Determination of natural frequency of soil vibration in Sarcheshmeh copper mine, using microtremor records


It is well known that ground-shaking site effect caused by an earthquake can vary significantly within a small distance. This is because at sites having soft soil and/or topographic and basement undulations, seismic energy gets trapped, leading to amplification of vibration that may cause considerable damage to man-made structures. Theoretical analysis and observational data have shown that each site has a specific resonance frequency at which ground motion gets amplified. Man-made structures having resonance frequency matching that of the site have the maximum likelihood of getting damaged. Hence, in order to construct seismically-safe structures, it is important to know the site response.
Various methods are available for the estimation of site response. The best method is to record strong ground motion caused by a large local earthquake. However, fortunately such events are not very frequent in many areas. Hence, for site response analysis, this method is not very practical. Another method is to carry out extensive seismic reflection and/or refraction surveys and geotechnical surveys; this method is extremely expensive and time-consuming. Recently microtremor data have also been widely used for estimation of site response. The advantage of this method is that it takes very little time for data collection. One does not have to wait for an earthquake to occur. Very few instruments are required; the data collection can be handled even with a single instrument.
The method involves recording microtremor data from the site to be investigated. It is assumed that signals from a hard rock site are carried equally well at all frequencies. On the other hand, a soft-soil site amplifies the signal at its resonance frequency, which depends on factors such as the soil type, basement configuration, etc. Hence, if the source and the path effects were removed from the spectra of the signal, then we should get a flat spectra at a hard-rock site and spectra showing peaks at resonance frequency at a soft-soil site.
In this study microtremor measurements were carried out in the Sarcheshmeh copper mine area at about 12 sites and the natural frequency at each site was estimated considering the main peak in the spectral ratio between the horizontal and the vertical component, the method called the NAKAMURA technique (H/V). Many experimental and theoretical studies have shown the reliability of microtremor measurements in site predominant frequency estimation. At each site traces have been collected 3 or 4 times and for all of them natural frequency based on SESAME project group standards have been analyzed. Then on the basis of natural frequency, site soil types have been determined and average Shear wave velocity for each site has been predicted. All results compared with available field observations.