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This study mainly concentrates on the determination of site effects for the Plio-Quaternary and especially alluvial soils of the Çubuk district and its close vicinity in Ankara, Turkey. In the study area, particularly in the vicinity of the asymmetric graben that has formed due to the tectonic activities along the NE-SW trending normal faults, a mi...
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... study area covers the Çubuk district and its close vicinity, mainly the northern part of the Çubuk Plain which is situated approximately 40 km north of Ankara. The location of the study area is given in Fig. 1. The area covers nearly 120 km 2 between the northern part of the Çubuk basin and the Çubuk district. The investigation was conducted at a moderately populated area with mostly residential settling, an international civil airport and a considerable amount of small to large in- dustrial buildings which has a major potential for ...
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... infer the bedrock geometry beneath the study area, to check the reliability of fundamental period distribution over the area and to corre- late these results, the bedrock geometry was created by using the Rockworks software V. 15. This 3D geological model was constructed by considering the deep borehole information compiled from the stud- ies conducted by MTA for thermal springs and particularly, DSİ (DSİ, 1979) for hydrogeological investigations, as well as the extracted and verified faults (Eker et al., 2012). ...
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... wave velocity profiles of the subsurface at the 41 sites were utilized as another complementary data along with the geotechnical borings (Fig. 4). As can be seen in Fig. 10, the shear wave velocity measurements prove the presence of a thick soft sediment layer for Plio-Quaternary sediments. These measurements characterize the sedi- ments generally at least up to a depth of 55 m due to the presence of the faults and their deformation zones (Eker, 2009;Eker et al., ...
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... the borehole database, five examples from the conducted and collected geotechnical borehole logs (Fig. 8) were examined with the corresponding shear wave velocity profiles (Fig. 10). The profiles were constrained to a depth of 30 m since the geotechnical borings did not penetrate greater depths (i.e., b 30 m) within the study area. It should be noted that the constructed shear wave velocity profiles gave highly compatible correlations with the geotechnical borings. When the logs are to be evaluated individually, ...
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... S data can give an inference to interpret the variations of the H/V parameters within the basin. Therefore, 2D V S profiles were created to compare with the fundamen- tal periods and the H/V amplitudes obtained from the microtremor survey. The trends of these profiles (A-A′, B-B′ and C-C′) can be seen in Fig. 4. In creating three 2D Vs models in Fig. 11, a digital elevation map of the study area was produced from 1:25,000 topographic maps of the General Command of Mapping. The upper surface limit of the models was adjusted with respect to the topography and it was exag- gerated in the vertical direction for each model. The effect of the local soil conditions along with the topographic ...
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... for each model. The effect of the local soil conditions along with the topographic elevation differences on the H/V results was examined by the help of these profiles. The variations of the fundamental periods and H/V amplitude are illustrated for each of the 2D V S models. The thickness of the deposits based on the Vs values is given in Fig. ...
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... A-A′ (Fig. 11a) runs through the thick sedimentary deposits located at the southern part (center of the basin) to relatively thinner ones present at the northern part of the area where the basin starts to get narrower. Throughout this section, especially the axial and marginal depositional system products can be easily identified with the aid of the ...
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... be easily identified with the aid of the V S information, and the sediment thickness and bedrock geometry can be inferred from the fundamental periods of the sites. When the shear wave velocity profiles of these measurement sites are to be examined (i.e., one of the sections was taken along the measurement on the Qua- ternary sites as given in Fig. 11a), it is clearly observed that coarse grained layers with high shear wave velocity results intersect the fine grained deposit with a relatively low velocity. This intersection could be either a wedge or a layer according to the depositional setting con- trolling the environment, namely, alluvial fan or braided river. Addition- ally, ...
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... since the trend of this intersected profile is not a straight line, some boundary effects are also observed in the H/V results. Especially, in Mob-013, -044 and -046 located on the western side of the deformation zone of the normal faults and at the Plio-Quaternary boundary, high H/V amplitude s are easily observed as the peaks of the curves in Fig. ...
