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A site condition survey is extremely important for the seismic fortification of major projects. The distribution of underlying weak interlayer in sites is extremely harmful to buildings. However, it is a technical problem to find out the distribution of weak interlayer in the overburden. The shallow velocity structure can directly reflect the change characteristics of a stratigraphic structure. In this paper, acquisition of background noise is conducted using a microtremor linear array method, and the distribution characteristics of two typical stratigraphic structures in Wuhan, Hubei Province, are obtained through an inversion of the apparent S-wave velocity; meanwhile, the equivalent shear-wave velocity and the overburden thickness are estimated, which provides a basis for site classification. The research results are as follows: (1) The two-dimensional profile of the apparent S-wave velocity obtained by the microtremor linear array method can be used for fine imaging of the stratum with weak interlayer, and its distribution form and velocity structure characteristics are highly consistent with those of the drilling data. (2) Compared to the borehole data obtained through in situ test, the error of the overburden thickness and the equivalent shear-wave velocity estimated by the inversion of the apparent S-wave velocity is only about 10%, and the estimated parameters can be directly used for site classification. These results can provide important parameters for seismic fortification of major projects, and also provide reference for the exploration of unfavorable geological bodies, such as weak interlayer in complex urban areas, in the future, which can have good scientific significance and popularization value. [ABSTRACT FROM AUTHOR]

Microtremor array measurements, and passive surface wave methods in general, have been increasingly used to non-invasively estimate shear-wave velocity structures for various purposes. The methods estimate dispersion curves and invert them for retrieving S-wave velocity profiles. This paper summarizes principles, limitations, data collection, and processing methods. It intends to enable students and practitioners to understand the principles needed to plan a microtremor array investigation, record and process the data, and evaluate the quality of investigation result. The paper focuses on the spatial autocorrelation processing method among microtremor array processing methods because of its relatively simple calculation and stable applicability. Highlights: 1. A summary of fundamental principles of calculating phase velocity from ambient noise 2. General recommendations for MAM data collection and processing using SPAC methods 3. A discussion of limitations and uncertainties in the methods [ABSTRACT FROM AUTHOR]

A site condition survey is extremely important for the seismic fortification of major projects. The distribution of underlying weak interlayer in sites is extremely harmful to buildings. However, it is a technical problem to find out the distribution of weak interlayer in the overburden. The shallow velocity structure can directly reflect the change characteristics of a stratigraphic structure. In this paper, acquisition of background noise is conducted using a microtremor linear array method, and the distribution characteristics of two typical stratigraphic structures in Wuhan, Hubei Province, are obtained through an inversion of the apparent S-wave velocity; meanwhile, the equivalent shear-wave velocity and the overburden thickness are estimated, which provides a basis for site classification. The research results are as follows: (1) The two-dimensional profile of the apparent S-wave velocity obtained by the microtremor linear array method can be used for fine imaging of the stratum with weak interlayer, and its distribution form and velocity structure characteristics are highly consistent with those of the drilling data. (2) Compared to the borehole data obtained through in situ test, the error of the overburden thickness and the equivalent shear-wave velocity estimated by the inversion of the apparent S-wave velocity is only about 10%, and the estimated parameters can be directly used for site classification. These results can provide important parameters for seismic fortification of major projects, and also provide reference for the exploration of unfavorable geological bodies, such as weak interlayer in complex urban areas, in the future, which can have good scientific significance and popularization value.

Palu City is an active seismic area in Indonesia due to the very active Palu-Koro fault system. The development of the city area, therefore, must consider the risks induced by the seismic activities. The risk assessment has to be supported by information on subsurface characteristics. The aim of this study is to investigate the characteristics of the subsurface of the area by considering the value of Vs30 (top 30 m shear-wave velocity). This parameter has been related to the estimation of the site’s ground shaking during the occurrence of an earthquake. The measurements taken in the deep soil sediment include the microtremor array, using the spatial auto correlations (SPAC) method, as well as the site’s dominant period measurement, using the horizontal-to-vertical spectral ratio (HVSR) method. All these parameters were local site parameters, which could be subsequently related to a description of the potential impact in an area near to the epicenter. The measurement of Vs30 was conducted in collaboration between the Indonesian Agency for Meteorology, Climatology, and Geophysics (Badan Meteorologi, Klimatologi, dan Geofisika) (BMKG) and the University of Indonesia (Universitas Indonesia) (UI); the overall surveys included Vs30 measurements at 44 sites, microtremor array surveys at 10 sites, and the dominant period measurements at 74 sites. The overall results indicated that there is a good correlation between Vs30 and the dominant period. In general, Palu City is predominantly a class-D site, but the northwest part of the Palu area is a class-C site.

Jakarta is located on a thick sedimentary layer that potentially has a very high seismic wave amplification. However, the available information concerning the subsurface model and bedrock depth is insufficient for a seismic hazard analysis. In this study, a microtremor array method was applied to estimate the geometry and S-wave velocity of the sedimentary layer. The spatial autocorrelation (SPAC) method was applied to estimate the dispersion curve, while the S-wave velocity was estimated using a genetic algorithm approach. The analysis of the 1D and 2D S-wave velocity profiles shows that along a north-south line, the sedimentary layer is thicker towards the north. It has a positive correlation with a geological cross section derived from a borehole down to a depth of about 300 m. The SPT data from the BMKG site were used to verify the 1D S-wave velocity profile. They show a good agreement. The microtremor analysis reached the engineering bedrock in a range from 359 to 608 m as depicted by a cross section in the north-south direction. The site class was also estimated at each site, based on the average S-wave velocity until 30 m depth. The sites UI to ISTN belong to class D (medium soil), while BMKG and ANCL belong to class E (soft soil).

