Microtremor Array Measurements Research Articles

Examination of shallow and deep S-wave velocity structures from microtremor array measurements and receiver function analysis at strong-motion stations in Kathmandu basin, Nepal

The Himalayan collision zone, where the Indian Plate subducts beneath the Eurasian Plate at a low angle, has caused many devastating earthquakes. The Kathmandu basin, situated in this region, is surrounded by mountains on all sides and is filled with distinct soft lake sediments with a highly undulating bedrock topography. The basin has been experiencing rapid urbanization, and the growing population in its major cities has increased the vulnerability to seismic risk during future earthquakes. Several strong-motion stations have recently been deployed in the Kathmandu basin. It is expected that the data captured by this strong-motion station array will further enhance our understanding of site amplification in sedimentary basins. Clear P-to-S converted waves have been observed in the strong-motion records. In this study, we investigate the medium boundary that generated these converted waves. First, we estimate the shallow velocity structures, which correspond to the topographic slopes or surface geology, beneath the strong-motion stations. We then apply a receiver function analysis to the strong-motion records. The receiver function indicates that the interface between the soft sediment and seismic bedrock serves as a boundary that generates converted waves. The obtained results can be used for tuning three-dimensional velocity structures.Graphical

Assessment of Site Effects and Numerical Modeling of Seismic Ground Motion to Support Seismic Microzonation of Dushanbe City, Tajikistan

In the territory of Dushanbe city, the capital of Tajikistan, detailed geological and geophysical data were collected during geophysical surveys in 2019–2020. The data comprise 5 microtremor array measurements, 9 seismic refraction tomography profiles, seismological data from 5 temporary seismic stations for standard spectral ratio calculations, 60 borehole datasets, and 175 ambient noise measurements. The complete dataset for Dushanbe was used to build a consistent 3D geologic model of the city with a size of 12 × 12 km2. The results of the seismological and geophysical surveys were compared and calibrated with borehole data to define the boundaries of each layer in the study area. The Leapfrog Works software was utilized to create a 3D geomodel. From the 3D geomodel, we extracted six 12 km long 2D geological cross-sections. These 2D geological cross-sections were used for 2D dynamic numerical modeling with the Universal Distinct Element Code software to calculate the local seismic response. Finally, the dynamic numerical modeling results were compared with the amplification functions obtained from the seismological and ambient noise data analysis. The 2D dynamic numerical modeling results allowed a better assessment of the site effects in the study area to support seismic microzonation and the determination of local peak ground acceleration changes in combination with regional seismic hazard maps. In addition, our results confirm the strong seismic amplification effects noted in some previous studies, which are attributed to the influence of local topographic and subsurface characteristics on seismic ground motions.

Subsurface imaging dataset acquired at the Garner Valley Downhole Array site using a dense network of three-component nodal stations

There is a growing need to characterize the engineering material properties of the shallow subsurface in three dimensions for advanced engineering analyses. However, imaging the near-surface in three dimensions at spatial resolutions required for such purposes remains in its infancy and requires further study before it can be adopted into practice. To enable and accelerate research in this area, we present a large subsurface imaging data set acquired using a dense network of three-component (3C) nodal stations acquired in 2019 at the Garner Valley Downhole Array (GVDA) site. Acquisition of this data set involved the deployment of 196 stations positioned on a 14 × 14 grid with a 5 m spacing. The array was used to acquire active-source data generated by a vibroseis truck and an instrumented sledgehammer, and passive-wavefield data containing ambient noise. The active-source acquisition included 66 vibroseis and 209 instrumented sledgehammer source locations. Multiple source impacts were recorded at each source location to enable stacking of the recorded signals. The active-source recordings are provided in terms of both raw, uncorrected units of counts and corrected engineering units of meters per second. For each source impact, the force output from the vibroseis or instrumented sledgehammer was recorded and is provided in both raw counts and engineering units of kilonewtons. The passive-wavefield data include 28 h of ambient noise recorded over two nighttime deployments. The data set is shown to be useful for active-source and passive-wavefield three-dimensional imaging and other subsurface characterization techniques, which include horizontal-to-vertical spectral ratios (HVSRs), multichannel analysis of surface waves (MASW), and microtremor array measurements (MAM).

