An urban scale study of ground response in Mellieħa using a Tromino seismograph

Abstract

The Maltese Islands are composed of a simple 4-layer sedimentary sequence, which were formed by deposition under the sea during the Oligocene and Miocene epochs. The project involved a microzonation study of the urban area of Mellieħa with respect to seismic site response. This area was mainly chosen because of its geology and topography. Although Mellieħa lies on Upper Coralline Limestone, which is underlain by a thick layer of Blue Clay, in some parts one may also find Blue Clay showing at the surface due to the erosion of the overlying layer. The main purpose of the study was to investigate the effect of Blue Clay on amplifying ground motion. A seismic site response survey was carried out, using the Horizontal to Vertical Spectral Ratio (HVSR) technique applied to ambient noise. This technique yield the site frequency and the corresponding amplification factor at a given point. The ambient noise was measured by a portable seismograph, Tromino® (Micromed S.p.A.). Twenty minute recordings of ambient noise using Tromino® were taken for 41 different sites distributed across Mellieħa I Santa Maria Estate. Most of the readings were measured on Upper Coralline Limestone while a few were measured directly on Blue Clay. The noise was analysed using the Nakamura Technique to give the Horizontal to Vertical Ratio (HVSR). The results show a clear correlation between the peak frequencies and the underlying geological structure. In agreement with previous studies in other areas of the Maltese islands, sites on the Upper Coralline Limestone show a consistent resonance frequency of 1 - 2 Hz attributed to lithological characteristics, while sites lying directly on clay exhibit higher frequencies (2.06 to 18.44 Hz) with more variability, as a result of the varying thickness of clay.