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Characteristics of H/V Spectrum
Abstract
Introduction The H/V spectral ratio is overviewed from its origin and on the application for practical disaster prevention.
In this paper, several case studies will be presented in which calibration method of FEM models via structural natural frequencies identified onsite were applied. We can expect reliable analysis results from the well-calibrated model. Calibration is required to make the model, which includes, at least indirectly, any imperfection that exists in the structure, and which cannot be registered in any other way. Possibilities of structural health monitoring will be shown also, by comparing the natural frequencies obtained by measurements at different times: before, during and after the structural rehabilitation. This method is not omnipotent, so, except for the advantages, we shall show limitations of method related to the type of structure, weather conditions and other. Our experience with low-rise structures with great stiffness will be compared with the published experience of other authors.
The paper compares various site-response estimation techniques using aftershock data of the 1989 Loma Prieta earthquake collected in Oakland, California. It examines and tests three site-response estimation techniques that do not rely on a reference site to estimate source and path effects. The first involves a parameterized source- and path-effects inversion. The second technique involves taking horizontal- to vertical-component spectral ratios of shear-wave aftershock data. The third estimate is formed by taking horizontal-to vertical-component ratios of ambient seismic noise, and these are shown to reveal the fundamental resonant frequency of the sediment sites. The highly frequency-dependent character of site response is well constrained, and the fact that non-reference-site-dependent methods are capable of revealing this is promising for site-specific hazard assessments in regions that lack adequate reference sites. -from Authors
SUMMARY This paper examined the relationship between the H/V spectral ratio of strong motion and the amplification characteristics or the H/V spectral ratio of microtremor. Both the data of strong motion and microtremor have been recorded at the lake, transition and hill zones in Mexico City. As a result, in case of strong motion, the amplification characteristic agreed with the H/V spectral ratio approximately. Moreover the H/V spectral ratio of strong motion agreed with that of microtremor at the same site. However, at some sites of soft ground on irregular basement, the shape of the H/V spectral ratio of microtremor and the strong motion are almost similar for each other but the H/V spectral ratio of microtremor is less than the other. At such a site, microtremor contains Rayleight wave component and also strong motion contains relatively less Rayleigh wave component. It suggests that Rayleigh wave has influenced to this situation.
Numerical simulation of noise is used to investigate the characteristics of the spectral ratio between horizontal and vertical components (H/V ratio) and its sensitivity to various parameters in order to better appreciate the reliability of the technique proposed by Nakamura (1989) to estimate site amplification effects from single station noise recordings. Noise is simulated as the signal produced at a single site by a set of superficial sources (unidirectional forces or dipoles) disposed all around with random amplitude and time delay. Individual signals from a single source are computed by the discrete wave number technique. Synthetic calculations for 15 soil profiles show that this ratio exhibits a single, clear peak, the location of which is independent of the source excitation function, but strongly correlated with the local geological structure: its frequency is very close to the S wave resonance frequency. This peak appears to be mainly controlled by the polarization curve of the fundamental Rayleigh waves, which in turn exhibits a sharp peak around the fundamental resonance mode of the sedimentary structure. A similar result is found for the H/V ratio obtained for incident plane SV waves. In contrast, the amplitude of this peak exhibits a poor correlation with the ground motion amplification of S waves at resonance frequency. It is shown to be related with a high sensitivity on the value of the Poisson's ratio in the uppermost layer presumed to be the noise source layer, and, though to a much lesser extent, on the mean distance between site and noise sources. It is concluded that Nakamura's method can clearly allow the resonance frequency of a given sedimentary site to be measured very efficiently and very cheaply, but that its use for deriving the amplification at resonance frequency seems still premature from a theoretical point of view.
Ambient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura's technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification factor.
A method which employs microtremor has been introduced for estimating dynamic characteristics of surface layers, in early 1950. Then usage of this method has received lots of criticism considering uncertainty about source of microtremor. After an introduction of the Nakamura's technique (H/V or QTS technique; Nakamura, 1989), many people have paid a renewed great attention for estimating dynamic characteristics of ground and structures using microtremor, since clear and reliable information was provided by very simple and inexpensive noise measurements. In recent years, although several researchers claimed that theoretical ground of this technique is not clear and consensus based on experiment couldn't be reached, there have been many successful experimental studies based on these technique. Many theoretical studies have been performed, for explaining the amount of types of waves included in microtremor and checking the applicability of the QTS technique. And some of them are suggested that the peak on H/V ratio can be explained with the fundamental peak of Rayleigh waves. From the output of these researches, explanation of microtremor with Rayleigh waves caused some confusion between users and the author decided to clear out this problem. The basic idea and the main goal of QTS technique are tried to be re-explained in present paper. The author's explanation about the effects of contents of Rayleigh waves in microtremor is also given. Other possible usage of products from QTS technique (predominant frequency and amplification factor) for hazard estimation is also given. As it is well known, occurrence of earthquake damage depends upon strength, period and duration of seismic motions. And these parameters are strongly influenced by seismic response characteristics of surface ground and structures. This reality makes investigation of vulnerability of ground and structures an important issue, before the earthquake occurs. For this purpose, vulnerability indices called K values were proposed by Nakamura (1996). K values are simply derived from strains of ground and structures. Formulation of K values for ground (Kg) and some application examples are also given in present paper. These new values give a chance to estimate vulnerabilities of all types of structures and ground, before the real damage occurs.
