The evaluation of the adjacency effect on the seismic response of homogenous hills

A brief review of previous studies shows that topographic features play an important role in the amplification (de-amplification) of seismic waves. Most researchers who have focused on topographic effects have paid attention to the distribution of amplification on isolated features, whereas what is seen in nature is often more complicated. It is known that 2D or 3D geological structures further contribute to the amplification of seismic ground motion. The main objective of this study is an investigation of the influence of adjacent hills on seismic responses. First, a parametric study has been performed, considering five models (consisting of one to five 2D symmetric homogeneous semi-sinusoidal hills), which are subjected to vertical incident waves. The 2D numerical results show that the largest values of spectral amplification for a series of 2D semi-sinusoidal hills are observed at the crest stations of the central hill, and it increases when the number of hills increases symmetrically on both sides of the central hill. This means that the spectral amplification increases from 3.3 for single-hill model to 4.37 for five-hill model. In the following, to validate the parametric results, three adjacent hills located in the city of Arak are studied using the horizontal-to-vertical spectral ratio analysis of microtremor recordings. The comparison with the amplification results obtained from the distinct element (UDEC) and boundary element (HYBRID) numerical approaches indicates that the central hill is more affected by adjacency, rather than the side hills. The spectral amplification for the crest station on the central hill is higher than the two other stations.