S. S. Rai

Crustal shear velocity structure of the south Indian shield

[1] The south Indian shield is a collage of Precambrian terrains gathered around and in part derived from the Archean-age Dharwar craton. We operated seven broadband seismographs on the shield along a N-S corridor from Nanded (NND) to Bangalore (BGL) and used data from these to determine the seismic characteristics of this part of the shield. Surface wave dispersion and receiver function data from these sites and the Geoscope station at Hyderabad (HYB) give the shear wave velocity structure of the crust along this 600 km long transect. Inversion of Rayleigh wave phase velocity measured along the profile shows that the crust has an average thickness of 35 km and consists of a 3.66 km s � 1 , 12 km thick layer overlying a 3.81 km s � 1 , 23 km thick lower crust. At all sites, the receiver functions are extremely simple, indicating that the crust beneath each site is also simple with no significant intracrustal discontinuities. Joint inversion of the receiver function and surface wave phase velocity data shows the seismic characteristics of this part of the Dharwar crust to be remarkably uniform throughout and that it varies within fairly narrow bounds: crustal thickness (35 ± 2 km), average shear wave speed (3.79 ± 0.09 km s � 1 ), and Vp/Vs ratio (1.746 ± 0.014). There is no evidence for a high velocity basal layer in the receiver function crustal images of the central Dharwar craton, suggesting that there is no seismically distinct layer of mafic cumulates overlying the Moho and implying that the base of the Dharwar crust has remained fairly refractory since its cratonization. INDEX TERMS: 7203 Seismology: Body wave propagation; 7205 Seismology: Continental crust (1242); 7255 Seismology: Surface waves and free oscillations; KEYWORDS: continental crust, Archean crust, receiver function, Indian shield

Deep crustal structure of the Indian shield from joint inversion of P wave receiver functions and Rayleigh wave group velocities: Implications for Precambrian crustal evolution

[1] The S wave velocity structure of the crust and uppermost mantle of the Indian shield has been investigated by jointly inverting P wave receiver functions and Rayleigh wave group velocities at 38 broadband stations in the subcontinent. The Indian shield is an amalgamation of several terranes of Archean and Proterozoic age that were partly flooded by Deccan Trap volcanism during Cenozoic times and that make up a natural laboratory for assessing models of Precambrian crustal evolution. Our results reveal significant variations in crustal thickness and deep crustal velocities: 45―50 km thick under the Archean West Dharwar craton and Southern Granulite Terrane, with lower crustal velocities around 4.1 km/s; 32―35 km thick beneath the Archean East Dharwar and Bundelkhand cratons, with lower crustal velocities around 3.8―3.9 km/s; 50―65 km thick under the Proterozoic Bhandara craton, with lower crustal velocities around 4.2―4.3 km/s; and ∼55 km thick under the Proterozoic Aravalli-Delhi belt, with lower crustal velocities around 4.2 km/s. S velocities in the 4.1―4.3 km/s range in the deep crust can be attributed to mafic lithologies, suggesting there has been no secular change in the Precambrian evolution of the south Indian shield. Moreover, pervasive mafic dike swarming throughout the Indian shield suggests that the layer of mafic cumulates is 2.5―1.6 Ga old and that it delaminated from some Archean terranes. Our interpretation is that mafic underplating of the terranes making up the Indian shield occurred in Proterozoic times and that a refractory root developed under the Archean terranes after the Proterozoic event.

Shear-Wave Structure of the South Indian Lithosphere from Rayleigh Wave Phase-Velocity Measurements

We investigate the upper mantle shear-wave speed structure beneath the south Indian shield by measuring and modeling fundamental mode Rayleigh wave phase-velocity dispersion. Observed phase velocities for the south Indian shield closely match those observed for the Canadian shield. We constrain the south Indian crust using published receiver function results and invert the dispersion data for upper mantle shear-wave structure. The ~155-km-thick seismic lithosphere of the south Indian shield is composed of a 35 km-thick, two-layer crust and a ~120-km-thick, high-velocity upper mantle lid. Beneath the Moho the average Sn wave speed is ~4.7 km sec–1. Both Sn travel times data and the dispersion data suggest a positive sub-Moho shear-wave speed gradient. Beneath the seismic lithosphere there is a low- velocity layer where the shear-wave speed drops to ~4.4 km sec–1.

