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Gupta S.,CSIR - Central Electrochemical Research Institute | Mohanty W.K.,Indian Institute of Technology Kharagpur | Prakash R.,Earthquake Risk Evaluation Center | Shukla A.K.,Earthquake Risk Evaluation Center
Pure and Applied Geophysics | Year: 2013

To better understand the relationship between crustal heterogeneity and seismotectonics in the National Capital Region (NCR), Delhi, India, we carried out local P- and S-velocity tomography beneath the NCR. First arrival times of the first P- and S-wave from 275 crustal earthquakes recorded by the Seismic Telemetry Network in and around Delhi of the India Meteorological Department, India, are inverted to obtain crustal P-, S-velocity and VP/VS variations in the region. Our tomographic images of the upper crust reflect well the surface geological and tectonic features. The Delhi fold belt is identified as low VP, high VS and low VP/VS. The Sohna hot spring region is appearing as low VP, low VS and high VP/VS correlating with the possible presence of fluid-filled rocks. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the low VP/VS region. © 2012 Springer Basel AG. Source

Mahajan A.K.,Wadia Institute of Himalayan Geology | Shukla A.K.,Wadia Institute of Himalayan Geology | Pandey A.,Wadia Institute of Himalayan Geology | Chauhan M.,Earthquake Risk Evaluation Center | And 2 more authors.
International Journal of Geotechnical Earthquake Engineering | Year: 2011

In this paper, shear wave velocity (Vs) investigations are carried out using Multichannel analysis of surface waves (MASW) method at ten representative sites in the NCT region, New Delhi. The analysis shows that the Vs obtained from the sites located on Alwar quartzites of Delhi Super Group ranges from 770 m/s to 2800 m/s, whereas on other sites located on lake/river sediments (Nazafgarh, Balsava and Akshar Dham) have Vs less than 180 m/s. The sites located on thick sediments shows Vs of the order of 180 m/s to 250 m/s. According to the soil classification, the sites covered can be classified under three categories: Class 'B' (Vs30 as >760m/s; JNU site and Asola site), class 'D' (Vs30>180 m/sec-360; Bhavana, Suhalpur, Ghazipur and Kirbi cantt. sites), whereas the sites located near lake/river sediments are classified as class 'E' (with very soft soil) and will be prone to liquefaction potential during strong earthquake shaking. Copyright © 2011, IGI Global. Source

Mandal H.S.,Earthquake Risk Evaluation Center | Khan P.K.,Indian School of Mines | Shukla A.K.,Earthquake Risk Evaluation Center
Natural Hazards | Year: 2014

In this study, soil response was carried out for the Greater Delhi region. A folded Proterozoic formation was identified as Delhi ridge, passes through its central part along SSW-NNE direction, and appears to be a main geomorphic feature for the study area. The Delhi ridge is an exposed quartzite rock of about 10-100 m wide and ~25 km long with gentler dipping both toward east and west. We have considered the exposed part as an outcrop side near the ridge axis and the dipping area as rigid base away from the ridge axis for ground motion study during the occurrence of the 25 November 2007 earthquake with magnitude M L 4.3 (Richter scale) that occurred at Delhi-Haryana State boundary. The degree of shaking was very strong and reported major cracks in the buildings near the epicenter area. We have studied the soil response parameters at the surface level, considering horizontally stratified soil layers above rigid base. The equivalent linear method was used for soil response analysis at 25 sites in Greater Delhi area. The peak amplification factors vary from 3.2 to 5.9 and peak resonance frequency varies from 1.2 to 5.3 Hz. The correlation among the peak amplification factor (A) and frequency (f) was empirically established as A = 0.36f + 3.60. Increasing peak amplification factor was found at sites with increasingly thicker alluvium deposit with lower frequency contains ground motion and vice versa. Seismic zoning map was also reconstructed for peak amplification factors and predominant periods for the study area for the mitigation purposes of earthquake damage. The average shear wave velocity up to 30 m soil depth is also obtained for site classification. The average velocity to 30 m [V̄s(30)] is a widely used parameter for classifying sites for predicting their potentiality to amplify seismic shaking. A lower value [V̄s(30)] thus yields a more conservative estimate of ground motion, which generally increases as V̄s(30) decreases. Present estimate of V̄s(30) varies from 315 to 419 m/s. In this study, we have identified two site classes C and D, as per National Earthquake Hazard Reduction Program. The city planner or engineers can directly use these data for site-specific assessment during retrofitting of the existing structure, demolition of the old buildings and design a new structure to avoid major destruction of the buildings due to future earthquake. © 2012 The Author(s). Source

Mandal H.S.,Earthquake Risk Evaluation Center | Khan P.K.,Indian School of Mines | Shukla A.K.,Earthquake Risk Evaluation Center
Journal of Asian Earth Sciences | Year: 2013

The study area lies between latitude 18-26°N and longitude 73-83°E, and mainly covers the Central India Tectonic Zone (CITZ). The frequency-dependent shear wave quality factor (Qs) has been estimated over the CITZ and its surroundings using Double Spectral Ratio (DSR) method. We have considered 25 local earthquakes with magnitude (ML) varies from 3.0 to 4.7 recorded at 11 stations running under national seismic network. The Fast Fourier Transformed (FFT) spectra were computed from the recorded waveform having time-window from onset of S-phase to 1.0s and for a frequency-band of 0.1-10Hz. Three different shear wave velocities (i.e., 3.87, 3.39 and 3.96km/s) were obtained over the study area based on a pair of earthquakes recorded at a pair of stations. The low Qs values of 51-96 at 1Hz (i.e., Qs=51f0.49; Qs=90f0.488 and Qs=96f0.53) were found in the area covering the Son-Narmada-Tapti (SONATA) lineament, CITZ, eastern part of the Satpura fold belt, Vindhyan and Gondwana basins, Godavari and Mahanadi grabens, and southern part of Gangetic plain. Intermediate Qs values of the order of 204-277 (i.e., Qs=204f0.56 and Qs=277f0.55) were noted in the cartonic areas, namely, Bundelkhand, Dharwar-Bhandara and Bastar. While the higher Qs values of 391-628 at 1Hz (i.e., Qs=391f0.49,Qs=409f0.48, Qs=417f0.48, Qs=500f0.66, Qs=585f0.65 and Qs=628f0.69) were found in the eastern part of the SONATA, CITZ, and the northeastern part of the Satpura fold belt. The low Qs values might be attributing to the more heterogeneous SONATA rift system. Low Qs values further may presumably be associated with lower-level of seismicity and apparently account for higher tectonic stress accumulation over long duration. The long-term accumulated stress is generally released through occasional triggering of moderate magnitude earthquakes in the SONATA zone. Surrounding the SONATA region, the higher Qs values possibly accounts for a more homogeneous subsurface structure along the SONATA zone. © 2013 Elsevier Ltd. Source

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