Microtremor Array Method Research Articles

Comprehensive geophysical, geotechnical, and geochemical assessments of an offshore landfill in Singapore

Comprehensive site investigation techniques, including Electrical Resistivity Tomography (ERT), Induced Polarization (IP), Multichannel Analysis of Surface Waves (MASW), and Microtremor Array Method (MAM), were integrated with geotechnical and geochemical tests of retrieved waste samples from Singapore’s operational offshore landfill. The properties of landfill wastes vary widely, including shear-wave velocities 127–248 m/s, densities 1.2–2.1 Mg/m3, resistivity 3.0–25.3 Ω∙m, and chargeability 48–82 mV/V. The natural clay layer underneath was clearly delineated and effectively mitigated leachate leakage. K-means clustering of the geophysical data facilitates precise mapping of waste distribution and quantities of recoverable metals based on quantitative criteria. This study illustrates a thorough case study adopting the new site investigation and characterization paradigm for an offshore landfill, which provides insights into leachate leakage detection and evaluations of landfill mining and resource recovery.

A Comparison of 1-D S-wave Velocity Profile Resulting from Microtremor Array Method and Lithology Profile from Borehole Data in Jakarta

The subsurface investigation is very important in seismic hazard assessment for the city scale because it greatly affects the propagation of seismic waves on the surface related to the amplification phenomenon caused by the thickness and hardness of the surface sediment. Local geology and geotechnical conditions for seismic hazard analysis are represented by site class which is determined based on the value of Vs30 or NSPT30, besides that it is also necessary to know the depth of the bedrock. In general, the availability of these data in the cities in Indonesia is very limited, so it is necessary to conduct a field survey which is one of the methods using the microtremor array survey method. This paper has presented the results of subsurface investigations in Kembangan Jakarta using the microtremor array method and their comparison with borehole data that was carried out at adjacent locations. The analysis results of those methods show a positive correlation between the 1-D shear wave velocity profile and lithology where the top layer has low S-wave velocity (Vs) correlated with soft rock. The S-wave velocity increases with depth which is correlated with hard rock that is indicated by lithology as hard sandstone. The results of this study are very useful for applying the array microtremor method in other cities considering it is very simple in operation.

Analysis of Liquefaction Triggering in the Nakdong-river Delta Sediments Based on Broadband Hybrid Ground Motion Simulation and Microtremor Array Method

We present a case study of liquefaction triggering analysis performed in the Nakdong-river delta region, based on the shear wave velocities obtained by the Microtremor Array Method and the ground motions generated by the broadband hybrid ground motion simulation. Assuming a M<sub>w</sub>6.2 earthquake occuring at the Dongrae fault, the ground motions at the studied sites exceeded the design spectra provided by the Korean Design Standard and many of the studied sites exhibited low factors of safety against liquefaction triggering. We found that the Idriss and Boulanger (2008) method yields larger Cyclic Stress Ratios (CSR), compared with the site response analysis based method. Cyclic Resistance Ratios (CRR) estimated from the shear wave velocity were sensitive to small changes in the velocity, which suggests that high accuracy and depth resolution are desired when the shear wave velocity is used to estimate the CRR.

New ground survey method : Development and application of offshore microtremor array method

2019年4月より再エネ海域利用法が施行されたことにより，洋上風力発電施設に係る地盤調査市場は大幅に拡大している。洋上における地盤の支持層分布あるいは工学的基盤深度の把握に必要なS波速度の測定は，ボーリング孔を利用したサスペンションPS検層が一般的に行われてきた。しかしながら，洋上風力発電事業（着床式）は年間平均風速7 m/s以上，水深10～40 m程度の海域を対象としており，厳しい環境下でのボーリング作業は容易ではない。著者らはこのような厳しい環境下でもボーリング調査を必要とせず，短期間でS波速度構造を把握することができる海底微動アレイ探査システムを開発した。本稿では，当手法の開発について概説した後，適用事例及び海底微動の特徴について示した。これらの結果より，得られたS波速度構造はPS検層および音波探査と整合的な結果が得られたことから海底微動アレイ探査の有用性が示された。

