EXPERIMENTAL EVALUATION OF THE H/V SPECTRAL RATIO CAPABILITIES IN ESTIMATING THE SUBSURFACE LAYER CHARACTERISTICS

Microtremor data have been carried out for microzonation studies and disaster mitigation in urban areas. Over a period of two decades, the Nakamura’s H/V spectral ratio method was recognized as a simple and cost effective method in seismological geotechnics. In order to identify the soil layers characteristics, microtremor measurements were performed in 6 different regions in Iran. These sites are located in different parts of the south, center and east of Iran. Regarding the study, 471 recorded microtremors on an area about 700 km² were processed. In addition, the results of 227 refraction tests, 386 electrical resistivity tests and 197 boreholes were evaluated. Based on the analysis results, the resonance frequency of each station was estimated from the peak of the H/V spectral ratio components. Conducting the boreholes or geophysical investigations enabled the thickness of the sedimentary cover to be determined. Different places were selected as study areas namely, Bam, Bushehr, Qeshm Island, Mashhad, South Pars, and Qom. In order to develop the sedimentary thickness, an attempt has been made to derive a formula to correlate the frequency of the horizontal-to-vertical, (H/V) spectral ratio peaks (f0) to the sedimentary cover thickness (h). The obtained equations in the different sites indicate that the relationship between these two parameters has a power form and it is significantly affected by the subsurface topography and material properties. It is also observed that both shape of basin and sedimentary thickness have a significant influence on the relation formula parameters.