JOURNAL J. GEOLOGICAL R. KAYAL ANDSOCIETY OTHERSOF INDIA Vol.78, July 2011, pp.76-80
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Evaluation of Crustal and Upper Mantle Structures Using Receiver Function Analysis: ISM Broadband Observatory Data J. R. KAYAL1, V. K. SRIVASTAVA1, P. KUMAR2, RIMA CHATTERJEE1 and P. K. KHAN1 1
2
Department of Applied Geophysics, Indian School of Mines, Dhanbad - 826 004 National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad - 500 007 Email:
[email protected]
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.
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
in the area, and the lower crust is the source area for the local earthquakes. These events are mostly of lower magnitude (M 5.8, recorded at the ISM-Dhanbad BB station. The selected earthquakes fall in the teleseismic distance range of 30-90 deg of epicentral distance. First, we rotated the vertical (Z), and the north-south (N) and east-west (E) components into radial (R) and transverse (T) components using the theoretical back azimuth of the event. We further rotated the vertical (Z), radial (R) and the transverse (T) components into the local ray coordinate system P, SV, SH components using the theoretically estimated incidence angle based on reference IASP-91 Earth model (Kennett and Engdahl, 1991). Finally, we derive the RF by time domain deconvolution of Sv by their respective P components.
(1)
Similarly, delay travel times for the reverberations PpPs can be given as: 1 1 TPpPs = Z − p2 + − p2 2 Vs2 V p
(3)
(2)
Fig.2. Schematic ray diagram showing ray paths for direct conversion (Ps) and two multiples PpPs and PpSs arising from a seismic discontinuity (Moho) at depth and from the free surface. The black inverted triangle is seismic station on the surface.
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The deconvolution is performed in order to equalize records of different events taking due care of different source time functions, influences of source depth and source structure, and of different magnitudes and source orientations. We performed deconvolution in time domain by finding an inverse time series of the P phase which transforms P approximately into spike on the P component. This inverse series is convolved with the Sv component, which results in the P-receiver function. In order to enhance the signal-to-noise ratio we stacked all the receiver function traces after moveout correction to remove the distance effect with a reference slowness of 6.4 sec/deg using the IASP-91 Earth model. The stacked RF is shown in Fig. 3 with the statistical errors estimated by the bootstrap technique (Efron and Tibshirani, 1998). Since the P-receiver functions register information regarding the direct conversions as well as multiples from shallow levels, we ascertain information which is greater than ±2σ (standard deviation) (Fig.3). RESULTS AND DISCUSSION
It is observed that the first positive phase occurs at about ~6 sec, which is from the Moho discontinuity at the crustmantle boundary, and its first and second multiples occur at ~19.8 sec (PpPs) and at 22-25 sec (PpSs) respectively (Fig.3). The crustal depth and average Vp/Vs are estimated using the equations 1-3. The Moho depth and average Vp/Vs for the crust below the ISM-BB observatory are ~41 km and ~1.80 respectively. Further, we inverted the stacked trace starting with a simple 1-D velocity model
(Fig.4). We found a reasonable 1-D velocity structure below the receiver. It is interesting to note that the stacked trace in Fig.3 also shows direct conversions from global upper mantle discontinuities at 410 km and 660 km. Although the 660 km discontinuity is within the noise level of data, the 410 km discontinuity is well-identified, and the travel time is delayed by 0.6 sec with respect to the IASP-91 global model. The delayed upper mantle discontinuity at 410 km suggests that the upper mantle below ISM station is slower/hotter. The well estimated crust-mantle boundary at a depth of ~41 km is comparable with the RF results in southern peninsular India (Kumar et al. 2001; Rai et al. 2005). Kumar et al. (2001) used 10 broadband station data of the national network for the period 1997-1999 of peninsular India including the Bokaro (BOKR) broadband station which is near to the ISM-Dhanbad (DHAN) broadband station (Fig.1). They estimated the Moho depth in the range 35-39 km in southern shield area, and 44-46 km in the northern shield area. They, however, could not resolve the Moho depth below the BOKR due to complexity in data and RF. Our study with the good quality broadband data show clear P-to-s (Ps) conversion, resolved the complexity and the estimated the Moho depth ~ 41 km, which is comparable with the estimated Moho depth in the Archaean terrain in peninsular shield area. The Dhanbad area in the northern Singhbhum craton, is a part of the Archaean terrain and its well estimated Moho depth at ~41 km adds a new information to our knowledge for the eastern part of the Indian shield, which was otherwise unknown or controversial in earlier studies.
Fig.3. Observed stacked RF (white wriggle) using 17 teleseismic events Mb > 5.8 recorded at the ISM-Dhanbad BB observatory with moveout corrections; the different phases are labeled. The gray wriggles are bootstrap family, the two black curvy lines represent ±2σ error bound. JOUR.GEOL.SOC.INDIA, VOL78, JULY 2011
EVALUATION OF CRUSTAL AND UPPER MANTLE STRUCTURES USING RECEIVER FUNCTION ANALYSIS
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Fig.4. Inversion results for the stacked trace at reference slowness of 6.4s/deg. (a) In the upper and lower panel, the solid wriggles represent the observed data. In the lower panel the dashed wriggle is the initial receiver function corresponding to the model shown in box b with dashed line. Matching of the observed and synthetic data is shown in the upper panel. The synthetic data are based on the model. (b). The model shown by the dashed line is our simple starting model and the curvy line represents the evolved crustal velocity model.
In our earlier study, we tried to understand source mechanisms and tectonics of the local earthquakes in Dhanbad area using the single BB station data of the ISMDhanbad observatory (Kayal et al. 2009). It was reported that the local earthquakes occur infrequently showing typical low shield seismicity, but the earthquake source zone is observed in the lower crust at a depth ~25 km, unlike the upper crustal source zone at shallower depth (