Relationship between Shear Velocities Recorded by Microtremor Observations and Seismic Cone Penetration Test Results

This research proposes a relationship between two methods such as a numerical approach by conducting a microtremor array observation and field survey by using the seismic cone penetration test unit (SCPTu). A database of shear-wave velocity (Vs) measurements was established using the microtremor array technique and seismic cone penetration test unit (SCPTu) on high-quality samples of rock and soft soil in Padang city, Indonesia. The study also demonstrates that the Vs values obtained from the different methods are consistent with the microtremor array technique. This technique may thus be deemed a valuable tool, as it can be used in engineering practice with confidence. Comparison of the Vs for different soils at the first layer between the microtremor array observation results and the SCPTu results exhibited the microtremor array method is unable to determine the Vs at the layer where its Vs changes dramatically, such as at the same layer as station UNP at 2 to 3.5m deep.

Active and passive seismic methods to explore areas of active faulting. The case of Lamezia Terme (Calabria, southern Italy)

This paper presents the results of a geophysical investigation aimed to illustrate the architecture of the shallow stratigraphic horizons across an area believed to be crossed by a NW-SE trending trascurrent fault called the Lamezia Terme-Catanzaro Fault. In spite of the available information on fault segments and tectonic activity associated with faulting, the local scale fault characteristics are currently poorly known and the surface displacements are difficult to detect in a such a densely populated area. A traditional integrated geophysical exploration method was proposed as an effective solution to assist in the location of structures in the subsurface where surface geological and morphological evidence is inconclusive in terms of defining the location and the potential of the local seismogenic sources, responsible for recurrent catastrophic earthquakes. Two active High Resolution (HR) seismic reflection/refraction profiles were acquired using an array of vertical geophones with a Swept Impact Seismic Source (VibSist-500) in order to obtain seismic imaging of the subsurface of the study area. Additionally, passive Microtremor Array Method (MAM), deep wells and commercial seismic profiles were used to better constrain the geological trend of deeper seismostrata. The integrated interpretation of both geophysical and geological data allowed to infer subsurface pictures of the investigated areas identifying the depth of the seismic bedrock and characterizing the buried morphology, enhancing lineaments with possible relevance to faults. Discontinuities in the main horizons in seismic sections indicate that faulting is present in a Pliocene unit and may extend to the shallow subsurface in Quaternary sediments. The results of this study can provide a basis for earthquake prevention and construction work and demonstrate the applicability of the seismic methods to explore buried hidden faults in the perspective of applications on studies of physical stratigraphy of syn-tectonic deposits.

Active and passive seismic surface wave methods for levee assessment in the Sacramento–San Joaquin Delta, California, USA

ABSTRACTSeismic surveys were performed using a novel application of combined active and passive surface wave methods to evaluate the integrity of levees protecting islands in the Sacramento–San Joaquin Delta, California, USA, from flooding. Delta islands have been undergoing rapid subsidence during the past century due to farming practices that have led to the loss of a surficial peat layer. A large earthquake on any one of several active faults in the region could cause multiple levee failures and extensive flooding in the Delta. Surface wave surveys were carried out along the crest of levees using the active method (2D multichannel analysis of surface waves) and along the base of levees using the passive method (microtremor array method). This allowed 2D seismic S‐wave velocity (VS) profiles to be prepared for each site by combining shallow data from the active survey along the crown of the levee with deeper data from a passive survey along the toe of the levee. TheseVS profiles reveal a low‐velocity peat layer beneath the levee body that was confirmed by geotechnical borehole logs. Lateral variability of the levee was evaluated using average velocity to the base of the levee versus along‐levee distance.