DeltaVs: A Method for Detecting Significant Layer Boundaries in Surface Wave Inversion Results

Surface wave testing is a powerful tool for noninvasive seismic site characterization. It includes a wide variety of active-source and passive-wavefield methods [e.g., spectral analysis of surface waves (SASW), multichannel analysis of surface waves (MASW), microtremor array measurements (MAM)] that can be applied to many different field conditions. The dispersion data collected with all these methods can be inverted to produce (one-dimensional) 1D shear wave velocity (VS) profiles of the subsurface. A critical part of this inversion procedure is the need to develop several trial model parameterizations with different numbers of layers as a means for investigating epistemic uncertainty and nonuniqueness when attempting to fit the experimental dispersion data. This is especially important when a priori information either is not available or does not extend to great enough depths to constrain the inversions. These trial parameterizations are used to vary the number of potential layers present in the subsurface models and to set the acceptable search ranges for depths to boundaries and stiffness in each layer. The inversion results are highly affected by the parameterizations. When performing high-quality inversions, it is important to use different parameterization options to fully explore potential conditions at the site and characterize the epistemic uncertainty of the inversion process. While this practice allows for a robust analysis of the experimental dispersion data and its uncertainty, it also generates many potential subsurface models for the site, which can represent a challenge for engineers when deciding which VS profiles best represent the true subsurface layering for use in subsequent analysis and design (e.g., seismic site response). This paper presents an approach, called the DeltaVs method, for systematically evaluating the depths of layer boundaries across all acceptable models determined from several different inversion layering parameterizations as a means to identify distinct clusters of boundaries. Using these clusters, the number and location of meaningful layer boundaries can be determined for many sites, thereby allowing for the elimination of some parameterization options containing more, or fewer, boundaries and a reduction in epistemic uncertainty. The distributions of layer boundaries within these clusters can then be used to determine statistics for the depths of layer boundaries that are consistent with the epistemic uncertainty of the models produced by the inversion procedure. The DeltaVs method is demonstrated on 12 synthetic ground models as well as one field data set.

The Trinidad and Tobago microzonation project: San Fernando

SUMMARYThe city of San Fernando is the industrial capital of the twin island nation of Trinidad and Tobago in the West Indies. Situated at the west coast, is in close proximity with the Central Range Fault, a major fault crossing the whole island and continuing offshore. Due to this proximity, but also because of the complex tectonics in the area with deep and shallow earthquakes, a detailed seismic hazard map deemed important. A microzonation study was carried throughout the boundaries of the city, using ambient noise techniques. 1200 single-site records of ambient noise, along with 16 microtremor array measurements were obtained. The results indicate that the alluvium deposits in the river basins along with a small part of reclaimed land appear to have velocities in the range 220–500 m s–1 and thickness from shallow to about 60 m. The marls, which cover the majority of the study area, exhibit shear wave velocities in the range of 700–1600 m s–1, depending on the level of compaction, with thickness that can reach almost 100 m. The hardest formations, Naparima Hill and San Fernando formations, in weathered state they appear with velocities above 1000 m s–1, while their unweathered state reaches 3000 m s–1. A parametric investigation which was conducted as an independent investigation to validate the experimental results, is in good agreement. An empirical susceptibility method to calculate the most vulnerable areas, reveals that areas close to coast and the river banks might experience much higher shaking values.

Microtremor array method using spatial autocorrelation analysis of Rayleigh-wave data

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.Highlights1. A summary of fundamental principles of calculating phase velocity from ambient noise2. General recommendations for MAM data collection and processing using SPAC methods3. A discussion of limitations and uncertainties in the methods

Three-dimensional microtremor array measurement with irregular receiver geometries in town and residential area