The phase velocity and the amplitude distribution are studied, including the cases of Poisson's ratio σ≠0.25, for Rayleigh type waves (M11 wave) which propagte along the surface of stratified double layer.
The velocities and the elasticity ratios used for the calculation are given in Table 1 and the numerical results in Tables 2 and 3.
Though the results are generally in consistent with those obtained by others in the past, that is, the cases of λ≠μ, the amplitude distributions are much complicated in the case that μ/μ' is equal to five or more. So the particle orbit at the surface changes twice times its direction with the increase in wave length.
The fact just mentioned is understood in comparison with the change of the particle orbit of the M21 waves in the existence domain.
The amplitude characteristics as the fundamental natures of microtremors are discussed at present paper. The experimental field is Hakodate City in Hokkaido.
The power spectral density functions of microtremors were compared with amplitude functions of Rayleigh wave (M11) computed from underground structure. Furthermore, theoretical ratio of the horizontal component to the vertical component of Rayleigh wave (M11) was compared with that of the observed microtremors. From these comparisons, it was shown that the amplitude characteristics of microtremor was mostly composed of Rayleigh wave (M11).
The spectral ratio between horizontal and vertical components (H/V ra-tio) of microtremors measured at the ground surface has been used to estimate fun-damental periods and amplification factors of a site, although this technique lacks theoretical background. The aim of this article is to formulate the H/V technique in terms of the characteristics of Rayleigh and Love waves, and to contribute to improve the technique. The improvement includes use of not only peaks but also troughs in the H/V ratio for reliable estimation of the period and use of a newly proposed smoothing function for better estimation of the amplification factor. The formulation leads to a simple formula for the amplification factor expressed with the H/V ratio. With microtremor data measured at 546 junior high schools in 23 wards of Tokyo, the improved technique is applied to mapping site periods and amplification factors in the area.
As methods for dynamic characteristics estimation of surface layers, investigation of boreholes and a method which employs microtremors are well known. Borehole investigation, one of the most accurate methods, is costly and time-consuming and is not available all the time. The method that employs microtremors is handy but has not produced satisfactory results to this day. This paper describes a new processing method that employs microtremor observations yet produces accurate estimates of the characteristics of the ground motion. The method uses a vertical component and horizontal components. As a result, the spectrum ratio of the horizontal components and the vertical component of the microtremors bears a resemblance to the transfer function for the horizontal motion of the surface layers.
Earthquake disaster never occurs whenever seismic force does not surpass durability of ground and structures. Thus continuous watch of earthquake motion is first of all necessary, in order to predict and monitor the occurrence of earthquake disaster. For the ground and structures exposed to earthquake motion, it is necessary to grasp their durability precisely by executing investigations in advance. From the view point of earthquake disaster prevention, to grasp the durability of ground and structures is even more important than to monitor the earthquake motion. In this paper, a new technique to investigate rapidly with precision durability against earthquake of various structures and surface ground by using microtremor. Validity of the proposed method has been examined by comparing the results of investigations by the new technique in the earthquake damaged areas (before or after the event) with actual earthquake damage experienced. If weak points of structures can be detected in advance by investigating the durability of various structures and ground, damage of structures can be decreased by taking appropriate countermeasures. Besides, secondary disaster can be also decreased very much by adopting precise immediate measures based upon accurate damage estimation when earthquake occurs.
半無限彈性體に表面層がある場合に於けるレーリー波の傅播に就ては巣で既にブロムウィッチ,ラプ等により論ぜちれ最近には妹澤博士によりて詳しく研究せられた.筆者は妹澤博士によるレーリー波の分散の結果を用ひて表面に於ける變位の水平成分と鉛直成分との比を波長と層の厚さとの函數として計算した.この比は表面層が下層より軟い場合には波長が層の厚さに比して非常に短い場合に於ても叉非常に長い場合に於ても共に普通のレーリー波の値に漸近的に近い値を示すが,波長と層の厚さとが同程度である場合には概して云へば著通のレーリー波のそれよりも大なる値となる.
Estimations of Amplification Characteristics of Surface Ground and PGA using Strong Motion Records
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