Propagation of regional seismic phases in the Indian shield: Constraints on crustal and upper mantle velocity models

The regional seismic wave field in the central Indian shield is studied, including propagation characteristics of various prominent phases well recorded in the broadband seismograms of the 21 May 1997 Jabalpur deep crustal (focal depth = 36 km) earthquake (M-w 5.8). The seismograms covering a distance range out to about 1650 km, although limited in number, seem to well reveal the prominent phases including the P-bar and Lg, as well as the high-frequency Pn and Sn and their coda. By detailed numerical simulations, the generation and propagation characteristics of the P-bar phase from earthquake sources at shallow and deep crustal depths are examined. It is found that the P-bar phase propagates efficiently at regional distances even for a deep crustal source depth. The P-bar phase is, nevertheless, very sensitive to the surface-layer velocity structure: extremely weak in the absence of a low-velocity surface layer and also slightly weak in the presence of a relatively high velocity surface layer. Analyses of the Jabalpur earthquake seismograms and the synthetics reveal that the P-bar phase is well generated and propagates efficiently in the Deccan Traps (V-p, 4.8-4.9 km/sec) covered regions (i.e., even without a lower velocity surface layer) surrounding the earthquake epicenter and the recording stations. Further, the anatomy of the P-bar phase is revealed by several computational models, its time window composed of several multiple PmP reflections as well as PmS conversions, the principal boundaries being the Moho and the free surface. The first observations of high-frequency Pn and Sn phases in the Indian shield are recognized from the Jabalpur earthquake seismograms. By numerical simulations, consistent with these observations including the coda, a lamellae model with alternating high- and low-velocity layers for the structure of the Indian continental uppermost mantle is inferred. A complete synthesis of the regional seismograms is obtained based on the inferred models of the P- and S-wave velocity structure of the crust and upper mantle in the central Indian shield. Comparison of the synthetics with the Jabalpur earthquake seismograms reveals a consistent fit to various observations, including the Lg phase (group velocity window, 3.6-3.2 km/sec). The low-pass filtered seismograms at the regional distances are further found to reveal coherent phases consistent with predicted reflection times from discontinuities in the upper mantle. A plausible P velocity model inferred from these observations reveals velocity discontinuities at average depths of 230, 320, 430, and 680 km, suggesting significant stratification of the upper mantle B region in the Indian shield.