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... alignment of the other sections was selected to trend normal to the alluvium depositional area in the sediments deposited in different ages (Fig. 11b and c). The marginal depositional system which is domi- nant at the lateral boundaries of the major course of the Çubuk river is composed of terrace and alluvial fan conglomerates deposited by debris flow and braided rivers based on slope variation (Fig. 11b and c). Due to the nature of the marginal depositional setting, the size of the ...
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... normal to the alluvium depositional area in the sediments deposited in different ages (Fig. 11b and c). The marginal depositional system which is domi- nant at the lateral boundaries of the major course of the Çubuk river is composed of terrace and alluvial fan conglomerates deposited by debris flow and braided rivers based on slope variation (Fig. 11b and c). Due to the nature of the marginal depositional setting, the size of the grains is larger than that of the axial depositional system that consists of fine grained alluvial plain sediments such as sand, silt and clay. Therefore, the shear wave velocity results of the marginal deposits were higher than that of the axial ones (Inazaki, ...
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... the basin margins were monitored in terms of H/V amplitude, shear wave velocity and fundamental period. As can be seen from two profiles (Profiles B-B′ and C-C′) presented by Fig. 4, towards the center of the basin, the basin effect may manifest itself with a decreasing tendency of the H/V amplitude. This observation can be seen in Mob-021 (Fig. 11b) and Mob-074 and -087 (Fig. 11c) in the curves constructed. In these curves, higher H/V amplitudes can be seen at the hanging walls of the ...
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... in terms of H/V amplitude, shear wave velocity and fundamental period. As can be seen from two profiles (Profiles B-B′ and C-C′) presented by Fig. 4, towards the center of the basin, the basin effect may manifest itself with a decreasing tendency of the H/V amplitude. This observation can be seen in Mob-021 (Fig. 11b) and Mob-074 and -087 (Fig. 11c) in the curves constructed. In these curves, higher H/V amplitudes can be seen at the hanging walls of the ...
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... experimental ap- proaches should be combined to implement a study like this one, in particular for those sites located in moderate seismically active areas such as the Çubuk region. Additionally, the outputs of the microtremor survey complement the study and provide an advantage compared to using only one parameter of the seismic design codes. Fig. 11. Comparison of the three interpolated shear wave velocity profiles along with the microtremor measurements. The blue lines show the area where the shear wave velocity and microtremor measurements were implemented. Note that the trends of these profiles (A-A′, B-B′ and C-C′) can be seen in Fig. 4. (For interpretation of the references ...
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Citations
... In this sense, the single-station MHVSR method (Nakamura 1989) could be considered an alternative or complementary to resistivity and seismic methods for underlying mapping. For measuring fundamental site frequency (f r ) and seismic microzonation, the MHVSR method is frequently used as a passive geophysical method (Eker et al. 2015;Wood et al. 2019). This method is based on the comparison of the amplitudes of the horizontal and vertical components (H/V) of microtremors or external environmental noise. ...
Tortum Lake, in Turkey, which is a natural feature and a large landslide barrier lake, was formed due to a natural landslide disaster. In this context, areas with the potential to create landslides in Tortum Lake and the waterfall region are identified and displaced with zoning. Microtremor horizontal-to-vertical spectral ratio method and seismic refraction studies were used, and by considering the results, the areas with high landslide potential were determined. It aims to choose an effective study area by determining the areas with high landslide capability and revealing the boundaries of the geotechnical project. Areas with high landslide potential were compared with those where landslides have happened, and it showed that this method can be used for determining sliding risk before it happens. Results showed that regions with low VS30, combined with the slope, create the potential for landslides. Afterward, when the map of ground amplification values obtained from the microtremor data was reviewed, it was found that the physical properties of areas with high ground amplification values would be the same as those of areas with low VS30 values. When these two maps are reviewed, it is observed that areas with low VS30 values and high ground amplification values are in the same locations as those with high sliding potential. The correct determination of the geotechnical project work area is important both to draw attention to the right area to be designed and to avoid wasting unnecessary time and money on unnecessary areas.
