PREDICTION OF DEPTH- DEPENDENT FREE SURFACE EFFECTS ON GROUND MOTION

Abstract

In this report a finite difference model is analyzed to demonstrate free surface effect. A simple plane wave model is adequate to explain depth dependent free surface effect on ground motion. A FORTRAN program for SH wave is used to simulate the model, our model is homogeneous one layer model having density 2700kg/m3 and SH wave velocity 1847 m/sec, in which 25 receivers are applied vertically at an interval of 20 m, so last receiver is at 480 m depth. The model has been discretized with a grid size 20m in both the direction. Source is generated at a depth of 1.5 km from surface. Responses at surface and at various depths are compared with respect to each other for 10 corner frequencies 0.5, 0.55, 0.6, 0.7, 0.8, 1.0, 1.5, 2.0, 2.5 and 4 Hz. It is generally observed that the corner frequency in the earthquake records decreases with the increase of earthquake magnitude. The plots of responses in time domain as well as in frequency domain are plotted by Sigma Plot 2010 Software. Further model is analyzed for effect of incident angles and type of source (point source, rupture and anti- rupture) to study the variation of free surface effect with depth. From different graphs it is found that at free surface amplitude is amplified, in SH wave case amplitude at depth is half of the amplitude at surface. The results are compared with other studies for validation of our results, which gave similar results.