Response Spectra Ratio Research Articles

Amplification of earthquake ground motion by steep topographic irregularities

AbstractThe problem of amplification of seismic waves by surface topographic irregularities is addressed through analytical and numerical investigations. First, a closed‐form expression for estimating the fundamental vibration frequency of homogeneous triangular mountains is obtained, using Rayleigh's method. Subsequently, numerical modelling based on the spectral element approximation is used to study the 3D seismic response of several real steep topographic irregularities excited by vertically propagating plane S‐waves. A topographic amplification factor is obtained for each case, by a suitable average of the ratio of acceleration response spectra of output vs input motion. The 3D amplification factors are then compared with those derived by 2D analyses as well as with the topographic factors recommended in Eurocode 8 for seismic design. Copyright © 2002 John Wiley & Sons, Ltd.

Evaluation of Site Factors for Seismic Bridge Design in New York City Area

Site factors for seismic bridge design in the New York metropolitan area are evaluated. Several profiles from Brooklyn, Queens, and Manhattan, matching soil categories D and E as defined in the recent New York City Department of Transportation and NEHRP provisions, are analyzed using 1D wave-propagation theory. Dynamic soil properties are derived using state-of-practice correlations with standard penetration resistance and compared to available in situ geophysical measurements. Three different rock motions are used, each modified from real records to match 500- and 2,500-year probabilistic spectra for the region. Results are presented in terms of dimensionless ratios of surface and rock response spectra. The effect of impedance contrast between soil and rock on soil amplification is examined. It is shown that, although seismic hazard in the area is moderate, large surface motions can be generated because of strong site amplification effects that exceed those in the western United States. Derived spectra a...

Statistical study of spatial variation of response spectrum using free field records of dense strong motion arrays

Spatial variation of acceleration response spectra is examined using strong motion records for a large number of events from dense accelerometer arrays at Chiba in Japan and SMART-1 in Lotung, Taiwan. The effects of earthquake component, structural damping, earthquake magnitude, focal depth, epicentral distance, structural time period, and station separation on the intra-event variation of response spectra are examined first through an empirical analysis and then through a least-square regression fit for parametric study. A very large scatter of the response spectra ratio is observed for both arrays, especially for SMART-1 array. The mean values of the ratio vary from 10 to 20 per cent for Chiba array while they vary from 25 to 50 per cent for SMART-1 array. The coefficients of variation of the ratio range from 5 to 25 per cent for Chiba array and 30 to 50 per cent for SMART-1 array. The correlation among response spectra is found to be inversely proportional to station separation and shows frequency dependence. For larger time periods, the correlation is lower and not higher. The correlation is also lower for UD earthquake component as compared to the two horizontal components. For higher damping ratio, the correlation among spectra is higher. The effect of the earthquake magnitude, focal depth and epicentral distance on the spatial variation is complex. The three parameters having implicit interdependence, considering their combined effect, a positive contribution to the value of ratio of response spectra is observed in the case of larger earthquake events. Furthermore, as mentioned above, the spatial variation for SMART-1 array is much larger than that for Chiba array. This difference can be attributed mainly to the difference in distance between the instruments in the two arrays. However, some of the difference is considered to be due to site specific characteristics. Copyright © 1999 John Wiley & Sons, Ltd.

Site effects in the city of Thessaloniki (Greece) estimated from acceleration data and 1D local soil profiles

AbstractIn this study, the site effects on seismic ground motion in the city of Thessaloniki (Greece) are estimated by applying experimental methods on acceleration data and theoretical modeling. The technique of standard spectral ratio (SSR) is applied to a reference station located on rock, while the horizontal-to-vertical spectral ratio (HVSR) technique is applied to earthquake records (entire record length including P and S waves) as well as on noise recordings. In addition, the SSR method is also applied to the vertical components. The results from all methods are compared in terms of resonant frequencies and amplification levels. The fundamental resonant frequency is identified by all methods, while the average amplification level is generally underestimated when the HVSR technique is used. An attempt is made to correlate the site amplifications computed in this study with the observed macroseismic intensities reported at the same sites for the 20 June 1978 earthquake. A relation of the form, δI=α+b⋅log[SSℛ¯(f)] is derived, where δI is the intensity increment with respect to the intensity of the reference station and SSR(ƒ) is the mean amplification factor obtained at each station using the SSR technique for a certain frequency band. In the numerical approach, we construct complete strong-motion synthetics using the modal summation method for the P-SV waves up to frequencies of 10 Hz. As input, four point sources are used, located at different distances and azimuths from the stations. Ratios of response spectra of the local 1D over the regional 1D synthetic seismograms are calculated. The obtained mean spectral amplifications are compared with those derived from experimental data, and the two sets are found to be consistent at most stations.

Site amplifications in the city of Benevento (Italy): comparison of observed and estimated ground motion from explosive sources

A hybrid approach is applied to construct 2-D synthetic seismograms for explosive sources. The computation of the signals has been performed with a hybrid technique, that couples the modal summation method, describing the P-SV propagation from the source to a target area, to the finite difference method, allowing the computation of the 2-D response of the target area. We use this technique to study the site response in the city of Benevento (Southern Italy). The results are compared both with those obtained for the same model, but considering an extended double-couple source and SH wave propagation and with those derived from a direct analysis of experimental data obtained during the Benevento Seismic Risk Project, funded by the Commission of European Communities. The type and depth of the source and the source-receiver distance can provide different excitations, but the response of the target area, in terms of ratios of response spectra, remains in general stable in the high frequency range (>2 Hz). The explosive source does not excite lower frequencies and is therefore unable to provide estimates of site effects in this frequency range. We also show that it is important to consider both P-SV and SH waves when assessing site effects. Finally, a very good definition of a structural model from geological data is fundamental in order to explain the observed data in the time domain. The observed data validate our synthetic modelling for evaluating the site effects and for a tentative seismic microzonation of the city of Benevento.

東京低地における強震観測

The average amplification characteristics of a sedimentary layer with a thickness of several kilometers in the Tokyo lowlands are estimated by using the SMAC's records obtained during the moderate earthquakes which occurred beneath the Kanto district. The average amplification characteristics are given by the average of ratios of the zero-damped velocity response spectra of surface strong-motion to those of bedrock motion.The zero-damped velocity response spectra of bedrock motion are given through the prediction models which were constructed by using data obtained by accelerometers installed at the bottoms of deep boreholes in the pre-Tertiary basement. The amplification factors obtaines at about 1Hz, are found to depend on soft alluvial layer ; high amplification factors as high as 20 are observed. The estimated response characteristics of the representative sedimentary layer in the Tokyo lowlands are explained by taking account of a frequency-dependent Q, i, e., Q = 50f (f =0.5-2 Hz) and Q=100 (f = 2-10Hz).