受振器を調査範囲内で格子状に配置し，利用可能な受振器のペア数と受振方位を十分に確保する3次元微動探査の適用が進んでいる。しかし，市街地や住宅地では受振器を設置する場所が制限されるため，格子状に密に受振器を配置することは難しい。そこで，アクセス可能な道路のみを利用し，直線，L字，T字，十字状に受振器を密に配置した小アレイと，格子状に受振器を配置した大アレイを組み合わせた不規則配置の3次元微動アレイ探査の適用を試みた。現場測定を実施した場所は，東日本大震災の際に液状化被害を受けた茨城県神栖市深芝地区の520m×640mの範囲である。小アレイの受振器間隔は5m，大アレイの受振器間隔は40mを基本とし，1展開につき45～50台の受振器を利用して測定した。両アレイによって得られた合計57展開の記録を用い，CMP-SPAC法により3次元S波速度構造を推定した。単独のアレイと両者を組み合わせたアレイの記録から求められたそれぞれの分散曲線を比較したところ，小アレイのみ，あるいは大アレイのみでは不足する解析可能な周波数範囲を，両者を組み合わせることで相互補完できていることがわかった。これより，広い周波数帯域で表面波の位相速度を決定でき，浅部から深部までのS波速度構造を推定することができることがわかった。得られた結果は，既存調査による道路被害分布や配水管被害点，噴砂の発生箇所，採掘跡地の分布と非常によく対応しており，その有用性が示された。今後，市街地や住宅地のように受振器を設置する場所が制限される場合でも，不規則配置での微動探査を行うことで，比較的に簡便に広域の地盤のS波速度構造を推定できると考えられる。また，得られた結果は，たとえば液状化評価に用いる地盤モデル作成に利用でき，防災ハザードマップの作成に寄与できると思われる。

Estimation of Shear-Wave Velocity Structures in Taichung, Taiwan, Using Array Measurements of Microtremors

Near-surface S-wave velocity structures (VS) are crucial in site-effect studies and ground-motion simulations or predictions. We explored S-wave velocity structures in Taichung, the second-largest city in Taiwan by population, by employing array measurements of microtremors at a total of 53 sites. First, the fundamental-mode dispersion curves of Rayleigh waves were estimated by adopting the frequency–wavenumber analysis method. Second, the surface-wave inversion technique was used to calculate the S-wave velocity structures of the area. At many sites, observed phase velocities were almost flat, with a phase velocity of approximately 800–1300 m/s in the frequency range of 0.6–2 Hz. A high-velocity zone (VS of 900–1500 m/s) with a convex shape was observed at the shallow S-wave structures of these sites (depths of 50–500 m). On the basis of the inversion results, we constructed two-dimensional and three-dimensional contour maps to elucidate the variations of VS structures in Taichung. According to VS-contour maps at different depths, lowest S-wave velocities are found at the western coastal plain, whereas highest S-wave velocities appear on the eastern side. The S-wave velocity gradually decreases from east to west. Moreover, the S-wave velocity of the Tertiary bedrock is assumed to be 1500 m/s in the area. According to the depth-contour map (VS = 1500 m/s), the depths of the bedrock range from 250 m (the eastern part) to 1550 m (the western part). The thicknesses of the alluvium gradually decrease from west to east. Our results are consistent with the geology of the Taichung area.

Investigation of shallow S-wave velocity structure and site response parameters in Taiwan by using high-density microtremor measurements

Site effect is considered a critical component affecting ground motions, and the shallow velocity structure is a primary factor for determining the site effect. The shallow velocity structure should be carefully evaluated to mitigate earthquake hazards. Thus, we use microtremor array measurements as well as the inversion of microtremor horizontal-to-vertical spectral ratios (MHVSRs) to investigate shallow S-wave velocity (Vs) profiles in Taiwan, which is one of the most seismically active areas of the world. On the basis of the diffuse field assumption, Vs profiles can be efficiently inverted. To reduce the nonuniqueness of MHVSR inversion, Vs profiles obtained using microtremor array measurements and from an engineering geological database were adopted as model constraints. Finally, this study included 3587 inverted MHVSR Vs profiles and 65 inverted Vs profiles from microtremor arrays. The results were used to create a detailed updated VS30 map of Taiwan and to map the depth contours of Z1.0; thus, the relationship between VS30 and Z1.0 was determined. We proposed a new parameter, HR, as a proxy for VS30, that is defined as a ratio of the average MHVSR across high- and low-frequency bands. This parameter was correlated to the predominant frequency without subjective selection. Moreover, we constructed a pseudo-three-dimensional shallow Vs model of Taiwan, which describes main shallow structural features and provides complete details for plain areas.