Palu City is an active seismic area in Indonesia due to the very active Palu-Koro fault system. The development of the city area, therefore, must consider the risks induced by the seismic activities. The risk assessment has to be supported by information on subsurface characteristics. The aim of this study is to investigate the characteristics of the subsurface of the area by considering the value of Vs30 (top 30 m shear-wave velocity). This parameter has been related to the estimation of the site's ground shaking during the occurrence of an earthquake. The measurements taken in the deep soil sediment include the microtremor array, using the spatial auto correlations (SPAC) method, as well as the site's dominant period measurement, using the horizontal-to-vertical spectral ratio (HVSR) method. All these parameters were local site parameters, which could be subsequently related to a description of the potential impact in an area near to the epicenter. The measurement of Vs30 was conducted in collaboration between the Indonesian Agency for Meteorology, Climatology, and Geophysics (Badan Meteorologi, Klimatologi, dan Geofisika) (BMKG) and the University of Indonesia (Universitas Indonesia) (UI); the overall surveys included Vs30 measurements at 44 sites, microtremor array surveys at 10 sites, and the dominant period measurements at 74 sites. The overall results indicated that there is a good correlation between Vs30 and the dominant period. In general, Palu City is predominantly a class-D site, but the northwest part of the Palu area is a class-C site. [ABSTRACT FROM AUTHOR]

In this study, microtremor array measurements were conducted at 45 sites in the Western Plain of Taiwan. The arrays were approximately 30 m or 60 m in radius, depending on the site. The maximum-likelihood frequency–wavenumber method was adopted to obtain the phase velocities of Rayleigh waves, and then a genetic algorithm technique based on an inversion scheme of the fundamental mode of the Rayleigh waves’ dispersion curves was applied to calculate a preliminary S-wave velocity (Vs) profile at each site. Because a layer of thick sediment covers the bedrock in the Western Plain of Taiwan, microtremor arrays in this size range cannot estimate the structure of the entire sediment. Therefore, this study implemented further inversion of the horizontal-to-vertical spectral ratios of the microtremors to estimate the deeper structures up to the bedrock of Vs greater than 1000 m/s. Previously logged velocity profiles for different depths at or near our study sites were collected and compared with the Vs profiles derived from our microtremor array measurements; the results were found to be highly comparable. Therefore, we could delineate the depth distributions for the layer depths for Vs = 600 m/s and 1000 m/s in this region. The depth for Vs = 600 m/s is approximately 50 m in the piedmont area and approximately 300 m at the coastline; moreover, the depths for Vs = 1000 m/s increase from 200 m in the piedmont area to approximately 1000 m at the coastline. The depths for Vs = 1.0 km/s (Z1.0), which is an important parameter that accounts for the basin effect in recent ground motion prediction equations, are consequently available at the study sites. The distribution of Z1.0 as a function of Vs30 indicates higher similarity to that in Japan than in the San Francisco Bay area. [ABSTRACT FROM AUTHOR]

Microtremor recordings are a very useful tool for microzonation studies because of simple data acquisition and analysis. The Spatial Autocorrelation Method (SPAC) proposed by Aki (1957), may be used to constrain the velocity structure underlying the site with microtremor array measurements, and the site effect (dominant period and amplification) can be calculated. In this paper the SPAC method is applied to Ciudad Universitaria, Mexico City. Results are compared with those obtained by Kagawa (1996) with the F-K method. The velocity structure inferred using SPAC method is consistent with Kagawa´s results. We compare the transfer function obtained from the velocity model estimated by SPAC method with the transfer function from Kagawa´s velocity model, and from spectral ratios of earthquake data. We conclude that the velocity structure of a site can be estimated from microtremor recordings by the SPAC method.

Several powerful earthquakes have struck Padang during recent years, one of the largest earthquake event was an M 7.6 that occurred on September 30, 2009 and caused more than 1000 casualties. Following the event, A 12-site microtremor array investigation to gain a representative determination of the soil condition of subsurface structures in Padang has been conducted. From the dispersion curve from the array observations, the central business district of Padang corresponds to relatively soft soil condition with Vs30 less than 400 m/s. Because only one accelerometer was existing, the 2009 Padang earthquake was simulated to obtain peak ground acceleration for all sites in Padang city. By considering the damaging data of the 2009 Padang earthquake for engineered houses, Seismic risk vulnerability estimation of non-engineered houses for rock, medium and soft soil condition can be obtained, and also estimate the loss ratio based on the ground response, seismic hazard of Padang and the existing damaged to non-engineered structure houses due to Padang earthquake in 2009 data for 500 return periods of earthquake events. [ABSTRACT FROM AUTHOR]

Abstract: The Central Weather Bureau (CWB) and National Center for Research on Earthquake Engineering (NCREE) launched a project to build an engineering geological database for strong-motion stations in Taiwan in 2000. The project measures shear-wave velocity using the suspension PS-logging method. In this study, we conduct array measurements of microtremors and apply the stress wave propagation method (SWPM) at seven free-field strong-motion stations in Ilan County to estimate shallow shear-wave velocity structures. We focus on the sediment layers of the top 100m to compare the shear-wave velocity structures of the three different methods. There are some misfits among the results of the three different methods; so we calculate the values of Vs30, Vs100 and plot S-wave travel-time curves of these methods for each site to analyze the misfits effectively. This analysis helped us to prove the efficiency of the microtremor array method in investigating shear-wave velocity structures in the shallow subsurface. Moreover, the horizontal-to-vertical ratios of microtremors for each survey point show the existence of divergence at the same site. We considered this as evidence that misfits are caused by the heterogeneous nature of sediments and also due to the nature of the methods as being one-, two- and three-dimensional. Furthermore, the average shear-wave velocity structure of microtremor arrays may be more representative of the whole site. [Copyright &y& Elsevier]