SITE EFFECTS USING HVSR AND SSR TECHNIQUES IN RAMSAR CITY

Over the last century, a lot of cities have expanded dramatically and specialists expect a similar trend for at least the next decades. In many parts of Iran, urban areas are usually built on soft sediments. Because this vast country is a part of Alpine-Himalayan seismic belt, so not only urban, but countryside and villages are also prone to strong ground shaking. The surface of the Earth is always in vibration, even without earthquake. The vibrations at low seismic frequencies called “microtremors”, exist everywhere. Site responses computed for a large number of events, allows to define unknown source position and plausible mechanism of the future ground motions. One way that is based on ambient vibrations are the H/V method of Nakamura (1989), has been proven to be a convenient technique to estimate the fundamental frequency of soft deposits. This method is based on the Fourier spectral ratio of the horizontal and vertical components of seismic noise wavefield. The relation between this ratio and the frequency is called the HVSR curve and has a peak within dominating frequencies of the record. The peak amplitude is used for the amplification factor estimate, and its frequency is the principal frequency of surface layer. The ability of the HVSR technique to provide a reliable data related to site response has been repeatedly shown in the past (Duval et al., 2001). We do use this approach for few reasons, first, the instruments and analysis are simple. Second, it can be performed at any time and any place. Third, it does not produce such environmental trouble as active seismic does (Bard, 1998). Each signal recorded in a site has the simultaneous effects of sources, paths, and sites. One has to eliminate the source and the path to determine the effect of soil (Huang & Teng, 1999). For this reason and based on the idea presented by Nakamura (1989), trajectory effects can be eliminated with a particular type of normalization on microtremors. In addition, peaks were achieved by spectral analysis need clarification, as well. In other words, processing the peaks obtained by recording a few minutes of field noise are needed significantly to perform validation steps and clarity for the maximum peaks. It may also have the maxima of industrial origin that must be checked. To do this, standards have been used based on European Comprehensive Research known as SESAME published in the year 2004, to analyzed data separately for each station and compared with the criteria. Finally, the estimated frequency is compared with other methods.

کلیدواژه ها:

Site effects ، Microtremors ، Spectral ratio ، Resonant frequency ، Amplification