D.D. Singh

Crust and upper mantle velocity structure beneath north and central India from the phase and group velocity of Rayleigh and Love waves

Abstract Crust and upper mantle velocity structure is determined using the fundamental mode Rayleigh and Love waves for 23 different paths across north and central India. The earthquakes occurring in Kashmir, Nepal Himalaya, Burma and nearby regions and recorded at Hyderabad, New Delhi and Kodaikanal seismic stations are used for dispersion studies. Frequency Time Analysis (FTAN) using the multiple filter technique is done for the fundamental mode surface waves to estimate group velocities from the time periods of 13–71 s. The single station technique is used to estimate Rayleigh and Love wave phase velocities at time periods of 15–75 s. Velocity structure down to a depth of 120 km is determined using both Rayleigh and Love wave phase and group velocities. The crustal thickness is found to be 50 km, with 4 km sedimentary thickness, for the northern India across Burma to New Delhi path, and 44 km for the central India across Kashmir, Nepal Himalaya and northeast India to Hyderabad and Kodaikanal path. A low-velocity zone ( V p = 8.4 km/s, V s = 4.15 km/s ) is found from 100 km below the surface. Velocity structure shows that the shield structure of the peninsula extends up to northernmost India below sub-Himalayan regions with an overlying sedimentary thickness of 4 km. The lower value of observed group velocity and higher phase velocity across northern and central India may be caused by the presence of varying sedimentary thickness in the northernmost part of India (the Indo-Gangetic Plain) below the sub-Himalayan region. The observed lower value of Rayleigh wave group velocities below 40 s time periods can be explained by assuming a thin layer of a high contrast material with a P-wave velocity of 4.6 km/s, an S-wave velocity of 2.5 km/s, a density of 3.24 g/cm3 and a thickness of 3 km, centred above the Moho.

Spectral analysis of body waves for earthquakes in Nepal Himalaya and vicinity: Their focal parameters and tectonic implications

Abstract The source characteristics of eight recent earthquakes which occurred in Nepal Himalaya and had body wave magnitudes between 5.2 and 6.2 have been investigated. Analyses of P- and S-wave spectra of these events are made using Brunes seismic source model to estimate the various source parameters. These are: seismic moment, source dimension, stress drop, average dislocation, apparent stress, and the radiated energy. The stress drop for earthquakes occurring in Nepal Himalaya is found to be high in general, indicating that high stresses are prevailing in the region. Four new focal mechanisms have been determined. These, and the other available focal mechanisms indicate that the northward thrusting is predominant. However, a focal mechanism with a large component of normal faulting is found for the Kinnaur earthquake of January 19, 1975 ( m b = 6.2, seismic moment = 5.6 · 10 26 dyne-cm, fault length = 41.8 km, radiated energy = 0.36 · 10 22 erg, stress drop = 27 bar, apparent stress = 2 bar, dislocation =136 cm, fault area =1372 km 2 ). Some other solutions with large strike-slip and normal fault components have been reported earlier. We believe that these earthquakes are associated with faults and folds trending normal and oblique to the Himalayan trend, for which geological evidence has been presented in recent literature.

Shear wave velocity structure over the eastern Indian subcontinent

Abstract Fundamental-mode surface wave generated by twelve earthquakes, which occurred in northeastern India and nearby regions and were recorded at the Shillong (SHL) and Chiengmai (CHG) seismic stations are used to determine the shear wave velocity structure beneath these regions. The Frequency Time Analysis method is used to determine the group velocities for periods ranging from 4 to 50 s for fundamental-mode Rayleigh and Love waves. A high shear wave velocity (4.2–4.7 km s −1 ) is estimated in the lower 30–34 km of the crust. Crustal thickness is found to vary from 36 to 56 km with an overlying 2–4 km sedimentary thickness of shear wave velocity of 2.55–2.75 km s −1 . The shear wave velocity in the upper 10 km of crust is 3.5–3.7 km s −1 below the sedimentary layer. The inferred high shear velocity for the lower crust beneath these regions suggests an oceanic affinity and they are different from the Indian shield structure. The lower crust must have an oceanic origin as derived from the reminiscent of a certain oceanic lithosphere. The available data do not permit us to estimate the upper mantle structure precisely; however, it shows a higher upper mantle shear velocity of 5.1–5.3 km s −1 . This may be indicative of active tectonism beneath these regions in the uppermost mantle. The Moho boundary is not sharply defined here. The increase in crustal thickness may be due to the collision of the Indian and Eurasian plates in the north and subduction of the Indian plate beneath the Burmese Arc in the east.

