Numerical Modeling of the Adjacency Effect on the Seismic Response of Homogeneous Semi-Sine Hills

Knowing the factors that increase or decrease the effects of seismic waves is one way to minimize the damage caused by earthquakes. In recent decades, many researchers have studied these factors, including one of the most important ones: topographic effects. The effect of surface features on ground motions, known as the topographic effect, has been studied using various methods that have emerged in the last few decades. Most studies on topographic features focus on single features, while nature shows more complexities in surface features. Therefore, this research aims to investigate the effect of the neighborhood of homogeneous semi-sine hills on seismic response. Semi-sine features are the most common form of topographic features in nature, making them important in the application of study results. The research uses the boundary element method and HYBRID code for 2D numerical modeling. In the first part, the seismic behavior of a semi-sine hill with a shape ratio of 1, height of 500 m, and half-width of 500 m is studied. Then, the number of hills is increased to five to investigate the effect of adjacency. The seismic behavior of the hills in three models M1, M3 and M5, and for two groups of stations located on the crest and foot-hill are compared with each other in the time and frequency domain. Finally, the impact of the angle of the incident wave on the adjacency effect and the seismic response of the adjacent features is investigated.The following are the results obtained from this research:As the number of hills increases, the incident wave propagates in the domain by re-reflecting from other features. According to the presented time domain results, the wave reflected in the adjacent hills generates a new peak in the S1 stations. In the frequency domain graphs, an increase in the value of the spectral amplification ratio is observed along with the frequency shift towards higher frequencies.The crest of the middle hill in the M5 model, specifically the S1-M5 station, had the highest value of spectral amplification. This indicates that as the number of hills increased from one to five, the amount of amplification increased at the crest of the middle hill.Adjacent hills also cause intensification in the amplification and de-amplification values at the bottom of the hill.The comparison of the graphs of the spectral amplification ratio of the two models M1 and M5 under vertical and oblique incident waves with an angle of 30 degrees shows that the angle of the incident wave has an effect on the “adjacency effect” and causes an increase in the spectral amplification ratios.The results for station S1 in both models under oblique and vertical incident waves indicate that the frequency shift in the peak of the spectral amplification graphs decreases under oblique incident waves with an angle of 30 degrees.Due to the population growth and urban expansion, settlements around cities have increased, especially in mountainous areas around cities such as Pardis City in Tehran province. It is important to pay attention to the effect of adjacency and its impact on seismic response in the design of buildings, both on the crest and heel of hills. The results of this research show that the crest of hills in the adjacent state has a significantly higher spectral amplification ratio than a single hill. However, building codes only briefly reference the effect of topography