Limits and Benefits of the Spatial Autocorrelation Microtremor Array Method Due to the Incoherent Noise, With Special Reference to the Analysis of Long Wavelength Ranges

AbstractTo examine the upper limit of the analyzable wavelength ranges (upper limit wavelength, ULW) of the spatial autocorrelation (SPAC) method in microtremor surveys, we have derived a formula evaluating stochastic errors of a phase velocity estimate obtained by the SPAC method. We then followed our previous paper to evaluate biases generated in a phase velocity estimate that come from the presence of the incoherent noise so that we could discuss which is the crucial factor determining the ULW. As a result, the incoherent noise is considered as the crucial factor. In the consequence, we proposed a formula to evaluate signal‐to‐noise ratio (SNR) and to determine phase velocity with the effects of the incoherent noise compensated. We used observed data to validate the above theory. Through the validation procedure, we obtained an approximation equation , where NULW means ULW normalized by array radius and A is a constant. The parameter A took a value from about 1/2 to 2 in the cases of this study. Finally, an additional and simple analysis was executed, which indicates that the incoherent noise not only limits the SPAC analysis but also can provide benefits: they are possibly used for evaluating the attenuation of soils. A microtremor array survey with the traditional SPAC method of Okada (2003, https://doi.org/10.1190/1.9781560801740) can be a mobile tool for evaluating the very local attenuation at a site of interest (i.e., gaining a high spatial resolution), in frequency ranges suited for earthquake engineering and by using simple seismic arrays with short time observations.

Evaluation of seismic site classification for Kahramanmaras City, Turkey

This paper presents a study on the seismic site classification map using the geophysical tests in Kahramanmaras city located at a place where African, Anatolian, and Arabian plates meeting in southern-central Turkey. Generating seismic site classification maps in accordance with National Earthquake Hazards Reduction Program (NEHRP) has become a more significant criterion for earthquake hazard estimations. The SPT-N values obtained from the field studies at 287 boreholes within the upper 30 m were used to describe the subsurface conditions in the region. The shear wave velocity (VS) values in the study area were obtained by implementing Multichannel Analysis of Surface Waves (MASW) and Microtremor Array Method (MAM) measurements tests. An approach proposed by Boore (Boore, Bull Seismol Soc Am 94:591–597, 2004) for the cases where the VS measurements do not reach 30 m depth has also been adopted by correlating the shallow shear wave velocity with VS30. The resulting site classification maps estimate that the study area is predominantly classified as soil site class C, while the small areas were rarely classified as soil site class D and B. Furthermore, a systematic analysis based on a comparative study of the present research and the published correlations for seismic site classification with VS30 values has been carried out using Geographical Information System (GIS). Evidently, the VS30 based seismic site classification maps could be effectively used by researchers and engineers for the purpose of land-use planning and urban development in earthquake-prone regions.

Estimation of Vs Structure of Krueng Aceh and its Suburb Basin of Aceh Province, Indonesia, Derived from Microtremor Measurements

The Aceh and Seulimeum Faults flank the Krueng Aceh Basin in Indonesia, and the shear-wave velocity (Vs) structure of the basin is not extensively available. Understanding the Vs structure is very important in order to figure out how the basin structures seemingly appear, and this can eventually be used to generate a microzonation map for other forthcoming studies. To provide this, Vs was measured over an area approximately covering Banda Aceh City and its surroundings, by setting two lines consisting of eight points projected in the NW–SE and SW–NE orientations. This research aims to facilitate the approximation of the Vs structure characteristics of the Krueng Aceh Basin using the microtremor array method (MAM). Triangular configurations were set by deploying four seismometers following an M-station geometry for three different array sizes (i.e., 3, 10, and 30 m in distance). The data were then processed by utilizing the spatial autocorrelation (SPAC) technique. The result shows that the Vs structure generally dips down from SE to NW, and it gradually declines from SW to NE. The combination of these Vs structures tends to be oblique toward the SW–NE direction. This form may be affected by the Aceh Segment Fault which is more active than the Seulimeum Segment Fault. The average maximum penetration depth and Vs are 603 m and 947.5 m/s in the SE–NW orientation, and 650 m and 958 m/s in the SW–NE direction. Generally, the thickness of the strata is greater in the upstream area compared to the downstream area. Their composition consists of alluvium (A) at the uppermost layer and diluvium (D) at the underlying layers. Then, all of the identified strata are aged from the Pleistocene to Tertiary Pleistocene (Tp). These characteristics of the strata could potentially cause surface damages as a result of site effect responses when an earthquake is occurring.

