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Rayleigh-Wave Group-Velocity Studies beneath the Indian Ocean

¹ National Geophysical Research Institute
Hyderabad – 500007 (A.P.) India

The fundamental mode Rayleigh wave generated by 16 earthquakes, which occurred in the Indian Ocean and were recorded at 14 seismic stations of Asia, Africa, and Australia, are analyzed to estimate the Rayleigh-wave group velocity at periods of 10 to 60 sec by using the multiple filter analysis technique. In addition to this, Rayleigh-wave group-velocity data available in the literature across different propagation paths of the Indian Ocean forming a dense distribution of seismic wave traverses have been considered for tomographic studies. The group-velocity distribution maps have been obtained at periods of 10, 20, 30, 40, 50, and 60 sec for the Rayleigh wave. The minimum value of the group velocity of 2.0 km/sec is centered near the Rodriguez triple junction (25° S, 70° E) at 10 sec and higher value (3.4–4.0 km/sec) at 20 sec and greater period. As we move in the north-northwest direction from the Rodriguez triple junction, the Rayleigh-wave group-velocity isolines at 10 and 20 sec are also increasing in the northern direction in both the east and west sides of the ridge axis. Another minimum value of group-velocity isolines (1.9 km/sec) is centered near (25° S, 90° E) at 10 sec, and there is a gradual increase in the northern direction across the Ninetyeast Ridge. These observations are in accordance with the increase in the age of the ocean floor and there is a gradual increase in the group-velocity isolines. This can be interpreted as due to the hot uprising convection current, the material shows low shear-wave velocity along the ridge axis, and farther away from the ridge axis the material has become cooler with higher shear-wave velocity.

The inversion of Rayleigh-wave group-velocity across the Ninetyeast Ridge shows 120-km-thick LVZ (low-velocity zone) of shear-wave velocity of 4.38–4.78 km/sec at a depth of 40 km from the water surface, whereas other parts of the Indian Ocean show 44-km-thick LVZ with shear-wave velocity of 4.12–4.17 km/sec at a depth of 40 km from the water surface. The high shear-wave velocity below Moho (4.19–4.75 km/sec) beneath the Ninetyeast Ridge indicates the presence of a cold and dense lithosphere beneath it.