JOUR.GEOL.SOC.INDIA, VOL78, JULY 2011 76 J. R. KAYAL AND OTHERS Evaluation of Crustal and Upper Mantle Structures Using Receiver Function Analysis: ISM Broadband Observatory Data J. R. KAYAL 1 , V. K. SRIVASTAVA 1 , P. KUMAR 2 , RIMA CHATTERJEE 1 and P. K. KHAN 1 1 Department of Applied Geophysics, Indian School of Mines, Dhanbad - 826 004 2 National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad - 500 007 Email: jr_kayal@yahoo.com Abstract: A three-component broadband seismograph is in operation since January 2007 at the Indian School of Mines (ISM) campus, Dhanbad. We have used the broadband (BB) seismograms of 17 teleseismic events (M ≥ 5.8) recorded by this single BB station during 2008-09 to estimate the crust and upper mantle discontinuities in Dhanbad area which falls in the peninsular India shield. The converted wave technique and the Receiver function analysis are used. A 1-D velocity model has been derived using inversion. The Mohorovicic (Moho) discontinuity (crustal thickness) below the ISM observatory is estimated to be ~41 km, with an average Poisson ratio of ~0.28, suggesting that the crust below the Dhanbad area is intermediate to mafic in nature. The single station BB data shed new light to the estimate of crustal thickness beneath the eastern India shield area, which was hitherto elusive. Further, it is observed that the global upper mantle discontinuity at 410 km is delayed by ~0.6 sec compared to the IASP-91 global model; this may be explained by a slower/hotter upper mantle; while the 660 km discontinuity is within the noise level of data. Keywords: Broadband seismograms, Teleseismic events, Receiver function, Crust and upper mantle, Moho discontinuity. in the area, and the lower crust is the source area for the local earthquakes. These events are mostly of lower magnitude (M<3.0) and infrequent. In this study, we have analysed teleseismic seismo- grams to estimate the crustal thickness and upper mantle structure beneath the ISM-Dhanbad observatory using the converted wave technique and the Receiver function analysis. The converted waves at the Mohorovicic (Moho) discontinuity and at the deeper upper mantle structures shed a new light to our understanding of the local crustal structure and the global model on deeper mantle structure. Receiver Function (RF) Analysis Receiver functions (RF) are the time series, derived from the three-component BB seismograms, which show the relative responses of the Earth structure beneath the receiver. The P-receiver function waveforms essentially consist of compressional-to-shear (Ps) converted waves and the multiples reverberations beneath the seismometer (Burdick and Langston, 1977; Vinnik, 1977; Kind et al. 1995; Gurrola et al. 1995). It is a robust seismological technique and routinely used to decipher the crust and upper mantle structures of the Earth. INTRODUCTION A CMG 40T seismograph connected to a DM24 data acquisition system is functioning at the Indian School of Mines (ISM) observatory, Dhanbad (23.875° N, 86.444° E) since January 2007. This broadband (BB) observatory was initially established under a research project scheme, Department of Science and Technology (DST), Govt. of India. The observatory is now run by the ISM authority under the supervision of the Department of Applied Geophysics. The seismograph is recording 100 samples per second at high gain with amplification factor 1.0. The observatory is situated on a hard rock in the metamorphic terrain of Chotanagpur granite gneiss, Archaean shield of peninsular India (Fig. 1). The record quality is very good, and it is one of the best broadband seismic observatories in the country (Kayal et al. 2009). Many regional and teleseismic events are recorded at this observatory. The local events are, however, much less in the area. Kayal et al. (2009) reported source parameters and source mechanisms of the local earthquakes recorded by this single BB station by waveform inversion (Fig.1). They reported that the local earthquakes occurred by left lateral strike-slip mechanism in the lower crust at a depth of ~25 km. North-south compressional and east-west tensional stresses are dominant JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.78, July 2011, pp.76-80 0016-7622/2011-78-1-76/$ 1.00 © GEOL. SOC. INDIA