Shear Wave Velocity (Vs) at Reactor Building, Experimental Power Reactor Indonesia

Experimental Power Reactor (RDE) is a Gen IV Reactor type with Hydrogen Gas Cooler. Despite this type of reactor has high safety performance, earthquake hazard should be demonstrated. Detail Engineering Design Activity on RDE has been conducted in the past 3 years. In the end of this phase, preliminary parameter design such as shear wave velocity (Vs) should be defined. This parameter correlated with subsurface condition which has high uncertainty. This study is conducted in order to estimate values of Vs. Generally, the data collection is carried out through geotechnical investigation but this method cost more time and resources. In the recent decades, another method has been widely introduced which is geophysical passive source Microtremor Array Measurement (MAM) with Spatial Auto Correlation (SPAC) method. This method can be used to estimate values of Vs and can be used as preliminary reference to define the position of borehole before construction phase getting started. The result shows, the location of reactor building is estimated to have 5 soil layers with varying Vs value. The Vs value of the first soil layer is about 152 m/s started from the surface to 8 m depth. The second soil layer has 169 m/s Vs value started from 8 m to 20 m depth. The third soil layer, started from 20 m to 36 m depth, has 384 m/s Vs value. The next layer as the fourth layer of soil, started from 36 m to 70 m depth with a value of Vs around 526 m/s. The last soil layer with a depth 70 m to 100 m, has Vs value of 667 m/s. Based on these Vs value estimation from surface to 30 m depth, the average value of the shear wave velocity (Vs 30) is m/s. Thus, reactor building is located in the site class SD with medium soil categories according to SNI 1726-2012. The foundation design and excavation planning phase, this information is needed.

Seismic site classification from HVSR data using the Rayleigh wave ellipticity inversion: A case study in Singapore

Shear wave velocity (Vs) profile is one of the critical geotechnical measurements of soil layers for seismic hazard assessment and liquefaction potential evaluation. Enhancing the effectiveness of in-situ Vs profiling by reducing time and cost is of great interest. For that reason, this study aims at assessing Vs profile generation from a single-station three-component geophone with additional borehole log data for constraining parameter space. Based on multichannel analysis of surface waves (MASW), and microtremor array Measurements (MAM) conducted previously at seven sites located in Bukit Timah Granite, Singapore, this study utilized HVSR signals for Rayleigh wave ellipticity (ellipticity curve) inversion with additional inversion constraint using borehole log data. The resulting Vs profiles and reference Vs profiles from MASW and MAM were quantitatively compared using average Vs of 30 m (Vs30). The profiles generated from ellipticity curve inversion revealed a good agreement with Vs reference profiles. Vs30 based site classification results also indicated a good fit of two test results. Therefore, HVSR measurements for further ellipticity curve inversion, with already available borehole log data for constraint, is considered as a promising cost and time-effective site classification approach.

Evaluation of Uncertainties in Site Response Analysis of Deep Soil Profiles in South Carolina Coastal Plain

ABSTRACTThe South Carolina Coastal Plain consists of deep soil sediments over basement bedrock. The depth of basement bedrock varies from being present at the surface to a depth of more than 1200 m at the southern tip of the state. A large variation exists in the thickness of the sediment, which impacts the seismic site response analyses of the Coastal Plain, particularly in areas where the availability of deep shear-wave velocity profiles is limited. This study evaluates the impact of variations in the shear-wave velocity profiles for two sites in the South Carolina Coastal Plain. The shear-wave velocity profiles were measured using different geophysical methods, including a combined multichannel analysis of surface waves and microtremor array measurement (MASW-MAM) method and P–S suspension logging. The equivalent-linear site response analyses were conducted by applying a synthetic earthquake motion at the depth of the B–C boundary (a depth of competent rock in which the shear-wave velocity is 760 m/s). The results are presented in terms of the amplification factor and its standard deviation. Results show that the average shear-wave velocity at the first 30 m (VS30), the shear-wave velocity contrast at the interface of the base layer and the B–C boundary, and the depth to the B–C boundary have a significant impact on the amplification factor and its variability, particularly for the amplification factor at periods higher than 0.1 s. The MASW-MAM method provided significantly lower VS30 values than the P–S suspension logging method at one of the two sites. Consequently, an additional peak in the amplification factor was observed for the site that had a low VS30, and the corresponding period was close to the resonant period of the loose, surface deposit.