... Several authors have advocated the use of T 0 or conversely f 0 as the principal parameter in the definition of seismic site classes [36,45,94,[110][111][112]. Gobbi et al. [100] investigated the use of f 0 and velocity gradient within 30 m depth (B 30 ) as proxies of site amplification in conjunction with V S, 30 . ...
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Extensive geophysical surveys (MHVSR and MASW) conducted for the microzonation of the Kashmir Valley, Himalayas, revealed unexpected dynamic characteristics at certain sites pointing out the weak relationship between fundamental frequency (f0) and time-averaged shear wave velocity over 30 m depth (VS,30). Unusual low-frequency amplification at stiff soil sites and high-frequency amplification at weathered rock sites was obtained. On the contrary, high-frequency amplification was attained at a soft soil site over shallow bedrock. Instances of topographic amplification on slopes, hills, and valleys were encountered. Consequently, the commonly used VS,30-based single-proxy site classification failed to explain these atypical site effects, thus underscoring the caution to be exercised for site classification in geologically complex regions. These findings motivated the documentation of the anomalies and the search for the most suitable site characterisation scheme for the geological deposits of the Kashmir Valley. The coupled MHVSR-VS,30 proxy approach accomplished the best results for the study region.
... ELA was used to model non-linear soil parameters in terms of equivalent linear soil properties. The earlier researchers used peak ground acceleration (PGA), spectral acceleration (SA), and maximum strain (%) to predict the ground motion (Kramer, 1996;Yoshida et al., 2001;Eker et al., 2015;Putti and Satyam, 2018;Hashash et al., 2020;Soltani-Azar, 2022). Due to the unavailability of strong ground motion in Ethiopia, the Northridge ground motion (Mw 6.69) was consulted as an input motion from the Pacific Earthquake Engineering Research Center (PEER, 2010) database (http://peer. ...
The study area is located in one of the most earthquake prone regions in southern Ethiopia, which is characterized by small-to-intermediate earthquake occurrences causing damage to buildings. Predicting liquefaction hazard potential and local site effects are imperative to manage earthquake hazard and reduce the damage to buildings and loss of lives. The objectives of this work were to perform the equivalent linear response analysis (ELA) and shear wave velocity (Vs.)-based liquefaction hazard analysis and classify the site into different seismic site classes based on the European and American codes. The SPT-N and Vs.30 values showed the site falls in the C and D classes based on the NEHRP (2015) code but falls in the B and C classes based on the EC8 (2003) code. The susceptibility of liquefaction was evaluated using grain size analysis curves. Moreover, peak ground acceleration (PGA), spectral acceleration (SA), and maximum strain (%), which are very critical to understanding the local site effects, were estimated by the DeepsoilV.7 program. The cyclic stress ratio and cyclic resistance ratio were used to calculate the factor of safety (FS). A liquefaction potential index (LPI), probability of liquefaction (PL), and probability of liquefaction induced ground failure (PG) were used to assess the probability of liquefaction. The peak ground acceleration (g) values ranged from 0.166 to 0.281 g, whereas spectral acceleration (g) was found to be high at 0.1–1s. The liquefaction susceptibility screening criteria revealed that the study area is highly susceptible to liquefaction. FS is < 1 for a liquefied site, but FS is > 1 for non-liquefied sites. In comparison to non-liquefied sites, the liquefaction forecast site has a liquefaction potential index value of 0–54.16, very likely high PL, and high PG. The findings will be helpful in the design of structures and in solving practical challenges in earthquake engineering.
... Many authors have worked in the evaluation of these site effects in many cities all over the world (e.g. Eker et al. 2012;Eker et al. 2015 in Turkey; Bala 2014 in Bucharest; Yilar et al. 2017 in Boston; Maringue et al. 2017 in Chile). ...
