An investigation on the effect of topography on ground motion amplitudes, using 3D simulation of seismic waves in Tehran

Tehran, the capital of Iran, is located in the foothills of Alborz Mountains in a very seismically active region and is surrounded by many active quaternary faults with the potential for devastating earthquakes. This city has a daytime population of 12 million people and is the political and economical capital of Iran as well. These facts together with the existence of neighborhoods with old buildings that are poorly constructed increases the importance of different studies to better characterize the nature of ground shaking from future probable earthquakes in the city.
With the increasing computational power in recent years, the seismic waveform simulation has become one of the preferred methods for studying the seismic hazard in regions like Tehran. The topography effect, in this regard, is one of the components of site effects that need to be included in hazard assessment studies. It is a known fact that the surface topography has significant effects on the earthquake ground motion, especially in mountainous areas with ridges and valleys. As has been observed in the annals of past earthquakes and numerical simulations, topography, In general, increases the ground shaking amplitude on mountain tops and reduces the ground motion amplitude in valleys.
The Spectral Element Method, combining the power of Pseudo Spectral methods with the geometrical flexibility of Finite Element Method, is one of the best methods for modeling the seismic wave propagation in regions such as Tehran, with notable surface topography.
In the present research, the Spectral Element Method was employed in order to simulate three point sources and three extended source earthquake scenarios both within and around Tehran city and to investigate the role of surface topography on the ground shaking inside the city. The topography effect was investigated by comparing the results of simulations with and without incorporating the surface topography in meshes; the resulting amplification was presented as color maps in the region.
Simulations were performed via SPECFEM3D software package which implements the Spectral Element Method to simulate the seismic wave propagation in the region. The first step in using the Spectral element Method was to create quality meshes in the study area; in this regard, CUBIT program is employed so as to create the hexahedral meshes in the model area. The extent of the model area was 100 x 60 kilometers horizontally and, vertically, from the ground surface to the depth of 60 kilometers with limits of latitude and longitude of 35.5 to 36.5 degrees and 51 to 52 degrees, respectively.
The peak ground acceleration amplification maps were presented for the topography effect in the frequency range of 0.01 to 1 Hz. The findings indicated that the topography effect inside Tehran city is dependent upon the earthquake scenario and the resulting amplification from topography effect inside the city is generally low and negligible. On average, the amplification resulting from the topography inside the city was between -10% to % which could reach as high as ±30% in certain earthquake scenarios at certain locations within the city. In mountainous areas near the city, we observed amplification on the peaks and de-amplification in the valleys; the amplification fell between -50% to %.