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.