Crustal Heterogeneity and Seismotectonics of the National Capital Region, Delhi, India
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ure and Applied Geophysics
Crustal Heterogeneity and Seismotectonics of the National Capital Region, Delhi, India SANDEEP GUPTA,1 WILLIAM K. MOHANTY,2 RAJESH PRAKASH,3 and ATINDRA KUMAR SHUKLA3 Abstract—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. Key words: Seismic tomography, National Capital Region (NCR), Delhi, seismic velocity, seismicity.
1. Introduction The National Capital Region (NCR), Delhi of India, which includes the National Capital Territory, Delhi (NCT), covers an area of 30, 242 sq. km. The region surrounding Delhi is characterized by eight tectonic units like (a) frontal folded zone (b) area of Delhi folding (c) Rajasthan shelf (d) Lahore–Delhi Ridge (e) Punjab shelf (f) Delhi–Hardwar ridge (g) West Uttar Pradesh shelf (h) Sarada depression (KARUNAKARAN and RANGA RAO, 1979) (Fig. 1). The
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National Geophysical Research Institute (CSIR), Hyderabad 500007, India. E-mail: [email protected] 2 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, West Bengal 721302, India. E-mail: [email protected] 3 Earthquake Risk Evaluation Center, India Meteorological Department, Lodhi Road, New Delhi, India. E-mail: [email protected]; [email protected]
NCR represents the northern most part of the Aravalli ranges. The terrain of Delhi in general is flat, covered by older and newer alluvium except for a low NNESSW trending ridge known as the Delhi fold belt (DFB) is exposed in the southern and central part of Delhi. The DFB seems to be a prolonged extension of the NNE-SSE trending Aravalli Mountain (RAIVERMAN et al., 1983; VALDIYA, 1976) The DFB marks the division between the Indus river system in the west to the Ganga–Yamuna basin in the east. The rocks exposed in Delhi belong to the Alwar series of Delhi supergroup (±1,500 million years in age) and are mostly quartzites interbedded with mica schist. The older alluvium comprises silt and clay. The newer alluvium consists of unoxidised sand, silt and clay occurring on both sides of the Yamuna river. Another important structural element of the region is the NW– SE trending Lahore–Delhi Ridge which passes through Delhi, and is flanked by basins on either s
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