The central area of Chile’s Valparaiso Region has been classified as a seismic gap for a major earthquake, which makes it very important to understand the seismic hazard of the zone. Generally, seismic codes consider a qualitative classification of sites to estimate the possible damage in the case of an earthquake scenario. Estimating the values of acceleration could be very important to prevent damages and increase preparedness for these rare events. In this research, a qualitative and quantitative estimation of seismic hazard is performed in the study area (Valparaiso region between Papudo and San Antonio 32°–34° S). This is achieved through an integrated and relatively economical approach which considers the information from Geology, Geophysical experiments (Gravity and seismic methods), and Geotechnical analyses. The results of the geophysical survey and geology information allow dividing the zone into five site types through a new proposal of site classification that depends not only on the Vs30, but also on the sites predominant period (T0), which is an innovation of this work for the Chilean code. The Peak Ground Acceleration (PGA) values in the study zone were estimated using a Ground Motion Predictive Equation developed for the Chilean subduction zone. Additionally, we consider three different seismic scenarios according to the history of events in Central Chile. The results of this quantitative analysis show PGA values up to 0.52 g for the median and 1.2 g for the 84th percentile of the scenarios. Overall, the highest accelerations (PGA) are in zones with low shear wave velocities (< 500 m/s), a long predominant period (> 0.4 s) and where geology establishes the presence of low stiffness soils. The comparison of response spectra from the model against records from 2010 Maule and 1985 Valparaiso earthquakes shows available models tend to overpredict the intensities.
... Studies in the last few decades have demonstrated that local site conditions, particularly those close to earthquake-prone areas, can generate substantial amplification and spatial variations of earthquake ground motion that considerably affect the level of ground shaking (e.g., Pratt et al. 2003;Ajala and Persaud 2021). Hence, the amplification of ground motion due to local site effects (i.e., basin geometry, topography, and ground motion resonance) plays a crucial role in enhancing seismic damage (Rodríguez-Marek et al. 2001;Bakir et al. 2007; Koçkar and Akgün 2012;Núñez et al. 2013;Eker et al. 2015;Koçkar 2016). Almost all of the destructive earthquakes in the last three decades (e.g., earthquake events such as Kobe 1995;Chi-Chi 1999;Kocaeli and Düzce 1999;Sichuan 2008;New Zealand 2010;Van 2011;Tohoku 2011) have brought particular attention to the significance of site effects. ...
... This dispersive character of surface waves can be effectively utilized to produce a one-dimensional velocity model for a particular site (Rodríguez-Marek et al. 2001;Herak 2008;Boaga et al. 2010;Roy et al. 2013;Pegah and Liu 2016). These estimations have been prepared by using data obtained through array applications, where it should be noted that in some situations where the topography and basin structure are more complex (i.e., tectonically deformed areas in near-fault regions), 2D and 3D basin models are required to account for the lateral heterogeneities or the complex structures that may strongly affect the local ground shaking pattern (Gazetas et al. 2002;Singh et al. 2011;Piatti et al. 2013;Eker et al. 2015;Wang et al. 2016;Narayan and Kamal 2018;Cushing et al. 2020;Mori et al. 2020). ...
... Several studies (Ibs-von Seht and Wohlenberg 1999;Özalaybey et al. 2011;Uebayashi et al. 2012;Eker et al. 2015) have shown that the resonance frequency obtained from microtremor measurements can be used to map the thickness of sediments. In this part of the study, some microtremor measurements were recorded using a single mobile station, and these records were processed by the H/V technique (Nakamura 1989) to verify basin depth and develop 2D Vs profiles. ...
