Spectral Ratios Of Microtremors Research Articles

Subsurface structure identification at the blind prediction site of ESG6 based on the earthquake-to-microtremor ratio method and diffuse field concept for earthquakes

We participated in the blind prediction exercise organized by the committee of the blind prediction experiment during the 6th International Symposium on Effects of Surface Geology on Seismic Motion (CBP-ESG6). In response to the committee's request, we identified the ground velocity structure from microtremors observed at a target site as the first step of the exercise. First, we calculated the horizontal-to-vertical spectral ratio of microtremors (MHVR) at the target site from the distributed microtremor data collected in the vicinity of the target site in Kumamoto Prefecture. Then, we converted the MHVR into a pseudo horizontal-to-vertical spectral ratio of earthquake (pEHVR) using the previously proposed and validated earthquake-to-microtremor ratio (EMR) method, where an empirically obtained EMR is used to convert MHVR into pEHVR. Next, we inverted the S-wave and P-wave velocity structures based on the pEHVR and the diffuse field concept for earthquakes. The theoretical EHVR calculated from the identified velocity structure reproduced the pEHVR quite well in the frequency range of 0.1–22 Hz. After the collection of the blind prediction results by all the participants, the CBP-ESG6 released the observed earthquake records, a preferred model based on the P–S logging data from the in-situ borehole measurement combined with the generic deeper structure, and the average of all the predicted structures by the participants. Notably, our inverted structure was found to be close to the preferred model and the averaged one of all the blind prediction participants, despite some minor differences in the horizontal site amplification factor around the maximum peak frequency at 0.8–1 Hz.Graphical

Estimation of the Thickness of Multi-layer Soils from the Higher Peak of H/V Spectral Ratio

Based on inversion of the horizontal-to-vertical spectral ratio of microtremors (H/V), the empirical H/V spectrum of surface waves propagating on a layered half-space was studied and the effects of soil layers on fundamental and higher peak frequencies were considered. Using the Nakamura H/V spectral method and comparing it with the analytical site-to-reference (RF) method, inverse analysis is performed. These procedures were carried out at several stations around Iran. The results indicate that the transfer function of upward propagation is equal to the downward transfer functions, and is independent of the direction of wave propagation. Furthermore, the resonant peaks depend on the sedimentary cover depth; the next peaks in the H/V spectrum (the peaks after the resonant peak) were generated either in the higher mode of resonant amplification in thick layers and/or from wave interference, subsurface topography and different impedance between two adjacent thin layers as well as resonance amplification. In addition, the analysis indicates that fewer deep sedimentary layers instead of multi-layers do not lead to variation in transfer functions. In deep-deposit layers, the soil layer difference can amplify the amplitude, and does not create a new peak in H/V spectral ratio.

Direct evaluation of S-wave amplification factors from microtremor H/V ratios: Double empirical corrections to “Nakamura” method

The Horizontal-to-Vertical spectral ratios of microtremors (MHVR) have been utilized as a convenient tool to extract a predominant frequency at a target site. The so-called “Nakamura” method, which was proposed by Nakamura in 80's, assumed that MHVR provides us directly the S-wave amplification factor of earthquake in the horizontal component, that is, Horizontal-to-Horizontal spectral ratio (HHR) with respect to the bedrock, although the validity of the method had never been proved. Recently, based on the diffuse field concept (DFC) proposed by Sánchez-Sesma and others in 2011 MHVRs are found to correspond to the square root of the ratio of the imaginary part of the displacement for a unit harmonic load in the horizontal direction with respect to the corresponding one in the vertical direction. With the same DFC for body waves Horizontal-to-Vertical spectral ratios of earthquake (EHVR) correspond to the ratio of the horizontal motion for a vertical incidence of S wave with respect to the vertical correspondent of P wave, as revealed by Kawase and others in 2011. Thus there should be a systematic difference between EHVR and MHVR because of the difference in their primary contribution of wave types. We first calculated the ratios of EHVR with respect to MHVR (EMR) at 100 strong motion stations in Japan. Then we normalized frequency by the fundamental peak frequency at each site and calculated the average of EMRs for five categories based on their fundamental peak frequencies. Once we got empirical EMRs for five categories we transformed MHVRs into pseudo EHVRs. At the same time we calculated the average Vertical-to-Vertical spectral ratios (VVRs) for the same sites using the generalized spectral inversion technique (GIT) of Nakano et al. (2015) [16]. Finally the S-wave amplification factor, HHR, of earthquake ground motion at the site were calculated from MHVR with double corrections using EMR and VVR for the corresponding category. We compared these final empirical prediction with the observed HHRs from GIT to find quite high correlations and small overall residuals. The proposed method to get HHR from MHVR with these double empirical corrections can be considered as a natural but significant extension to the so-called “Nakamura” method.

