Determination of Natural Period of Ground for the Kerman City (South East of Iran) by Microtremors

Microtremors were recorded on 38 locations within the Kerman city urban area in order to develop a basis for preliminary seismic microzonation studies. A site on exposed bedrocks, east of the city, was also measured to evaluate the spectral characteristics of sources of microtremors. All recordings were done using portable 3-component digital seimograph with 96dB dynamic range, capable of recording microseisms induced by natural and artificial sources, and high-gain short-period (1-sec) seismometers. Measurement were carried out during night-time to ensure quality recoding and reduced unwanted environmental noises.Extensive numerical processing and analysis were performed to obtain reliable power density spectra. Transfer function of the alluvial plain at measurement points was estimated by reducing the effect of the sources of tremors from calculated spectra. Nakamura technique was applied to eliminate the effect of ambient seismic sources. Results are presented in terms of a zonation map of dominant periods for the alluvial basin. The spatial distribution of dominant periods indicates a complex underground structure beneath the city. On average, the dominant periods increase from east to west, and from north to south. The results are constrained by the location of few late cretaceous outcrops, east of the city. Further detailed subsurface geotechnical and geophysical investigations are rquired to draw a light on the geological structures beneath the city.