Source parameters of the Burma-India border earthquake of July 29, 1970, from body waves

Abstract The source parameters are determined for the Burma-India border earthquake of July 29, 1970, from body-wave spectra. We obtain seismic moment [ M o (P) = 4.83 , M o (S) = 3.40 ] · 1026 dyne cm, source dimension [ r (P) = 22.5, −r (S) = 27.7 ] km, radiated energy [ E R (P) = 7.19 , −ER (S) = 1.35] · 1020 ergs and the stress drop = 11 bars.

Q-structure beneath the north and central Indian Ocean from the inversion of observed Love and Rayleigh wave attenuation data

Abstract The fundamental-mode Love and Rayleigh waves generated by 57 earthquakes which occurred in the north and central Indian Ocean (extending to 40°S) and recorded at Indian seismograph and other WWSSN stations such as HOW, SHL, VIS, MDR, HYB, KOD, CHG, TRD, POO, BOM, GOA, NDI, NIL and QUE are analysed. Love and Rayleigh wave attenuation coefficients are estimated at periods of 15–100 s using the spectral amplitude of these waves for 98 different paths across the Bay of Bengal Fan, the Arabian Fan, and the north and central Indian Ocean. The large standard deviations observed in the surface wave attenuation coefficients may be a result of regional variation of the attenuative properties of the crust and upper mantle beneath these regions. Love wave attenuation coefficients are found to vary from 0.000 03 to 0.000 45 km −1 for the Bay of Bengal Fan; from 0.000 03 to 0.000 85 km −1 for the Arabian Fan; and from 0.000 03 to 0.000 35 km −1 for the north and central Indian Ocean. Similarly, Rayleigh wave attenuation coefficients vary from 0.000 03 to 0.0004 km −1 for the Bay of Bengal Fan; from 0.000 06 to 0.0007 km −1 for the Arabian Fan; and from 0.000 03 to 0.0007 km −1 for the north and central Indian Ocean. Backus and Gilbert inversion theory is applied to these surface wave attenuation data to obtain average Q −1 models for the crust and upper mantle beneath the Bay of Bengal, the Arabian Fan, and the north and central Indian Ocean. Inversion of Love and Rayleigh wave attenuation data shows a high-attenuation zone centred at a depth of > 120 km ( Q β ≈ 125) for the Bay of Bengal Fan. Similarly, a high-attenuation zone ( Q β ≈ 40–70) occurs at a depth of 60–160 km for the Arabian Fan at 100–160 km ( Q β ≈ 115) for the Indian Ocean off Ninetyeast Ridge, and at 80–160 km ( Q β ≈ 80) for the Indian Ocean across the Ninetyeast Ridge. The Q β −1 models show a lithosphere thickness of 120 km beneath the Bay of Bengal Fan. Similarly, lithosphere thickness of 70, 100 and 80 km is estimated beneath the Arabian Fan, and the Indian Ocean off Ninetyeast Ridge and across Ninetyeast Ridge, respectively. The base of the lithosphere is identified as the depth at which there is a significant increase in the Q β −1 value, which attains its maximum value in the asthenosphere. The thinning of Indian lithosphere beneath the Arabian Fan suggests high temperature below Moho depth (60 km from surface) which has caused a high-attenuation zone at this shallow depth.

Crust and upper-mantle velocity structure beneath the northern and central Indian Ocean from the phase and group velocity of Rayleigh and love waves

