HORIZONTAL-TO-VERTICAL SPECTRAL RATIO AND GEOLOGICAL CONDITIONS - THECASE OF GARNER-VALLEY DOWNHOLE ARRAY IN SOUTHERN CALIFORNIA

The aim of the present article is to further check the use of the hori zontal-to-vertical (h/v) spectral ratio, which has been recently sugge sted as an indicator of site effects. The data set consists of 110, th ree-component, high sensitivity accelerograms, recorded at five differ ent depths by the Garner Valley Downhole Array (GVDA), in southern Cal ifornia, with peak ground accelerations 0.0002 g less than or equal to a(g) less than or equal to 0.04 g, magnitudes 3.0 less than or equal to M(L) less than or equal to 4.6, and hypocentral distances 16 km les s than or equal to R less than or equal to 107 km. First, the stabilit y of the (h/v) spectral ratio is investigated by computing the mean fo r the whole data set in different depths. The (h/v) spectral ratio on the surface is compared with the surface-to-depth standard spectral ra tio, with theoretical S-wave transfer functions derived from the verti cal geotechnical profile, as well as with the (h/v) spectral ratio of synthetic accelerograms generated by the discrete wavenumber method. B oth theoretical and experimental data show a good stability of the (h/ v) spectral ratio shape, which is in good agreement with the local geo logical structure and is insensitive to the source location and mechan ism. However, the absolute level of the (h/v) spectral ratio depends o n the wave field and is different from the surface-to-depth spectral r atio. Consequently the (h/v) spectral ratio technique provides only pa rtially the information that can be obtained from a downhole array. Bu t surface-to-depth ratios may also be misleading because they combine effects at surface and at depth.