Horizontal To Vertical Spectral Ratio Technique Research Articles

Structure, soil–structure response and effects of damage based on observations of horizontal-to-vertical spectral ratios of microtremors

The microtremor horizontal-to-vertical-spectral-ratio (HVSR) technique is widely used in the urban environment to assess the fundamental frequency response of the ground. Extensive literature exists about case histories using HVSR for microzonation in several cities, but no systematic studies have been devoted to check the presence of soil–structure interaction effects, and even less attention to study building behaviour after earthquake damage. To evaluate the above-mentioned effects, a series of experiments are reported in this article. We first made a series of microtremor measurements on buildings and civil structures to evaluate the reliability of fundamental frequency determinations. Then, we considered several case studies to evaluate the effect of soil–structure interaction in estimates of site response in the presence of tall buildings. Finally, an experiment on the frequency change due to damage was performed. It was possible to confirm that HVSR is able to detect building fundamental modes and once known the building frequency, it is also possible to detect the presence of soil–structure interaction. Thus, once the presence of the building natural frequency is identified, it is possible to infer the site response from free field measurements. We also found that the HVSR technique is equally useful for detecting structural damage by determining the frequency shift of the buildings.

HINTS ABOUT SITE AMPLIFICATION EFFECTS COMPARING MACROSEISMIC HAZARD ESTIMATE WITH MICROTREMOR MEASUREMENTS: THE AGRI VALLEY (ITALY) EXAMPLE

We approach from a new standpoint the problem of estimating seismic hazard for some towns and villages located in Val d'Agri area (Southern Italy) that in the past have been affected by several seismic events. The estimates are carried out using a method that is based on the analysis of site seismic history extracted from macroseismic catalogues. To study the influence of site effects two different procedures have been performed: in the first, seismic hazard estimates have been deduced from epicentral data only, in the second, intensity data actually observed at the site are also considered. The difference between the two estimates can be correlated with local variations of seismic response due to local geological features which are responsible for possible cases of amplification. In order to validate the presence of such correlation, seismic hazard estimates have been compared to site amplification measurements obtained by using the HVSR (Horizontal to Vertical Spectral Ratio) technique. Our findings reveal a good correlation between seismic hazard enhancements and the presence of site amplification effects. The application of this kind of analysis to the Val d'Agri area has pointed out that the joint estimates of site seismic hazard enhancement and HVSR measurements could be a helpful tool to identify problems related to seismic microzonation.

The HVSR technique reveals pervasive nonlinear sediment response during the 1994 Northridge earthquake (Mw 6.7)

The Northridge earthquake triggered a considerable number of nonlinearsite-response studies. The standard approach involves the standard spectralratio (SSR) method, requiring a suitable reference site (rarely available).Recently, Dimitriu et al. (1999, 2000) proposed the use of the(nonreference) horizontal-to-vertical spectral-ratio (HVSR) technique toassess nonlinear site response. The purpose of this article is to test it onNorthridge-earthquake data.Accelerograms from five sediment sites exhibiting low-frequency resonanceswere used. Hypocentral distances and recorded peak horizontal groundaccelerations (PHA) are between 19.9 km–27.7 km and381 cm/s2–827 cm/s2. For each recording, HVSRs werecomputed in a 6-s moving window, spanning the entire recording and thuscovering a wide range of excitation levels.All five sites are found to exhibit statistically significant negative correlation(r between –0.70 and –0.91) between the site's dominant-resonancefrequency, fres, determined visually from the HVSRs, and the mean(in the 6-s window) horizontal ground acceleration, MHA. Assuming alayer-over-halfspace site structure, it is possible to relate the variation offres to the change in the sediment-layer shear modulus, G,by Gs/Gw = [1-(fress-fresw]/fresw]2, where superscripts `s' and `w' represent the valuescorresponding to `strong' and `weak' motion. Shear-modulusdegradation, Gs/Gw, is a measure of the nonlinear responseof the (softest) layer(s) contributing the most to the resonance; the resultssuggest that these are the upper 30 m of sediments. Substituting thevalues of fres for the `strong' – (MHA > 80 cm/s2) and`weak' – (MHA < 35 cm/s2) motion ranges yielded realisticvalues of Gs/Gw for the sites considered, allowing to rankthem in accordance with the magnitude of their nonlinear response.

