Site effect assessment using KiK-net data: part 2—site amplification prediction equation based on f0 and Vsz

Abstract

This paper presents empirical correlations between amplification factors and simple site parameters derived from a large subset of the KiK-net data. The amplification factor is estimated from the ratios between the surface and down-hole horizontal response spectra, corrected for the varying depths and impedance of the down-hole sites (Cadet et al. in Site effect assessment using KiK-net data-part 1-a simple correction procedure for surface/downhole spectral ratios, 2011). Several site parameters are selected on the basis of their simplicity and availability at relatively low cost. They are the shallow time-average velocities V SZ , with z equal to 5, 10, 20 and 30 m, and the fundamental frequency f 0 . The amplification factors are then correlated with each of the individual site parameters; four other "twin-parameter"-couples (f 0 , V SZ )-are also considered and the correlation with amplification factors is performed through a normalization of the frequencies by each site fundamental frequency. The quality of the correlations is given by a misfit compared with the original data variance. The largest variance reduction is obtained with twin-parameter characterizations, out of which the couple (f 0 , V S30 ) proves to provide the lower misfit. The performance of single parameter correlations is relatively lower; however, the best single parameter proves to be the fundamental frequency, which provides smaller misfit than the Vsz parameters. A comparison is also performed with the amplification factors recommended in European regulations, showing that it is possible right now to significantly improve both the site characterization criteria and the associated amplification factors, for use in building codes and microzonation studies.

Surface wave surveys for seismic site characterization of accelerometric stations in ITACA

“…Together with V S,30 (Borcherdt 1994), which is the primary information with respect to site classification according to EC8 (CEN 2004), other lumped parameters could be useful for a more detailed seismic site classification (e.g. : Bray and Rodríguez-Marek 1997;New Zealand Standard 2004;Pitilakis et al 2006;Cadet et al 2011). In particular for each site, the following information is reported: -testing setup: active (A), passive (P) or both (A + P); -average shear wave velocity for the uppermost 30m (V S,30 ); where the soil model obtained by surface wave tests does not reach 30 m depth, the V S,30 is estimated using the correlation proposed by Figini (2006); -depth H of conventional seismic bedrock (V S > 800 m/s), if reached in the test; -average shear wave velocity above the seismic bedrock V S,H , evaluated in terms of traveltime, with a formula similar to the one used for V S,30 (inverse of the weighted average of the slownesses); -fundamental resonance frequency of the site: either the experimental value from HVSR (if available) or the value estimated as V S,H /4H on the basis of the two previous columns (the latter is marked with a "star"); -EC8 classification on the basis of shear wave velocity profile from surface wave tests.…”

“…The V S,30 can be evaluated very efficiently with surface wave methods also because its average nature does not require the high level of accuracy that can be obtained with seismic borehole methods (Moss 2008;Comina et al 2010). Moreover an estimate of the V S,30 can be obtained from direct interpretation of surface wave dispersion curve (Brown et al 2000;Albarello and Gargani 2010).…”

Northern Aegean Earthquake (Mw=6.9): Observations at three seismic downhole arrays in Istanbul

“…But, obviously the recording at the deepest sensor will be affected by the downward incident waves reflected from the upper layers. Hence in this case the ratio will be 2A a /(A b þB b ), B b representing the reflected portion of the incident wave at the bedrock level [21][22][23][32][33][34]16,17]. This means that an adjustment is needed to determine the amplification ratio more accurately.…”

“…However, though there are several approaches proposed, a unified procedure is not available yet. An extensive treatment of the topic can be found in a study published by Heloise et al [16,17].…”

“…Hence it is quite customary to report these curves. However, it should be kept in mind that the use of spectral ratios calculated using the downhole arrays have limitations and may produce misleading results [21][22][23][32][33][34]16,17]. The reason for the error contained in these curves is due to the contribution of the reflected waves, namely the down-going waves, to the up-going incident waves.…”