Earthquake site-response study in Giumri (formerly Leninakan), Armenia, using ambient noise observations

We have tested whether ambient noise observations reveal useful information regarding site response at sediment sites in Giumri (formerly Leninakan), Armenia. This was done by comparing the noise observations with spectral-ratio site-response estimates based on aftershock data of the 7 December 1988 earthquake. We show that neither the horizontal-component noise spectra alone, nor spectral ratios taken with respect to a nearby bedrock site, reveal useful information regarding site response. However, spectral ratios of horizontal components taken with respect to vertical components (Nakamura's method) exhibit a prominent peak that corresponds to the fundamental resonant frequency observed in the aftershock data. Based on this and other recent investigations, we conclude that Nakamura's method may be a reliable procedure for determining the fundamental resonant frequency of sedimentary deposits from ambient noise observations.