the Himalayan seismic belt (HSB), to the north of the main central... more
ABSTRACT The 18 September 2011 Sikkim Himalaya earthquake of Mw 6.9 (focal depth 50 km, NEIC report) with maximum intensity of VII on MM scale (www.usgs.gov) occurred in the Himalayan seismic belt (HSB), to the north of the main central thrust. Neither this thrust nor the plane of detachment envisaged in the HSB model, however, caused this strong devastating earthquake. The Engdahl–Hilst–Buland (EHB) relocated past earthquakes recorded during 1965–2007 and the available global centroid moment tensor) solutions are critically examined to identify the source zone and stress regime of the September 2011 earthquake. The depth section plot of these earthquakes shows that a deeper (10–50 km) vertical fault zone caused the main shock in the Sikkim Himalaya. The NW (North-West) and NE (North-East) trending transverse fault zones cutting across the eastern Himalaya are the source zones of the earthquakes. Stress inversion shows that the region is dominated by horizontal NNW-SSE (North of North-West-South of South-East) compressional stress and low angle or near horizontal ENE-WSW (East of North-East-West of South-West) tensional stress; this stress regime is conducive for strike-slip faulting earthquakes in Sikkim Himalaya and its vicinity. The Coulomb stress transfer analysis indicates positive values of Coulomb stress change for failure in the intersecting deeper fault zone that produced the four immediate felt aftershocks (M ≥ 4.0).
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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... 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.
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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... 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.
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... 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...
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A multiple inverse method has been used to separate stresses from a set of focal mechanism data of Indo-Myanmar region. Four parameters are determined by the inversion; one for the shape of the stress ellipsoid and three for the direction... more
A multiple inverse method has been used to separate stresses from a set of focal mechanism data of Indo-Myanmar region. Four parameters are determined by the inversion; one for the shape of the stress ellipsoid and three for the direction of principal stress axes. Most prominently, the area is affected by a stress state with a horizontal NE-SW directed maximum compression with low stress ratio which has induced thrust and strike-slip faulting in the region. Study of variation of stress pattern in three different depths ranges (0-45 Km, 45-90 Km and 90-150 Km) in the study region shows rotation of maximum principal stress axis towards North at higher depths.
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... Saurabh Baruah. ... Res. Lett. 8, no. 11, 1126-1128.[Web of Science][GeoRef]. Dasgupta, S., P. Pande, D. Ganguly, Z. Iqbal, K. Sanyal, NV Venaktraman, S. Dasgupta, B. Sural, L. Harendranath, K. Mazumadar, S. Sanyal, A. Roy, LK Das, PS... more
... Saurabh Baruah. ... Res. Lett. 8, no. 11, 1126-1128.[Web of Science][GeoRef]. Dasgupta, S., P. Pande, D. Ganguly, Z. Iqbal, K. Sanyal, NV Venaktraman, S. Dasgupta, B. Sural, L. Harendranath, K. Mazumadar, S. Sanyal, A. Roy, LK Das, PS Misra, and H. Gupta (2000). ...
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ABSTRACT An attempt has been made to examine an empirical relationship between moment magnitude (M W) and local magnitude (M L) for the earthquakes in the northeast Indian region. Some 364 earthquakes that were recorded during 1950–2009... more
ABSTRACT An attempt has been made to examine an empirical relationship between moment magnitude (M W) and local magnitude (M L) for the earthquakes in the northeast Indian region. Some 364 earthquakes that were recorded during 1950–2009 are used in this study. Focal mechanism solutions of these earthquakes include 189 Harvard-CMT solutions (M W ≥ 4.0) for the period 1976–2009, 61 published solutions and 114 solutions obtained for the local earthquakes (2.0 ≤ M L ≤ 5.0) recorded by a 27-station permanent broadband network during 2001–2009 in the region. The M W–M L relationships in seven selected zones of the region are determined by linear regression analysis. A significant variation in the M W–M L relationship and its zone specific dependence are reported here. It is found that M W is equivalent to M L with an average uncertainty of about 0.13 magnitude units. A single relationship is, however, not adequate to scale the entire northeast Indian region because of heterogeneous geologic and geotectonic environments where earthquakes occur due to collisions, subduction and complex intra-plate tectonics.
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... PANKAJ M. BHATTACHARYA,1 JR KAYAL,2 SAURABH BARUAH,3 and SS AREFIEV 4 ... The Assam syntaxis zone is also seismically active and produced the great 1950 AssamTibet earthquake (Ms 8.7) (TANDON 1954). ...