Shear‐Wave Velocity in the Seattle Basin to 2 km Depth Characterized with the krSPAC Microtremor Array Method: Insights for Urban Basin‐Scale Imaging

Shear‐Wave Velocity in the Seattle Basin to 2 km Depth Characterized with the krSPAC Microtremor Array Method: Insights for Urban Basin‐Scale Imaging

Compensating for the Impact of Incoherent Noise in the Spatial Autocorrelation Microtremor Array Method

Compensating for the Impact of Incoherent Noise in the Spatial Autocorrelation Microtremor Array Method

Preliminary Estimation of Engineering Bedrock Depths from Microtremor Array Measurements in Solo, Central Java, Indonesia

In the last decade the city of Solo, located in Central Java, Indonesia, has grown significantly and become a major city. Many industries and hotels have been built in the city and its surroundings. This study aimed to determine the engineering bedrock depths in Solo, an important parameter in seismic hazard analysis. The microtremor array method was used to obtain 1D S-wave velocity profiles and construct layer depth maps. The spatial autocorrelation (SPAC) method was used to calculate the dispersion curves, while the S-wave velocity structure was derived using a genetic algorithm (GA). The results of the S-wave velocity structure in Solo show that there are four stratigraphic layers, where the engineering bedrock depths in Solo exist within the range from 145 to 185 m. The shape of the bedrock basin is elongated in an east-west direction.

Estimation of shallow S-wave velocity structure using microtremor array exploration at temporary strong motion observation stations for aftershocks of the 2016 Kumamoto earthquake

Shallow S-wave velocity V S profiles were estimated for 26 temporary strong motion observation sites surrounding the epicenters of a sequence of the 2016 Kumamoto earthquake. The microtremor array method was used to gather the dispersion characteristics of Rayleigh waves. V S profiles were obtained by inverting the dispersion curves for each site and those of three permanent strong motion stations that recorded the sequence of seismic events. The shallow V S profiles near two of the permanent strong motion stations in the town of Mashiki were almost identical. However, the V S profiles at other stations varied. The V S profiles were found to have the common feature of the uppermost low-velocity layer being widely distributed from Mashiki to the village of Minami-Aso, and it was especially thick in the areas that suffered heavy damage. This low-velocity layer was a major contributor to the site amplification. The horizontal-to-vertical spectral ratios of the microtremors indicate that both the shallow soil and deep sedimentary layers may control the site response characteristics over a broad frequency range.

Surface wave surveying for near-surface site characterization in the East San Francisco Bay Area, California

Seismic surface wave methods are effective tools for estimating S-wave velocity in urban areas for near-surface site characterization and geologic hazard assessment. A surface wave survey can provide quantitative site-specific measurement of physical properties needed for the design of earthquake-resistant structures. We successfully used a combined active and passive seismic surface wave method to estimate the S-wave velocity in the upper 30 m at sites with a range of geologic conditions. At five of the six sites, multichannel analysis of surface waves (MASW) and microtremor array method (MAM) methods were used. The MAM method could not be used at one site due to insufficient ambient noise. Data from the active method (MASW) contained higher frequencies that contributed to higher resolution of the near-surface zone, whereas passive data (MAM) contained lower frequencies that provided deeper penetration. Phase velocities from the two methods were in good agreement in the frequency range where they overlapped. Surface wave dispersion curves from the two methods were used to prepare an initial velocity model, and a nonlinear inversion was performed to obtain an improved velocity-depth profile. The use of a multimethod data set provided greater confidence in velocity measurements. The six sites of this study may be classified as belonging to two main groups based on S-wave velocities and geologic materials. Two sites are located in the East Bay Hills on Mesozoic bedrock, and four sites are located on Holocene sedimentary units. The highest [Formula: see text] was [Formula: see text] (class C), at a site with fractured and weathered bedrock exposed in a geotechnical trench at 1–2 m depth. The four sites on Holocene sedimentary units have [Formula: see text] values ranging from 207 to [Formula: see text] (class D).