Directionally dependent horizontal-to-vertical spectral ratios of microtremors at Onahama, Fukushima, Japan

As observational evidence of 3-D microtremor horizontal-to-vertical spectral ratios (MHVRs), previous studies have shown that a significant directional dependency is observed in and around Uji campus, Kyoto University, Japan. This directional dependence is considered to be the result of 2-D basin structure. In this study, we observed microtremors around a strong motion observation site of the Port and Harbor Research Institute in Onahama, Japan, and found that directional dependence of MHVRs exists in some parts of the area around the site. The directional dependence is more apparent and has a higher dominant frequency, at around 5 Hz, relative to those observed in Uji, at around 0.5 Hz. We defined a parameter γ, which we refer to as the “directionally dependent coefficient” to indicate the magnitude of difference between the two orthogonal components which implies the directional dependence of the MHVRs. We rotated the axes and calculated γ for each angle and searched for the orientation that gave the largest γ at a point. Points for which the axis with larger MHVR amplitude among the two axes is oriented in the NS direction are aligned in the NS direction, while points for which the axis with larger MHVR amplitude is oriented in the EW direction are aligned in the EW direction. The distribution of points with large γ formed a T-shaped distribution. We calculated the analytical and numerical MHVRs in order to simulate the observed MHVRs and succeeded in showing the existence of a narrow wedge. From these results, we conclude that a wedge-like lateral heterogeneity exists in the shallow subsurface of the studied area, parallel to the direction of the axis of the larger MHVR amplitude.Graphical abstract.

상시미동의 수평/수직 스펙트럼비를 이용한 예당댐체의 지진응답특성 분석

상시미동의 수평/수직 스펙트럼비를 이용한 예당댐체의 지진응답특성 분석

Applicability of Theoretical Horizontal-to-Vertical Ratio of Microtremors Based on the Diffuse Field Concept to Previously Observed Data

Abstract Horizontal-to-vertical spectral ratios of microtremors (MHVRs) have been interpreted as representing either the Rayleigh-wave ellipticity or the amplitude ratio of the sum of Rayleigh and Love waves in a horizontally layered structure. However, based on the recently established diffuse field concept, the theoretical form of MHVR has been proposed to be the square root of the ratio between the imaginary part of the horizontal Green’s function on the surface and that of the vertical one. The theory assumes that the energy of a wavefield inside the earth will be equipartitioned among the various states in 3D space. In the case of microtremors, this may occur for randomly applied point-force loadings on the surface after sufficient lapse time to allow multiple scattering. Recent works on diffuse fields suggest that equipartition may arise in several ways, but understanding the emergence of equipartition in realistic settings requires further scrutiny. In the meantime, the resulting formula is quite simple, and its meaning has theoretical support from deterministic exact solutions. As references, we use observed microtremor data from several sites that were reported previously and validate the diffuse field method (DFM) as an alternative method to explain observed MHVR. We use only sites with reliable velocity structures to compare different methods quantitatively. As a result, we found that the DFM solutions with the corresponding 1D layered structures well explain the observed MHVRs for most of the sites. Thus, we believe that MHVR can be used to invert a 1D velocity structure by using DFM as a theoretical tool.

