Abstract
Ambient-noise methods, and the horizontal-to-vertical spectral ratio (HVSR), whether from earthquakes (eHVSR) or mircrotremors (mHVSR), have gained much popularity in the field of site response over the last decade. These methods can be used either for direct prediction or to inform the velocity structure utilized in more conventional site response analyses. This paper describes a field study and subsequent analyses undertaken in the Lower Hutt sedimentary basin of New Zealand. The field study involved collecting 50 ambient-noise or microtremor measurements across the entire basin over the same time window that microtremor measurements were being recorded at strong-motion stations in the basin for use as reference stations. Additionally, microtremor array measurements (MAM; involving ~24 ambient-noise measurements per site) and multi-channel analysis of surface waves (MASW) were conducted at five sites to better quantify deep and shallow velocity structure of the basin. In total, microtremor data were collected at 154 locations. This paper focuses on the use of the microtremor data collected for direct prediction of site response in the sedimentary basin. The hybrid standard spectral ratio approach is tested in this region. A rigorous validation study was performed at strong motion stations at which microtremor measurements were made for use as reference basin sites in the hybrid spectral ratio method. The prediction accuracy and uncertainty of the method, when using synchronized versus unsynchronized data between the reference basin sites and temporary target sites, are compared. The hybrid spectral ratio method predicts well the observed site amplification between nearby, deep basin sites for f < 5 Hz, when synchronized data are used. Predictions around the fundamental frequency of the basin (corresponding to periods of 1- 2 seconds) worsen when unsynchronized data are used due the influence from environmental and anthropogenic factors on microtremor amplitudes. Finally, the method is used to predict the site response, relative to a rock reference site, at all basin sites at which synchronized temporary microtremor data were collected. Early efforts to spatially interpolate the observations and predictions are discussed.
