• Rambhatla G Sastry

      Articles written in Journal of Earth System Science

    • 3-D GRACE gravity model for the 2011 Japan earthquake

      Rambhatla G Sastry Mahendra K Sonker

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      The GRACE mission has contributed to the seismic characterization of major earthquakes in offshore regions of the world. Here, we isolate satellite gravity signal (μGal range) for the Japan Earthquake of 2011 using a difference method. Contrary to the existing gravity models, we propose a unit vertical pyramid based five-layer 3-D thrust fault model, which extends to the hypocenter and honors the ocean water layer and sea floor upheaval also. Our model partly uses existing seismological information (hypocenter depth of 32 km, rupture length of 300 km and vertical slip of 4 m), provides a snapshot of episodic subduction of the Pacific Plate below the Atlantic Plate and its gravity response closely matches the observed gravity (RMS error of 3.4012×10−13μGal), fully accounting for co-seismic mass redistribution including sea surface deformation. Our inferred rupture length, rupture velocity, average seismic moment magnitude and momentum, respectively, are 300 km, 4.49 km/s, 1.152×1021 −1.8816×1021 N m and 2.319×106 GNs, which fairly agree with the literature. Further, our model inferred momentum at the sea floor corresponds to an area pulse that led to Tsunami generation.

    • Statistical analysis of geo-electric imaging and geotechnical test results – a case study

      Rambhatla G Sastry Sumedha Chahar Manohar N Viladkar

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      For conjunctive use of geoelectric imaging and geotechnical site investigations in geotechnical characterization of major civil engineering construction sites, an objective assessment of influencing factors is important. Here, we present multiple regression analyses of both geoelectric (Electrical Resistivity Tomography, ERT; Induced Polarization Imaging, IPI) and geotechnical site investigations (Standard Penetration Test, SPT) for two profiles at a construction site for CGEWHO Complex in Greater Noida region, Delhi to assess the role of influencing formation factors like sand, fines and water content. Achieved results show that SPT ‘N’ and IPI are well predicted by a linear multiple regression. On an average, the nonlinear regression has improved predicted SPT ‘N’, resistivity and chargeability by 28.55%, 22.45% and 9.58%, respectively. The influence of sand and fines content is more than thatof water content in the prediction of chargeability and SPT ‘N’. RMS error is less in prediction of IPI chargeability (average error of 1.96%) in comparison to SPT ‘N’ value (average error of 11.35%). As factors affecting chargeability (IPI) and SPT ‘N’ are similar, non-invasive IPI can be used along with few geotechnical site investigations for detailed geotechnical site investigations.

    • Co-seismic grace gravity-based 11-layered 3-D thrust fault model for the Sumatra earthquake 2004

      Rambhatla G Sastry Mahendra Kumar Sonker

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      Our co-seismic Gravity Recovery and Climate Experiment gravity data (level 2 ‘RL_05’ data product ‘GX-OG-_2-GSM) for the Sumatra earthquake 2004 is obtained by differencing monthly gravity field average for November 2004 from that of January 2005 and band-pass filtering (17${-}$30$^{\circ}$ and orders) in the spectral domain. Here, we propose a 11-layered three-dimensional (3-D) thrust fault gravity model based on different co-seismic rupture models in the literature. It honours co-seismic deformation of the ocean surface, ocean bottom and subsurface earth medium, different earthquake parameters and hypocentre information (${\sim}$ 35 km below mean sea level). Our forward gravity response matches well with the observed gravity (RMS error of 0.06257 $\mu$gal (6.26%)) data and our model allowed an independent computation of rupture length, instantaneous velocity, average seismic moment and momentum, which are, respectively, 1560 km, 2.9 km/s, 4.53 $\times$ 10$^{22}$ N m and 9.7 $\times$ 10$^{17}$ kg m/s. These parameters fairly agree with those in the literature. The computed momentum indeed corresponds to an area pulse (9.7 $\times$ 10$^{17}$ kg m/s) at ocean bottom that led to a tsunami generation. Thus, the proposed multi-layered 3-D gravity model in traditional fashion fully accounts for co-seismic gravity signal of the Sumatra earthquake 2004.

    • Geoelectric versus MASW for geotechnical studies

      Rambhatla G Sastry Sumedha Chahar

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      We explore the role of non-invasive multi-electrode electrical resistivity imaging (ERT) and induced polarisation imaging (IPI) as an alternative to multichannel analysis of surface waves (MASW) for geotechnical site characterisation in view of their higher near-surface spatial resolution. By using regression equations, we assess the relative performance of ERT, IPI and MASW in predicting geotechnical test results (standard penetration test (SPT), dynamic cone penetration test (DCPT) and static cone penetration test (SCPT)) in a site investigation on our IIT Roorkee Campus, India. The results indicate that the average root mean square (RMS) errors in predicting SPT based on ERT, IPI and MASW are 16.95%, 21.9% and 28.03%, respectively. Likewise, the average RMS errors in predicting DCPT based on ERT, IPI and MASW are 15.4%, 15.3% and 56.99%, respectively, and the average RMS errors in predicting SCPT based on ERT, IPI and MASW are 20.15%, 18.65% and 36.49%, respectively. In view of higher resolution for near-surface investigations, ERT/IPI seems to score over MASW in geotechnical site investigation studies. So, a leading role for non-invasive and cost-effective ERT/IPI in geotechnical site investigations is envisaged.

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