The effect of input ground motion location on design spectrum in a seismic ground response analysis performed for an alluvial site

Site response analyses are performed to estimate the change in the characteristics of the input rock motion occurred during the vertical propagation of seismic waves and to obtain the local variation in intensity and spectral characteristics of surface ground motion. The rock outcrop motions which are selected by considering the source mechanism, fault distance and earthquake characteristics that are compatible with the standard design ground motion level. It is generally agreed that time domain simulated or spectral matched ground motions may be applied as input ground motions at seismic bedrock in the dynamic response analysis of soil layers under earthquake excitation and thus shear stresses and deformations, acceleration-time histories and site-specific design spectrum can be obtained for surface layers. In “Turkish Building Earthquake Code (TBEC), 2018” it is being stated that the input ground motion can be exerted at the base of a soil layer lying at a shallower depth in comparison with seismic bedrock depth by using the method of spectral matching the accelerograms to the target spectrum of corresponding soil layer where the input motion is applied. In this study, by evaluating and combining the findings of the geotechnical borings and microtremor array measurements, the shear wave velocity profiles extending to seismic bedrock (VS≥760 m/s) were obtained for the investigation sites. The results of the site response analyses that were conducted by using the two different methods proposed in TBEC (2018) were interpreted. At the first method, in deeper 1D dynamic analyses, the input ground motions were applied at seismic bedrock defined as site class ZB where the shear wave velocity (VS30) is higher than 760 m/s2. Whilst at the second method, the inputs were applied at the base of the shallower 1D soil column having site class either ZC or ZD where VS30 is in the range of 360 m/s2 < VS30 < 760 m/s2 and 180 m/s2 < VS30 < 360 m/s2 respectively. As a result, the acceleration spectra obtained on the surface of deep and shallow models were compared for 31 strong ground motion records, selected in accordance with the design spectra corresponding to seismic code depending on the location of the investigation area and local site classes.

Site Amplification Analysis of Dushanbe City Area, Tajikistan to Support Seismic Microzonation

Being a country exposed to strong seismicity, the estimation of seismic hazard in Tajikistan is essential for urbanized areas, such as the rapidly growing capital city Dushanbe. To ensure people’s safety and adequate construction work, a detailed seismic microzonation is the key to proper hazard planning. Existing estimations of seismic hazard date back to 1978; they are based on engineering geological investigations and observed macroseismic data. Thereupon relies the Tajik Building Code, which considers seismic intensities according to the Medvedev–Sponheuer–Karnik Scale, MSK-64. However, this code does not accurately account for soil types, which vary considerably in Dushanbe—not only by their nature, but also due to increasing anthropogenic influences. In this study, we performed a series of analyses based on microtremor array measurements, seismic refraction tomography, and instrumental data recording from permanent stations for standard spectral ration and from mobile seismic stations for the horizontal to vertical spectral ratio in order to provide a comprehensive full-cover microzonation of Dushanbe accounting for soil types. Our results identify several critical areas where major damage is likely to occur during strong earthquakes.

Shallow One-Dimensional Shear Wave Velocity Structure in Lhokseumawe, Aceh-Indonesia

A series of microtremor array measurements, consisted by thirty-one sites, has been designed and conducted in Lhokseumawe. There were three lines of microtremor measurement consisting triangular seismic array for each point to study shear wave velocity model. A Spatial Auto Correlation method was applied in order to reveal the 1D velocity structure beneath the three lines at each measurement point. The analysis successfully revealed one dimensional shear wave velocity model down to 30 meters depth. The results show that the highest shear wave velocity of 639 m/s at depth of 30 m while the lowest value of 45 m/s at depth of 5 m. The results indicate that 12 out of 13 sites have low shear wave velocity with a range between 100 to 200 m/s. Low velocity structures are mainly located in Banda Sakti subdistrict. The areas with low shear wave velocity are associated with less compact sediment material which are mostly located in paddy field and ponds areas.

Microtremor measurements in the India's holy city, Varanasi for assessment of site characteristics

Microtremor measurements are used for preliminary site effect of the Quaternary sediments of Varanasi in Indo-Gangatic plain, which is being planned as a Smart City of India. We evaluated site effects using single and arrays Microtremor measurements at various sites in the Varanasi City, Uttar Pradesh, India. At City, H/V spectral ratios using microtremor measurements indicate fundamental frequency range 0.37–0.63 Hz, corresponds amplification 5 to 14 times. This indicates that the deep thickness of the upper soft soil at about several hundreds of meters. Such huge amount of sediments deposition sources could be associated with fluvial rivers system in the area and sediment drain from Himalaya. The average peak frequency (~0.5) may reflect the Quaternary-Tertiary boundary. Microtremor array measurements at two sites were taken using circular arrays (30 and 60 m) which consists of three recording stations on the circumference of circle and one in the centre of circle. The Phase velocity dispersion of Rayleigh wave is calculated from array using SPAC method, and a 1-D shear wave velocity structure is determined by means of inversion processes. The top layers up to 50 m show shear velocities between 280 and 320 m/s and the velocities of underneath layer vary between 1500 and 1600 m/s till 100 m of depth. These results from the present study would be useful for future planning purposes as well as risk management and to reduce severity of earthquake effects in the study area.

