SITE RESPONSE AS A FUNCTION OF NEAR-SURFACE GEOLOGY IN THE SOUTH ICELAND SEISMIC ZONE

Site response measurements provide information on the amplification of ground motions generated by local conditions. Recent studies of large destructive earthquakes have shown that damage during the earthquakes are often caused by the amplification of seismic waves in near-surfac e sedimentary layers. The estimation of site response is therefore cri tical, in order to evaluate the true seismic hazard potential of a giv en area. We investigated local site amplifications in the South Icelan d Seismic Zone (SISZ). Nine digital seismographs were deployed, tempor arily, in an area of approximately 400 km(2), in the westernmost part of the SISZ. Among the 90 events recorded, 15 were used in this study, including a magnitude 3.1 (M-L) event and selected aftershocks, which occurred in the northern outskirts of the village Hveragerdi. Single Station Spectral Ratios (SSSR) of the recorded earthquakes revealed so me of the effects of local site conditions. Spectral amplification fac tors of 2-5 on average, can be expected in the SIL area, depending on the sediment type and thickness. Higher site amplifications occur in t he southern part of the study region, where the thickest sedimentary c over is found. Spectral amplification, related to topographical effect s, is observed at the bedrock reference station, Bjamastadir. Standard Spectral Ratios (SSR), with respect to the bedrock reference station, Bjamastadir, were also calculated for some stations, in order to comp are the two spectral ratio results. The two methods show a good correl ation at some stations, whereas at others they vary considerably. The comparison between the earthquake and ambient noise data, on the other hand, gave better correlation when the SSSR method is used.