Site Effect Study in the Southwest of Tehran Using Microtremor Measurements

Microtremor measurement, as one of the well known methods in site effect study and seismic microzonation, is a very convenient, simple, and economic tool to estimate the effect of surface geology on seismic motion. H/V (Nakamura's technique) has received great attention from all over the world with its simplicity together with quick information about dynamic characteristics of ground and structures. In this study, the Nakamura's technique has been used to estimate fundamental frequencies and amplification factors of 40 sites in the southwest of Tehran. Different techniques in Nakamura's method are applied on more than 480 time windows of microtremor measurements. The results at different sites indicate that the variation of resonance (fundamental) frequencies is fast in the SW and W-NW of the study region, whereas in the central and eastern parts this variation is slow. Essentially, the resonance frequencies differ from 1.5-2.5 Hz in eastern and central parts to 3.5-4.5 Hz in SW and W-NW. The estimated amplification factors indicate that the alluvial deposits in the SW of Tehran can amplify the magnitude of earthquakes up to 6 (+/-2) at resonance frequency.