The Crust-Mantle Structure of the Indian Peninsula from Surface Wave Dispersion

Abstract

Summary The observed variations of group velocity with period for fundamental Rayleigh and Love waves along two profiles, viz. Kodaikanal-New Delhi and Kodaikanal-Poona as well as along paths from a few epicentres in the neighbourhood of New Delhi to Kodaikanal have been used to determine the crust-mantle structure of the Indian Peninsula. The periods range from 6 to 80 s for Rayleigh waves and from 11 to 97 s for Love waves. Structural models obtained show that the total crustal thickness is 41 km in the central part of the Peninsula and increases to 52 km in the Western Ghats region; at the same time the thickness of the granitic layer also increases from 12 to 16 km. It has been noted that the assumption of a simple isotropic mantle does not give satisfactory fit to both Rayleigh and Love waves data at higher periods. A satisfactory fit has been obtained by considering the mantle between depths 60 and 160 km to be anisotropic with SV velocity about 5 per cent less than SH velocity. The crust-mantle structure of the Indian Peninsula was investigated by the author (Bhattacharya 1971, hereafter referred to as A) using the observed variations of group velocity with period of fundamental Rayleigh and Love waves. In A the interpretation of the group velocity data was made with the help of group velocity and its partial derivatives with respect to structural parameters for the model 200 (Savarensky, Glasko & Granit 1967) and the shield model (Bloch, Hales 8c Landisman 1969); the exact group velocity curves for the final models thus obtained after perturbing the initial model were not drawn. In the present work the interpretation of the above-mentioned group velocity data (which are slightly improved here by using the epicentral data of International Seismological Centre) is modified by computing theoretical dispersion curves with IBM 360/44 and using a program which utilizes the computational procedure described in another work by the author (Bhattacharya 1972). 2. Data The long period seismograms of Kodaikanal (KOD), Poona (POO) and New Delhi (NDI) belonging to USCGS World Wide Seismological Stations Network have been used in this study. The locations of the stations are KOD (lo 14' N, 77 28' E), PO0 (18 32' N, 73 51' E) and NDI (28 41' N, 77 13' E) and shown in Fig. 1. The