Local site characterization and seismic zonation study by utilizing active and passive surface wave methods: A case study for the northern side of Ankara, Turkey

This study encompasses dynamic soil characterization and site classification zonation mapping of the Plio-Quaternary and especially Quaternary alluvial sediments based on the current seismic codes. The study area is located in the Çubuk district and its surrounding area that is situated towards the north of Ankara, the capital of Turkey. The project site is located in a region which has the potential of being seriously affected by probable seismic events occurring along the Çubuk Fault Zone. The Çubuk Fault Zone is thought to be a continuation of the Dodurga Fault Zone and a sub-fault belt of the North Anatolian Fault System that is one of the most prominent fault systems in Turkey with a significant earthquake potential. Sediment characteristics were determined and soil profiles were characterized by surface wave methods at different locations. Non-invasive seismic methods were used to obtain a shear wave velocity profile at 41 sites and two measurements were taken at each site for passive and active surface wave methods. The Multichannel Analysis of Surface Wave Method (MASW) and the Microtremor Array Method (MAM), which gave comparable results, were used as active and passive surface wave methods, respectively. By combining these two techniques, the shear wave velocity profile of the site was obtained. Additionally, the MAM survey alone was implemented at 10 of the sites, especially to get information on the deeper parts of the soil profiles. The geological characteristics of these sedimentary units were compared with the geological and geotechnical boring and seismic site characterization studies to classify the soil deposits. Therefore, the average shear wave velocity results and the variable characteristics of the seismic data along the soil profiles were correlated with these boring studies. This was performed to develop site categories which took site conditions into account according to the design codes of IBC (International Building Code) 2006 and TSC (Turkish Seismic Code) 1998. Consequently, the regional site classification map of the north of the Çubuk basin considering site classes was assessed based on the average shear wave velocity (Vs30) results in IBC 2006 and the shear wave velocity data and thickness of the surface layer based on the Vs values in the TSC.

Bayesian inversion of microtremor array dispersion data in southwestern British Columbia

This paper applies Bayesian inversion, with evaluation of data errors and model parametrization, to produce the most-probable shear-wave velocity profile together with quantitative uncertainty estimates from microtremor array dispersion data. Generally, the most important property for characterizing earthquake site response is the shear-wave velocity (VS) profile. The microtremor array method determines phase velocity dispersion of Rayleigh surface waves from multi-instrument recordings of urban noise. Inversion of dispersion curves for VS structure is a non-unique and non-linear problem such that meaningful evaluation of confidence intervals is required. Quantitative uncertainty estimation requires not only a non-linear inversion approach that samples models proportional to their probability, but also rigorous estimation of the data error statistics and an appropriate model parametrization. This paper applies a Bayesian formulation that represents the solution of the inverse problem in terms of the posterior probability density (PPD) of the geophysical model parameters. Markov-chain Monte Carlo methods are used with an efficient implementation of Metropolis-Hastings sampling to provide an unbiased sample from the PPD to compute parameter uncertainties and inter-relationships. Nonparametric estimation of a data error covariance matrix from residual analysis is applied with rigorous a posteriori statistical tests to validate the covariance estimate and the assumption of a Gaussian error distribution. The most appropriate model parametrization is determined using the Bayesian information criterion, which provides the simplest model consistent with the resolving power of the data. Parametrizations considered vary in the number of layers, and include layers with uniform, linear and power-law gradients. Parameter uncertainties are found to be underestimated when data error correlations are neglected and when compressional-wave velocity and/or density (nuisance) parameters are fixed in the inversion. Bayesian inversion of microtremor array data is applied at two sites in British Columbia, the area of highest seismic risk in Canada, to study the ability to recover an accurate VS profile in relatively deep and shallow geological settings on the Fraser River delta in Greater Vancouver and in Victoria, respectively. A well-resolved VS profile to at least 110 m depth is determined at the Fraser River delta site for a power-law gradient parametrization. At the Victoria site, a layer with low VS and a weak linear gradient is indicated to 15–18 m depth, above much higher velocity material. Invasive VS measurements from seismic cone penetration testing and/or surface-to-downhole methods are used to assess the reliability of the Bayesian microtremor inversion results, with excellent agreement obtained at both sites.

Comparison of three different methods in investigating shallow shear-wave velocity structures in Ilan, Taiwan

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 100 m 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 Vs 30, Vs 100 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.