ANALYSIS OF SURFACE WAVE AND ITS • INVERSION FOR S-WAVE VELOCITY

Abstract

In this research work an attempt has been made to observe the variation in dispersion curves characteristic of the surface waves using broadband data of two stations along and across the 90°E ridge within the Indian sub-continent. The seismic events chosen are within the regional distance, shallow focused and with magnitude greater than 5.5 Mb. These earthquakes are located in Bay of Bengal area of northern Indian Ocean closer to Ninety East Ridge. Since dispersion curves give the average value along its ray path, our investigation focusses on observing the effect along the ridge and compare it with dispersion curves obtained across the ridge. Multiple filter technique is applied to compute the group velocities of the surface waves from the dispersion curves. The measured dispersion curves for this data set are within the period of 10-70 sec and 10-100 sec for Kodikanal and Shillong stations respectively. Group velocity is further computed using the weighted average and higher order polynomial fir to estimate trend of trajectory between station and epicenters of the earthquakes. It is observed that Love and Rayleigh group velocities in east-west direction is higher than Love and Rayleigh group velocities along the north-south direction. On both the directions the difference between Love and Rayleigh wave group velocities are higher at larger period. Whereas difference is less at smaller period. On the basis of number of available dispersion curves at higher period, selected period range for inversion lies in between 10-70 sec. The inversion process is approached by combining the Love and Rayleigh group velocities. The depth up to 90 km can be resolved. The observed low-velocity is explained by a under thrust wedge of low velocity material beneath the oceanic crust.