Preliminary analysis for evaluation of local site effects in Lima city, Peru from ground motion data by using the spectral inversion method

Abstract

Effects of local site, propagation path and source in ground motion records observed in Lima, Peru, were separated by the spectral inversion method proposed by Iwata and Irikura (1986 [1], 1988 [2]) to examine the relation between local subsurface conditions and local site amplifications in a frequency range from 0.5 to 20 Hz. S-wave portions of accelerograms in horizontal components observed at 5 stations for 11 events along the Pacific coast of Lima city, Peru, were analyzed. The Q factor was obtained from our inversion results as frequency dependent function QS( f) = 80.4 f 0.63. In terms of local site effects, stations located on alluvial gravel deposits were likely to suffer amplification at frequencies larger than 4 Hz, while one station (CAL site) located on soft soil sediment has different behavior of amplification. We also compared our results with 1-D theoretical computation, observed standard spectral ratio and observed H/V spectra in previous studies, finding that site responses determined by different methods are similar. In addition, we analyzed the relationship between average S-wave velocity in the top 10 meters and the average site amplification factor in a frequency range between 0.5 Hz and 10.0 Hz, showing a good correlation between the two parameters. We also calculated the average transfer function (AvTF) to compare it with the existing amplification map for Lima city, and found that our calculations differed from this map.