SITE AMPLIFICATION EFFECTS OBSERVED FROM MODERATE INTENSITY EARTHQUAKE RECORDS OBTAINED IN THE SMASCH ARRAY

Abstract: The SMASCH array for the observation of strong ground motion has been operating in the Santiago area since 1989. One of the objectives of this instrumentation project is to study the influence of local site conditions on observed earthquake ground motions. The array consists of seven free-field sites located on different soil conditions ranging from a reference rock outcrop site to a site located on soft silty ground. Using data from low to medium intensity events which occurred between 1989 and 2001, an evaluation of site amplification effects has been carried out. Fourier Amplitude spectra of the accelerograms for each site, for several different events, were computed. For this purpose only a limited portion of the record was used: that containing predominantly S waves. The spectral shapes were smoothed and amplitude ratios with respect to the rock site were calculated. For each site the averages of the amplification ratios were computed after grouping the events in three different sets: all events, events in two different magnitude ranges (4 to 5 and 5 to 6), and events having different epicenter location (NW azimuth and NE azimuth relative to the array location). From the results obtained it is apparent that there is a noticeable influence of the site soil conditions on the amplification value and also that this amplification can be quite different depending on the period range considered. The influence of magnitude in not very large, but is noticeable for several sites. The influence of azimuth is clear at some sites and not noticeable at others