ESTIMATION OF SEISMIC VELOCITY USING SURFACE WAVE DISPERSION

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 in all the direction around Bhopal station. The seismic events chosen are within the regional distance, shallow focused and with magnitude greater than 5 Mb. These earthquakes are located in all the direction around Bhopal station. Since dispersion curves give the average value along its ray path, our investigation focusses on finding the seismic velocity in all the direction and then comparing with each other. 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 10-100 sec for all the 9 cluster around Bhopal station. 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 are highest reaching 4.5km/sec for love wave in the cluster 9 in which events are present in bay of Bengal. Also in other clusters, in cluster no. 2 the group velocity of both love and Rayleigh wave exceed the 4km/sec. In cluster 4,5 and 6 difference between Love and Rayleigh wave group velocities are higher at smaller period. Whereas difference is less at higher period. But in cluster 9 and 10 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 10-95 sec. The inversion process is approached by combining the Love and Rayleigh group velocities. The depth up to 180 km can be resolved. As usual we found the moho at 12-15km for the oceanic crust for the cluster no.9 and in continental region moho is at highest depth of approx. 65km for cluster 4 which is slightly west of the north from Bhopal station or we can say hind-kush region. The depth up to 180 km can be resolved. The observed high-velocity for cluster 9 is explained by a higher density oceanic crust.