Quantification of effects of geometry of sediment bedrock interface on ground motion in 3D basin with circular free surface

This paper presents the effects of geometry of sediment bedrock interface (GSBI) of 3D basins with a circular outcropping free surface on the characteristics and the focusing of the basin-generated surface (BGS) waves and associated spatial variations of the average spectral amplification (ASA) and average aggravation factor (AAF). An increase of ASA towards the centre of the semispherical (SS-) basin as compared to the 2D basin revealed the focusing of the BGS-waves in the SS-basin. The obtained 2.8 times larger ASA at the centre of the SS-basin to that of 2D basin calls for a special attention in predicting the seismic hazard in such basins for earthquake engineering purposes. The analysis of the simulated results revealed that the amplitude amplification due to the focusing of the BGS-waves was highly affected by the GSBI. For example, the ASA at the centre of a trapezoidal basin with edge slope 20° was 50% larger than that at the centre of a trapezoidal basin with edge slope 45°. It is recommended to compute the AAF using the same components of ground motion as that of the incident wave to conservatively aggravate the ground motion to incorporate the 3D basin-effects in the seismic microzonation where it is over based on the 1D response of sediment column.