A Numerical Experiment on the Horizontal to Vertical Spectral Ratio in Flat Sedimentary Basins

— In this paper we study the seismic response of flat sedimentary basins and carry out numerical experiments to determine the extent to which we could go using the Horizontal to Vertical Spectral Ratio (HVSR) for a given site. The HVSR has been used by many researchers to characterize local conditions in terms of the dynamic response of the soil, and one of its variants, that proposed by NAKAMURA (1989) in which records of microtremors are used, is one of the most applied in recent years. We study the response of different configurations under incident waves coming from an explosive source using the Indirect Boundary Element Method (IBEM), and we investigate two cases: low- and high-velocity contrast. We compute the seismic response using the HVSR technique at various locations in the free surface of the basins, and compare it with the response calculated with the horizontal Sediment to Bedrock Spectral Ratio (SBSR) and with the Horizontal Component (HC) of the transfer function for the displacement at the same locations. The comparison shows that, in general, HVSR cannot provide the predominant period of a site due to the fact that this technique cannot predict accurately the Spectral amplification levels. On the other hand, the HVSR provides an erroneous response in the sedimentary basins which have a low-impedance contrast, with respect to bedrock, and with shape ratios like the one studied here, whereas it can reasonably well predict the fundamental local frequency when there is a high-impedance contrast, except in the center of the basin.

Evidence of nonlinear site response in HVSR from SMART1 (Taiwan) data

Deamplification of strong motion and the increase of the effective period of soil deposits are typical nonlinear effects; we seek them in SMART1-array data by applying the horizontal-to-vertical spectral ratio (HVSR) technique. The recordings, from four soil and one rock stations, represent 23 earthquakes (ML 4.9–7.0); PGA varies between 20–260cm/s2. For each station, mean HVSR curves are calculated for two PGA ranges: <75cm/s2 and >100cm/s2 (weak and strong motion). At the soil stations, the “weak” (linear) and “strong” (nonlinear) responses are significantly different. Below 1–1.8Hz, the nonlinear response exceeds the linear one. Above 2Hz, the nonlinear response drops below the linear one and above 4–6Hz below unity (deamplification). From 10 to 16Hz, the two responses converge. One soil site shows significant negative correlation between resonance frequency and ground acceleration. Such behaviour agrees with other empirical studies and theoretical predictions. Our results imply that the HVSR technique is sensitive to ground-motion intensity and can be used to detect and study nonlinear site response.

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.

EURO-SEISTEST strong-motion array near Thessaloniki, Northern Greece: A study of site effects

AbstractThe effects of local geology on the characteristics of strong ground motion have been, and continue to be, a field of active research. Despite the considerable efforts made so far, there are still several unresolved and controversial issues remaining. In particular, debates still continue over the limits of applicability of one-dimensional (1D) wave-propagation models. There are also unresolved questions related to the implementation and reliability of site-response estimation techniques such as the standard spectral ratio (SSR) and the horizontal-to-vertical spectral ratio (HVSR). This study addresses these issues on the basis of data from the EURO-SEISTEST strong-motion array at Volvi, near Thessaloniki, Greece. The data set used consists of accelerograms of 32 earthquakes, almost exclusively local, covering a magnitude range from ML 2.0 to 6.1. The range of recorded accelerations is 0.001 − 0.042 g.We implement two 1D wave-propagation models (the Haskell-Thompson matrix method and Kennett's reflectivity-coefficient method) and the HVSR site-response assessment technique. We test the applicability of the 1D models in a basin environment. We use both Fourier-amplitude and response spectra to compute HVSR and investigate how HVSR is influenced by the choice of the time window and smoothing procedure.We found the HVSR technique and 1D SH-wave modeling to perform reasonably well in a sediment-valley environment (the modeling was performed for a site in the center of the valley).