Group velocity tomography of the Indo-Eurasian collision zone

We present results of a Rayleigh and Love wave group velocity dispersion study of the Indo-Eurasian collision zone. Group velocity dispersion curves are measured and combined to produce dispersion maps for 10–70 s period Rayleigh waves from 4054 paths and for 15–70 s Love waves from 1946 paths. Group velocity maps benefit from the inclusion of data recorded at a large number of stations within India, an advantage over previous global studies. This has the largest impact at short periods as a result of the improved path length distribution. Synthetic tests are used to estimate resolution, which ranges from 3° to 5° on the continents for Rayleigh wave maps and from 5° to 7.5° for Love wave maps. Group velocities correspond well with known geological and tectonic features and show good correlation with sediment thickness at short periods. The cratons of the Indian Shield can be distinguished in the short-period and midperiod group velocities. Group velocities are slow across Tibet until 70 s whereas the cratonic cores of the Indian Shield appear as a high velocity anomaly at 70 s. Dispersion curves extracted from the Rayleigh wave group velocity maps are inverted for shear wave velocity as a function of depth for profiles across India and Tibet. The relationship between shear velocity contours and the Moho indicated by receiver function studies has been used to obtain a first-order estimate of crustal thickness across the collision zone. Results suggest a slow Tibetan midcrust and low sub-Moho velocities beneath the central and northeastern Tibetan Plateau.

Teleseismic tomographic evidence for contrasting crust and upper mantles in south Indian Archaean terrains

Abstract We compare the velocity structure of the crust and upper mantle of the Archaen granulite terrain of southernmost India with that of the Archaean granite-greenstone (Dharwar craton) terrain just to the north, through P-wave teleseismic travel time measurements. The teleseismic rays recorded by a coarse network of 11 portable seismic stations show an anomalous pattern of late arrivals (delays) over the granulite terrain in contrast to fast arrivals on the Dharwar craton. Such a pattern of time residuals amongst these Archaean terrains, which have remained inert in the last 2 Ga, may indicate the presence of compositional heterogeneity within the crust and upper mantle beneath them. Three-dimensional P-wave velocity tomography using teleseismic rays from a variety of azimuths indicates the existence of contrasting P-wave crust and upper-mantle velocity patterns: (1) in the crust (0–40 km) the western Dharwar craton and the granulite terrain have lower velocity (up to −3%) compared with the higher velocity (1–3%) observed over the eastern Dharwar craton, and (2) in the upper mantle (40–177 km), there is 2–3% lower velocity beneath the granulite terrain compared with the western Dharwar craton. The existence of such lateral velocity variation in the crust and upper mantle, and its preservation since late Archaean times, points to the presence of possibly thick and chemically distinct lithospheres that did not participate in mantle convection.

What triggers Koyna region earthquakes? Preliminary results from seismic tomography digital array

The cause for prolific seismicity in the Koyna region is a geological enigma. Attempts have been made to link occurrence of these earthquakes with tectonic strain as well as the nearby reservoirs. With a view to providing reliable seismological database for studying the earth structure and the earthquake process in the Koyna region, a state of the art digital seismic network was deployed for twenty months during 1996–97. We present preliminary results from this experiment covering an area of 60 × 80 km2 with twenty seismic stations. Hypocentral locations of more than 400 earthquakes confined to 11×25 km2 reveal fragmentation in the seismicity pattern — a NE — SW segment has a dip towards NW at approximately 45°, whilst the other two segments show a near vertical trend. These seismic segments have a close linkage with the Western Ghat escarpment and the Warna fault. Ninety per cent of the seismicity is confined within the depth range of 3–10 km. The depth distribution of earthquakes delimits the seismogenic zone with its base at 10 km indicating a transition from an unstable to stable frictional sliding regime. The lack of shallow seismicity between 0 and 3 km indicates a mature fault system with well-developed gouge zones, which inhibit shallow earthquake nucleation. Local earthquake travel time inversion for P- and S-waves show ≈ 2% higher velocity in the seismogenic crust (0–10 km) beneath the epicentral tract relative to a lower velocity (2–3%) in the adjoining region. The high P- and S-wave velocity in the seismogenic crust argues against the presence of high pressure fluid zones and suggests its possible linkage with denser lithology. The zone of high velocity has been traced to deeper depths (≈ 70 km) through teleseismic tomography. The results reveal segmented and matured seismogenic fault systems in the Koyna region where seismicity is possibly controlled by strain build up due to competent lithology in the seismic zone with a deep crustal root.