Vulnerability evaluation of the urban area using the H/V spectral ratio of microtremors

During the age of cities, buildings were constructed using different techniques, materials, styles and standard codes. Buildings have different responses to earthquake-related effects, depending on their materials and design rules. Soil layer specifications are recognized by different methods, including geotechnical, geophysical and the horizontal-to-vertical spectral ratios (H/V). The spectral ratio of microtremors was first introduced by Nakamura [14]. This method is useful in site characterization studies due to its low budget and easy use. In this paper, it is presented as a method for estimating the degree of vulnerability of structures using spectral ratio H/V. By applying a vulnerability study in the city of Shiraz, the predominant frequencies are derived for different parts of the city. The data is then used to define a coefficient of vulnerability for the earth. For this purpose, 11 different building structures including masonry, steel and concrete were selected. Then, the primary frequency and amplification of the microtremor H/V spectral ratio for buildings are obtained. Furthermore, the resonant frequency and amplification factor of the ground is calculated. The vulnerability index of soil Kg and the vulnerability of building Kb are defined. By comparing these coefficients, the damage factor η of buildings during an earthquake is obtained. These parameters are able to provide an inexpensive method for preparing a disaster program for an urban area. In this paper, the damage rate of different buildings in Shiraz City was obtained. In addition, the damage potential of the entire city area is calculated.

Examination of source fault model for the Gifu-Ichinomiya fault based on seismic intensity data

To investigate possible source fault models for the Gifu-Ichinomiya fault, we construct multiple source fault models of the 1891 Nobi earthquake, taking into account several different types of geometry for the Gifu-Ichinomiya fault, and conduct strong ground motion simulations. We choose the most plausible source model by comparing the distribution of simulated seismic intensities with the spatial distribution of a questionnaire-based intensity of 7, and with that of the damage ratio of wooden houses in the near-source region. Our results imply that the length of the source fault of the Gifu-Ichinomiya fault is relatively short and the dip angle is 75 degrees to the east. Because a seismic intensity of 7 is considered to arise from site amplification and the short distance from a source fault, we classified the points with a seismic intensity of 7 based on the H/V spectral ratio of microtremors by considering the contributing factors to the generation of a seismic intensity of 7. A linear distribution of the points, which might have relatively lower site amplification factors, in the northeastern part of the Nobi Plain implies that a part of the source fault of the 1891 Nobi earthquake existed in this area, as indicated through strong ground motion simulations.

Long-Period Ground Motions Observed in the Northern Part of Kanto Basin, During the 2011 off the Pacific Coast of Tohoku Earthquake, Japan

During the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0), strong ground motions were observed at many seismic stations in the Tokyo Metropolitan Area located about 200 km away from the southern edge of the earthquake source fault. Large earthquake responses in high-rise buildings having long natural periods of several seconds were also observed. The largest ground responses for a period of 4 to 5 seconds were observed locally in Oyama (K-NET TCG012) and Koga (K-NET IBR009) on the border between Tochigi and Ibaraki Prefectures in the northern part of Kanto basin. Geophysical information in these areas was not accurate enough, however, to evaluate these ground motions. To understand S-wave velocity structures, we performed array microtremors observations at TCG012 seismic station in Oyama. We applied the Spatial Autocorrelation (SPAC) method to array microtremors data for vertical components. Rayleigh wave phase velocity from 0.3 to 1.6 km/s was obtained for a period of 0.25 to 3 seconds. We inverted phase velocity to a S-wave velocity structure reaching to bedrock at a depth of 1.6 km, using a Genetic Algorithm. The estimated structure explained the first peak of the H/V spectral ratio of microtremors well by the ellipticity of fundamentalmode Rayleigh wave. To evaluate long-period ground motions observed around Oyama during the main shock, we estimated earthquake ground motions by 1-D analysis, showing agreements with and the differences from those observed. As a result, velocity calculated at IBR008 located midway between the Tsukuba Mountains and Oyama, explained that observed for main phases and later phases. However, velocity calculated at TCG012 did not explain that observed for later phases. According to the emphasis of airy phases for group velocity of Love wave using the estimated S-wave velocity structure and the principal axis for later phases obtained by PCA corresponding to the vibration direction of Love wave propagating from the earthquake source fault and through the Tsukuba Mountains, long-period ground motions of a period of 3 to 5 seconds observed at TCG012 lasting for 200 seconds after the arrival of main phases, consist of Love wave.

