Bulletin of the Seismological Society of America, Vol. 86, No. 2, pp. 306-319, April 1996 Horizontal-to-Vertical Spectral Ratio and Geological Conditions: The Case of Garner Valley Downhole Array in Southern California by N. Theodulidis, P.-Y. Bard, R. Archuleta, and M. Bouchon Abstract 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 suggested as an indi- cator of site effects. The data set consists of 1 t0, three-component, high sensitivity accelerograms, recorded at five different depths by the Garner Valley Downhole Array (GVDA), in southern California, with peak ground accelerations 0.0002 g _--< ag 0.04 g, magnitudes 3.0 ML 4.6, and hypocentral distances 16 km = R = 107 km. First, the stability of the (h/v) spectral ratio is investigated by computing the mean for the whole data set in different depths. The (h/v) spectral ratio on the surface is compared with the surface-to-depth standard spectral ratio, with theoretical S-wave transfer functions derived from the vertical geotechnical profile, as well as with the (h/v) spectral ratio of synthetic accelerograms generated by the discrete wavenumber method. Both theoretical and experimental data show a good stability of the (h/v) spectral ratio shape, which is in good agreement with the local geological structure and is insensitive to the source location and mechanism. However, the absolute level of the (h/v) spectral ratio depends on the wave field and is different from the surface-to-depth spectral ratio. Consequently the (h/v) spectral ratio tech- nique provides only partially the information that can be obtained from a downhole array. But surface-to-depth ratios may also be misleading because they combine effects at surface and at depth. Introduction The importance of local site geology in seismic design is nowadays well established. The evaluation of site effects on strong ground motion has been extensively studied during the last two decades and is thoroughly reviewed by Aki (1988, 1993) and Finn (1991). The most standard method for characterizing site amplification is the spectral ratio tech- nique. It requires a pair of instruments, one located at the site under investigation (generally on alluvium) and the other on a "reference site," preferably a nearby rock site; both instruments must simultaneously record the ground motion of a number of events. In many cases, it is difficult to find sites on bedrock that are close enough to alluvium ones. In addition, outcrops of bedrock sites are usually weathered, and the resulting superficial velocity gradient is capable of influencing the "reference" ground motion. In order to over- come these shortcomings, downhole arrays whose record- ings allow for a direct comparison of the response of the superficial layers to the response of the underlying bedrock have been developed. Data obtained from these arrays put some light into the efficiency of theoretical models used (Seed and Idriss, 1970; Joyner et al., 1976; Redpath and Lee, 1986) and into the interference between incident and re- flected wave field (Shearer and Orcutt, 1987; Blakeslee and Malin, 1991; Aster and Shearer, 1991) as well as into the near-surface attenuation or amplification (Hauksson et al., 1987; Malin et al., 1988; Seale and Archuleta, 1989; Liu et aL, 1992). Apart from the standard spectral ratio technique, re- cently, another nonreference site-dependent technique was introduced by Nakamura (1989) for the evaluation of site effects. This technique, originally applied to microtremors (Ochmachi et al., 1991; Field and Jacob, 1993; Field et al., 1993; Lachet and Bard, 1994), has also been applied to weak- (Lermo and Chavez-Garcia, 1993; Duval, 1994; Field, 1994) and, in some cases, to strong-motion studies (Lermo and Chavez-Garcia, 1993; Theodulidis and Bard, 1995). This technique, which is comparable with the so-called re- ceiver-function technique, applied to studies of the upper mantle and crust from teleseismic records (Langston, 1979), assumes that the local site conditions do not significantly influence the vertical component of the ground motion. The Garner Valley Downhole Array (GVDA) has been in full operation since the summer of 1989 and consists of 5 three-component accelerometers placed at depths of 0, 6, 15, 22, and 220 m. Archuleta et aL (1992, 1993), after a preliminary data analysis, observed for the frequency range 306