Abstract Fundamental-mode Rayleigh and Love waves generated by nine earthquakes, which occurred in the central Indian Ocean (with epicentres extending to 40° S) and recorded at seven WWSSN stations of central Asia, have been used to determine the phase and group velocity along various paths across the northern and central Indian Ocean. The dispersion characteristics of Rayleigh and Love waves show a thick crust of 23 km thickness. This anomalous crustal thickening, having quasi continental-oceanic structure, may be explained by assuming the gradual transformation of top mantle material into material having either crustal like velocity or slightly lower than Moho velocity (P-wave velocity 7.72 km s −1 , S-wave velocity 4.45 km s −1 ). The velocity structure down to a depth of 200 km has been obtained across the northern and central Indian Ocean. A low-velocity zone of 90 km with P-wave velocity of 7.85 km s −1 and S-wave velocity of 4.37 km s −1 is estimated to be centred at a depth of 78 km from the water surface. This low shear velocity in the mantle may be caused by the partial melting and elevated temperature there. The observed dispersion data for Rayleigh and Love waves also exhibit a strong anisotropy along different paths in the crust and upper mantle. The observed dispersion data for Rayleigh and Love waves that cross the Ninety-east Ridge are characterized by lower phase and group velocities, as compared to waves having paths in a more western direction than this ridge axis. However, the phase velocities for Rayleigh and Love waves are characterized by lower values across the Ninety-east Ridge, as compared to the normal oceanic structure.

Crustal structure of the peninsular shield beneath Hyderabad (India) from the spectral characteristics of long-period P-waves

Abstract The crustal transfer functions have been obtained from long period P-waves of thirteen teleseismic events recorded at Hyderabad (HYB), India. The crustal structure beneath this seismograph station has been obtained after comparing these functions with the theoretical crustal transfer functions which were computed using the Thomson-Haskell matrix formulation. The method is suitable and economical for determining the fine crustal structure. The crust beneath Hyderabad is found to consist of three layers with total thickness of 36 km. The thicknesses of top, middle and bottom layers are 21 km, 8 km and 7 km, respectively.

Q-structure beneath the Tibetan Plateau from the inversion of Love- and Rayleigh-wave attenuation data

Abstract The fundamental mode Love and Rayleigh waves generated by ten earthquakes and recorded across the Tibet Plateau, at QUE, LAH, NDI, NIL, KBL, SHL, CHG, SNG and HKG are analysed. Love- and Rayleigh-wave attenuation coefficients are obtained at time periods of 5–120 s using the spectral amplitudes of these waves for 23 different paths. Love wave attenuation coefficient varies from 0.0021 km−1, at a period of 10 s, to 0.0002 km−1 at a period of 90 s, attaining two maxima at time periods of 10 and 115 s, and two minima at time periods of 25 and 90 s. The Rayleigh-wave attenuation coefficient also shows a similar trend. The very low value for the dissipation factor, Qβ, obtained in this study suggests high dissipation across the Tibetan paths. Backus-Gilbert inversion theory is applied to these surface wave attenuation data to obtain average Qβ−1 models for the crust and uppermost mantle beneath the Tibetan Plateau. Independent inversion of Love- and Rayleigh-wave attenuation data shows very high attenuation at a depth of ∼50–120 km ( Q β ⋍ 10 ). The simultaneous inversion of the Love and Rayleigh wave data yields a model which includes alternating regions of high and low Qβ−1 values. This model also shows a zone of high attenuating material at a depth of ∼40–120 km. The very high inferred attenuation at a depth of ∼40–120 km supports the hypothesis that the Tibetan Plateau was formed by horizontal compression, and that thickening occurred after the collision of the Indian and Eurasian plates.

Source-mechanism of the burma—india border earthquake of october 17, 1969

Abstract The focal mechanism for the Burma—India Border earthquake of October 17, 1969 has been determined using the P-wave first motions, S-wave polarization angles and surface wave spectral data. A combination of thrust and strike-slip faulting is obtained along a plane with a strike N 34° W, dip 26° SW and slip angle 141°. The direction of rupture propagation is southward. This earthquake, which occurred at latitude 23°N, indicates north-south compression and change in the thrusting direction which is in general eastwest in the Burma region. This earthquake mechanism may suggest southward underthrusting of the Burmese block or contortion of the lithospheric block of the Indian plate. The source-parameters have been estimated for this event using the body and surface wave spectra. From the surface waves, calculated values of the magnitude, radiated energy, moment and apparent stress are 5.7, 0.21 × 10 21 ergs , 0.32 × 10 26 dyne-cm and 2 bar, respectively. From P-waves the seismic moment, fault length, stress drop and dislocation are determined to be 0.9 × 10 26 dyne-cm, 51 km, 2.4 bar and 15 cm, respectively.