Estimation of Long Period Ground Motion Earthquake Effects to High-rise Buildings at Hanoi and HoChiMinh Cities, Vietnam

Estimation of dynamic characteristics of subsurface such as predominant period, amplification factor, shear wave velocity play an important role in researchs of the effects of long-period ground motion earthquakes on high-rise buildings in big cities. However, in Vietnam, due to the lack of observation devices, there is a shortage of dynamic ground information. Consequently, in this paper, at first, the results of applying the Nakamura Method to data processing of microtremor observations in Hanoi and HoChiMinh City of Vietnam are presented. By using this method, the predominant period of the ground at the sites could be estimated from the horizontal to vertical (H/V) spectral ratios of microtremors. The obtained results show that the grounds of the mentioned two cities have considerable possibility of resonating with long-period ground motions. Then, the spectral accelerations in bedrock of Hanoi and HoChiMinh city were estimated by attenuation relationship of Uchiyama and Midorikawa 2006 with the far-source rupture faults that were Sumatra earthquake 2004 and Tha-Khaek fault line in Thailand. Next, the accelerations on ground surface of the above two Vietnamese cities were calculated by using spectral ratio of microtremor and inverse Fourier transform. Finally, those obtained accelerations were used to do the time history analysis applied for 8 high-rise buildings in Hanoi and HoChiMinh city. The results of this final step show that far-source rupture faults have considerable possibility to generate long-period ground motions that cause certain affects to the behaviors of high-rise buildings in Vietnamese cities.

Relationship between Building Damage Ratios and Ground Motion Characteristics during the 2011 Tohoku Earthquake

Fragility curves, the relationships between three kinds of building damage ratios, i.e., total collapse ratio (TCR), collapse ratio (CR) and damage ratio (DR), as well as ground motion characteristics, i.e., peak ground acceleration (PGA), peak ground velocity (PGV), Japan Meteorological Agency instrumental seismic intensity (IJMA) and spectral intensity (SI), were studied for the 2011 Tohoku Earthquake. It was found that the DR was related better to ground motion characteristics than the TCR and CR during this earthquake, and the correlation between the DR and any one of the PGV, IJMA or SI indices was almost the same. The DR during the 2011 Tohoku Earthquake reached almost the same level as that during the Kobe Earthquake for the same level of ground motions. There were some districts where the DRs did not always correspond to reasonable levels of ground motion characteristics. One reason for this was considered to be the representativeness of the ground motions over each district. Thus, one method of estimating ground motions at damaged sites was proposed by using the underground velocity structures identified from the H/V spectral ratios of microtremors, after we confirmed the consistency of the H/ V spectral ratios between earthquakes and microtremors. It is expected to be used for the construction of fragility curves with greater accuracy.

常時微動のH/Vスペクトル比と地形・地盤分類を用いたスペクトル増幅率の推定

常時微動のH/Vスペクトル比と地形・地盤分類を用いたスペクトル増幅率の推定

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.

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.

The new accelerograph network for Santa Fe De Bogota, Colombia and implications for microzonation

The new accelerographic network of Santa Fe de Bogotá is composed of 29 three-component stations with sensors at the surface and three additional six-component borehole stations with three sensors at the surface and three at depth (115, 126 and 184 m). In total, 32 stations have been operative in the metropolitan area of Bogotá since 1999. During this period of time, a significant number of weak motion are recorded and used for a preliminary analysis of local site effects. Using the SH-wave response spectra we verify the behavior of the different seismic zones proposed by the previous microzonation study of the city. A comparison between normalized SH-wave response spectra and the normalized design spectra for each zone clearly depicts that parts of the design spectra should be revised, as well as the boundaries between different zones may require some changes. The spectral amplification levels reach up to a factor of 5. The predominant periods obtained by the amplification spectra in different stations in the city, show variability from 0.3 to 3.0 s. A comparison is also made between the predominant periods obtained using H/ V spectral ratios of microtremors and those using weak motion. In general, microtremors tend to predict slightly lower values of dominant periods than those calculated by the weak motion spectra. However, there is a general correlation between the two data sets. Using the data recorded by one of the borehole station, an equivalent linear seismic response analysis was conducted. The modeled and recorded response spectra show similarities in period peaks, however, the modeled soil amplification is underestimated for periods less than 0.8 s. Since the available record is weak motion which represents mostly the linear response of the soils, further analysis is required.