Evidence for high velocity in Koyna seismic zone from P-wave teleseismic imaging

Three-dimensional P-wave velocity structure of the crust and upper most mantle beneath the Koyna Seismic Zone (KSZ) was determined by inverting 780 P-wave teleseismic travel times recorded by a 20 station digital network. The velocity image obtained through damped least square was tested for its resolution using checkerboard approach. The results of the inversion show strong lateral velocity variations of 6% to 7% within the study region. A high velocity anomaly (2% to 5%) in the upper and lower crust beneath the KSZ is flanked by lower velocity (−2% to −5%) anomalies on either side. The correlation of high-velocity with the seismic zone suggests that the earthquakes are occurring in more rigid rocks with increasing ability to store strain energy and release it as brittle failure.

High-velocity anomaly under the Deccan Volcanic Province

Abstract Tomographic modelling of P-wave residuals at 22 vertical-component seismograph stations operated in the Deccan Volcanic Province (DVP) in west-central India and the Dharwar Craton point to the existence of a large, deep (at least 300 km), high-velocity anomaly (1–3% faster) directly beneath most of the DVP and Dharwar Craton. There is also marginal evidence that the westernmost part of the DVP may be characterised by low velocity (up to −1.5%) to a depth of about 200 km. This coherent high-velocity anomaly under the DVP and its extension into the south Indian shield is viewed as a lithospheric root, an architecture characteristic of Precambrian shields.

Frequency-Dependent Lg Attenuation in the Indian Platform

We use seismograms from regional earthquakes recorded on digital seismographs in peninsular India to determine the frequency-dependent Q of Lg for the Indian platform. We measure Lg attenuation by determining the decay of spectral amplitudes with distance. The available data suggest some spatial variation in attenuation but a much denser ray-path coverage would be required to validate such observations. We, therefore, combine all the measurements of overlapping regions that span both the shield and intervening terranes to obtain an average value of attenuation for the Indian platform: Lg–Q = 665 ± 10 with the frequency exponent n = 0.67 ± 0.03. This average value of Lg attenuation for the Indian platform is similar to the average for other stable regions of the globe.

High velocity anomaly beneath the Deccan volcanic province: Evidence from seismic tomography

Analysis of teleseismicP-wave residuals observed at 15 seismograph stations operated in the Deccan volcanic province (DVP) in west central India points to the existence of a large, deep anomalous region in the upper mantle where the velocity is a few per cent higher than in the surrounding region. The seismic stations were operated in three deployments together with a reference station on precambrian granite at Hyderabad and another common station at Poona. The first group of stations lay along a west-northwesterly profile from Hyderabad through Poona to Bhatsa. The second group roughly formed an L-shaped profile from Poona to Hyderabad through Dharwar and Hospet. The third group of stations lay along a northwesterly profile from Hyderabad to Dhule through Aurangabad and Latur. Relative residuals computed with respect to Hyderabad at all the stations showed two basic features: a large almost linear variation from approximately +1s for teleseisms from the north to—1s for those from the southeast at the western stations, and persistance of the pattern with diminishing magnitudes towards the east. Preliminary ray-plotting and three-dimensional inversion of theP-wave residual data delineate the presence of a 600 km long approximately N−S trending anomalous region of high velocity (1–4% contrast) from a depth of about 100 km in the upper mantle encompassing almost the whole width of the DVP. Inversion ofP-wave relative residuals reveal the existence of two prominent features beneath the DVP. The first is a thick high velocity zone (1–4% faster) extending from a depth of about 100 km directly beneath most of the DVP. The second feature is a prominent low velocity region which coincides with the westernmost part of the DVP. A possible explanation for the observed coherent high velocity anomaly is that it forms the root of the lithosphere which coherently translates with the continents during plate motions, an architecture characteristic of precambrian shields. The low velocity zone appears to be related to the rift systems (anomaly 28, 65 Ma) which provided the channel for the outpouring of Deccan basalts at the close of the Cretaceous period.