Local site effects in the northern North Sea based on single-station spectral ratios of OBS recordings

In recent years, there has been a growing awareness of the importance of local site effects in earthquake damage. A number of studies of recent destructive earthquakes have illustrated the relative contribution of enhanced ground shaking due to unconsolidated sediment layers. Among the different methods used to estimate local site response, the spectral ratio of shaking at a sedimentary site with respect to a bedrock reference site, has been successfuly applied in different geological environments. In this study, a technique recently proposed by Nakamura (1989) is used to evaluate site response using spectral ratios of horizontal vs. vertical components of earthquake recordings from a temporary ocean bottom seismograph (OBS) network in the northern North Sea and a permanent OBS at Oseberg oil field. Comparison with results obtained from the standard spectral ratios, indicate that the method is applicable also to subsea conditions, and the estimates obtained in this study indicate similar amplification factors to those obtained previously from analytical techniques. The ambient noise data on the other hand, gave unstable results, probably due to different noise characteristics in the marine environment. The results obtained on the earthquake data, however, provide an encouraging alternative to previously used analytical techniques for estimating local site response.