Applicability of the H/V spectral ratio of microtremors in assessing site effects on seismic motion

Applicability of the H/V spectral ratio of microtremors in assessing site effects on seismic motion

Comparison of Site Response Characteristics Inferred from Microtremors and Earthquake Shear Waves

We investigated the validity of seismic site response characteristics es- timated from microtremors by comparing them with those of earthquake motions. For this purpose we observed microtremors as well as earthquake motions using large (5-km diameter) and small (0.5-km diameter) arrays deployed on soft sedi- ments. Specifically, we examined four estimates from microtremors: relative site amplification factors to incident shear waves, site amplification factors by the Naka- mura method, resonance frequency in horizontal-to-vertical spectral ratios, and horizontal-to-vertical spectral ratios. As a result of the comparisons, we obtained the following conclusions. The relative amplification factors can be inferred from horizontal-component ratios of microtremors to a reference site within a small area of several hundred meters. The horizontal-to-vertical spectral ratios inferred by the Nakamura method partly reflect site amplification factors, but do not agree with site amplification factors. A sharp-peak frequency in the horizontal-to-vertical spectral ratios is possibly the resonance frequency. The horizontal-to-vertical spectral ratios of microtremors either agree with those of earthquake motions at some array sites or are slightly smaller at the other sites.

Applicability of the H/V spectral ratio of microtremors in assessing site effects on seismic motion

AbstractThe authors examine the reliability of site response estimations obtained by the horizontal to vertical (H/V) spectral ratios of microtremors by means of cross‐validation with the ratio of the horizontal spectra of earthquake motion with respect to reference sites. The data comprise microtremor and ground motion records recorded at 150 sites of Yokohama strong motion array. The use of non‐supervised pattern recognition techniques aims to group the sites with more objectivity. Attributes defining the overall shape of the amplification spectra serve as input in the computation of Euclidean distance similarity coefficients amongst sites. The implementation of the Ward clustering scheme leads to the attainment of a meaningful tree diagram. Its analysis shows the possibility of summarizing the results into six general patterns. A good coincidence of site effects estimates at 80 per cent of the sites becomes apparent. However, this coincidence appears poor for sites characterized by H/V amplification ratios around 2 or smaller and predominant periods longer than 0.5 s. In such cases, the presence of stiff, sandy sediments in the soil profile proves common. To proscribe H/V estimations, relying solely on the small spectral ratios criterion seems inadequate. Copyright © 2001 John Wiley & Sons, Ltd.

Microtremors as a Geophysical Exploration Tool: Applications and Limitations

—The usefulness of microtremors as a geophysical exploration tool has been analyzed. This application is possible due to the relationship between the main resonance frequency of a given soil, obtained from the H/V spectral ratios of microtremors, its thickness and average shear velocity. We first measured the ambient noise at 33 sites and determined their main resonance frequency. Detailed geotechnical information was available for 23 of the sites, thereby allowing us to establish a quantitative relationship between the resonance frequency and the thickness of the soil, and indirectly between either of them and the shear velocity of the soil. The practical application of this relationship has revealed its usefulness in determining the surface structure of a valley with excellent accuracy, with an error of only 15% in the depths calculated. These errors are due to the simplification of the problem that this method implies: it requires that the shear velocity vary constantly with depth throughout the study region, which is evidently not always so, and that the input data themselves intrinsically have a certain degree of uncertainty. This method is therefore not valid when there is no mechanical contrast between the study soil and the underlying layer or when the shear velocity varies irregularly with depth in the study area.

The Microzonation of Perth, Western Australia, Using Microtremor Spectral Ratios

This paper indicates new microzonation maps of Perth, Western Australia, utilising microtremor spectral ratios. This metropolitan area has been developing in recent times on Perth Basin which is one of the most active seismic zones in the country. The authors carried out simultaneous measurings of microtremors over most of metropolitan Perth, using a 3 km grid as a basis and hard rock reference site throughout. They calculated spectral ratios of microtremors at deposit site to rock reference site and plotted and contoured on maps for 6 frequency bands from 0.2 to 5.0 Hz. Spectral ratio contours appeared to correlate well with various geological subsurface contours. They also showed that previously estimated earthquake risk estimates underestimated ground motions by up to a factor of two.