An approach to identify site response directivity of accelerometer sites and application to the Iranian area

In recent years, several workers have found numerous cases of sites characterised by significant azimuthal variation of dynamic response to seismic shaking. The causes of this phenomenon are still unclear and possibly related to combinations of geological and geomorphological factors determining a polarisation of resonance effects. To improve their comprehension, it would be desirable to extend the database of observations on this phenomenon. Thus, considering that unrevealed cases of site response directivity can be “hidden” among the sites of accelerometer networks, we developed a two-stages approach of data mining from existing strong motion database, to identify sites affected by directional amplification. The proposed procedure first calculates Arias Intensity tensor components from accelerometer recordings of each site, to determine mean directional variations of total shaking energy. Then, at the sites where a significant anisotropy appears in ground motion, azimuthal variations of HVSR values (spectral ratios between horizontal and vertical components of recordings) are analysed to confirm the occurrence of site resonance conditions. We applied this technique to a database of recordings acquired by accelerometer stations in the Iranian area. The results of this investigation pointed out some sites affected by directional resonance that appear correlated to the orientation of local tectonic lineaments, these being mostly transversal to the direction of maximum shaking. Comparing Arias Intensities observed at these sites with theoretical estimates provided by ground motion prediction equations (GMPE), the presence of significant site amplifications was confirmed. The amounts of the amplification factors appear correlated to the results of HVSR analysis, even though the pattern of dispersion of HVSR values suggests that, while high peak values of spectral ratios are indicative of strong amplifications, lower values do not necessarily imply lower amplification factors.