The debris avalanche in Donghekou area triggered by the 2008 Wenchuan (M8.0) earthquake: Features and possible transportation mechanisms

In 2008, the Wenchuan earthquake triggered many large landslides with rapid movement and long runouts, resulting in a great number of casualties. Although there have been many studies of the geographical features and initiation mechanisms of some catastrophic landslides, the movement mechanisms for many remain unclear. In this paper, we present a case study of a large landslide (debris avalanche) triggered by the 2008 Wenchuan earthquake in the Donghekou area, Sichuan Province, China. We made detailed field surveys of the geographical features of the landslide and carried out subsurface investigations of the landslide deposits using microtremor array measurement and electrical resistivity tomography (ERT). Based on the observed surficial features, shear-wave velocity (Vs) profiles and 2D electrical resistivity profiles of the landslide deposits, we estimated the possible thickness of landslide deposits at different locations, and also analyzed the possible landsliding mechanisms. We inferred that this landslide resulted from retrogressive failures on the source area, and the displaced landslide materials underwent transitional spreading with further entrainment of debris along the travel path. Multiple mud waves might have been formed in the substrate soil layers along the travel path due to the entraining of landsliding materials, and the landsliding materials might have presented channelized movements, indicating that different parts may have moved at different speeds. This kind of transportation mechanism may provide information for elevating the numerical simulation of landsliding, and also for reuse of deposit area of large landslides.

Practical configured microtremor array measurements (MAMs) for the geological investigation of underground space

Underground space utilization is becoming increasingly essential for modern metropolitan cities such as Singapore. Mapping a soil/rock interface using traditional borehole investigation methods is expensive and difficult, owing to the numerous physical constraints within a built-up city. Boreholes are often far apart, resulting in many unforeseen ground conditions during subsequent excavation. Geophysical methods are sometimes employed as possible alternatives for fast, economical, and efficient bedrock surveys. The goal of this study is to investigate the practical details of applying microtremor array measurement (MAM) as a non-invasive surface wave survey for mapping soil/rock interfaces in Singapore. Critical configurations in field data acquisition are examined, and practical recommendations for array construction are provided. In addition, 30 in situ MAM tests are carried out for two major geological formations in Singapore. From the results, a standard shear wave velocity (Vs) of 500 m/s is found to be suitable for interpreting the soil/rock interface, for the Bukit Timah Granite and Jurong formations. However, the method does not predict well when soft Kallang formation deposits are present. Other limitations are also discussed in the later parts of this paper. Conclusions and practical recommendations are discussed, providing constructive guidance to the industry. The proposed Vs-based method and associated guidelines and limitations can be used to create a digital geological database and are especially useful for rock profiling in an urban environment.

Estimating the shear wave velocity structure above the fresh bedrock based on small scale microtremor observation array

S-wave velocity and thickness of weathered strata are the important parameters to engineering construction. In this study, array measurements of microtremors were carried out at 50 sites in Jimo, China. It revised the observation array that a small scale array was placed in the center of traditional regular triangle observation array. The field data of Jimo verified that it is effective to identify the different degrees of weathered strata above fresh bedrock. Based on the spatial autocorrelation (SPAC) method, the dispersion curves were achieved. While the S-wave velocity structure was estimated using the fork genetic algorithm. By using small scale observation array, it identifies different strata including weathered layer in detail. In order to test the accuracy of inversion results, 5 sites were chosen to drill. Comparison between inversion results and boreholes, it shows agreement with logs data. At last, the S-wave velocity structures in different depths above the fresh bedrock were plotted. We delineated the S-wave velocity structure for the first time in this research region using revised measurement of microtremors signal. The depth of fresh bedrock fluctuates strongly which is affected by weathered. This method provides another means to distinguish the degree of weathered, depth of fresh bedrock, and S-wave velocity structure of shallow strata.

2D Microtremor array measurements for estimating of deep subsurface structural model in Tsukuba city

2D Microtremor array measurements for estimating of deep subsurface structural model in Tsukuba city