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Propagation of Crustal-Waveguide-Trapped Pg and Seismic Velocity Structure in the South Indian Shield

National Geophysical Research Institute
Hyderabad-500007, India
postmast{at}csngri.ren.nic.in.

Short-period vertical-component digital recordings of the seismic wave field generated by mine tremors and explosions recorded at the Gauribidanur array (GBA) reveal relatively high-frequency secondary energy, with a group velocity of about 6 km/sec accompanied by a long, incoherent coda. We interpret this part of the wave field including the coda as resulting from a crustal-waveguide-trapped Pg, and it is observable from a distance of about 150 km behind the first arrival Pn phase. These observations and extensive modeling results reveal the fine structure of the crust beneath the Archaean eastern Dharwar craton in the south Indian shield. A waveguide with random successions of high- and low-velocity layers in the upper crust, possibly with considerable lateral extent beneath the Closepet granite, explains the main features of the waveguide Pg, such as coda length and group velocity. Reflectivity synthetic seismograms for 1D lamellae structures in the upper crust are found to reproduce the observed features of a record section assembled from GBA seismograms extending to a distance of 240 km. The preferred models of P- and S-velocity structure reveal lower than normal V_P/V_S and Poisson's ratios: respectively, 1.63–1.65 and 0.20–0.21 in the upper crust, including the lamellae structure, and 1.70 and 0.235 in the lower crust. A Q_P/Q_S ratio nearly equal to, or even slightly less than, 1 in the upper crust is found to generate the synthetics consistent with the observed seismogram section. F-K spectral analysis of the observed P coda following the waveguide Pg, for travel paths mostly through the Closepet granite, convincingly reveals two dominant energy peaks, at near-on-azimuth as well as relatively off the source azimuth, both with an apparent velocity of 6.1 km/sec appropriate to the upper crustal depths. The inferred upper crustal structure modeled by alternating high and low velocity lamellae, together with the derived physical properties and the F-K analysis of the P coda, suggests a scattering waveguide at 5–15 km depth in this region. The scattering waveguide appears to be related to the large Closepet granite intrusion, which is believed to be a Precambrian suture between the western and eastern Dharwar cratons in the south Indian shield. Possible lateral extent of the modeled lamellae structure receives added support from a recently proposed hypothesis of a late Archaean mantle plume centered below the Closepet granite.

The inferred crustal P-velocity model for the eastern Dharwar craton, revealing a transitional Moho at a shallow depth of 34–36 km, is consistent with the models for other Archaean cratons. The observed Pn velocity of 8.2 km/sec and its gradient 0.013 sec^–1 compare fairly well with similar values found for other shield regions. V_P/V_S and Poisson's ratios are found to increase respectively from 1.71 and 0.24 below the Moho to 1.78 and 0.27 at about 60 km depth in the upper mantle.