The high seismicity and tectonic activity of the study area located in a near-fault region in Gölyaka, Düzce, results in a bedrock geometry highly complex in the sense of faulting and deformation. This makes this area very challenging in terms of a site response study that would aid seismic hazard assessment. This study develops a basin model to evaluate the site effects in the tectonically formed Plio-Quaternary fluvial sedimentary layers of the Gölyaka region. The selected site uniquely falls within the near-field domain of a section of the North Anatolian Fault System. To determine the presence of these lateral variations in the geology as well as the geometry of the basin over a wide area, surface seismic measurements and deep vertical electrical sounding along with geotechnical boring studies have been performed, and a 3D basin geometry model was developed. The basin model shows that the sediment thickness continues to a depth of approximately 250–350 m with an irregular geometry due to over-step faulting near the southern boundary of the basin. Consequently, this study confirms the spatial variations in the near-field area that depend on basin geometry, material heterogeneity, and topography, indicating dipping and nonuniform stratification in the velocity profiles. Furthermore, the conducted microtremor measurements were used to compare the natural periods of microtremor results, along with interpolated Vs profiles to validate estimated basin depths. In conclusion, this study indicates that a well-developed basin geometry that reflects the complex process associated with the characteristics of the near-fault region could be accurately and reliably determined by developing a 3D basin model to assess site response in an account for seismic hazard assessment studies.
... Various ground failures like liquefaction, structural failures or landslides can result from ground motion amplification, leading to excessive ground shaking. Various researchers have made a considerable attempt for seismic hazard microzonation study of other important cities in India and worldwide (Gupta et al. 2021, Brando et al. 2020, Pagliaroli et al. 2018, Eker et al. 2015, Ansal et al. 2010, Berilgen 2007, Mohanty et al. 2007, Van Eck et al. 2006, Papathanassiou et al. 2005. ...
Considering future earthquakes, the vulnerability of Agartala city is following an increasing trend mainly due to the recent surge in population density and significant infrastructure developments. The recent moderate intensity earthquake at Tripura, namely the 2017 Dhalai Earthquake (Mw 5.7), with its epicentre being the adjoining district of Agartala, resulted in liquefaction cases, sand blows and lateral spreads, which may be an alarm for the geotechnical engineering fraternity. The present study attempts to evaluate local soil sites’ effect in the form of developing site-specific ground motions using stochastic point-source program SMSIM based on past scenario earthquakes in this region and performing one-dimensional nonlinear ground response analysis (1D GRA) of a recently constructed flyover site of 2.3 km length at Agartala city. Further, the seismic vulnerability of the flyover structure incorporating the local site effect is also assessed herein. The results obtained are explained in terms of surface acceleration time history, the ratio of shear stress to effective vertical stress with depth, acceleration response spectrum, Fourier amplitude ratio with frequency, etc. 1D GRA presented contrasting results with higher values of acceleration in areas with stiffer soils due to attenuation of the seismic waves. In general, the amplification ratio (Af) evaluated from the present study indicated that the central portion of the flyover yielded high values and a high fundamental frequency value of 6.0 Hz, which might be detrimental for low-lying buildings. The generated synthetic motions of scenario earthquakes will be helpful for researchers and designers for earthquake resistant assessment or design of geotechnical structures in north-eastern India or similar sites in the absence of site-specific data. Finally, this study sheds important observations on seismic design guidelines of the structure located in this region, which may revamp the existing codal procedures.
... ELA is carried out to estimate the seismic wave propagation from bedrock to ground surface. The seismic wave propagation is estimated by the ground motion parameters using peak ground acceleration (PGA), 5% damped spectral acceleration (SA) and maximum strain (%) (Kramer 1996;Yoshida et al. 2001;Eker et al. 2015). The detail discussion of the ground motion parameters are given as follows. ...
Liquefaction is one of the geotechnical earthquake issues when a soil subjected to high strain due to dynamic loading and causes damage of the infrastructure. The main objective of this study is to predict the ground motion parameter and evaluate the potential of soil to liquefaction in Hawassa town. The site was characterized and classified based on the SPT-N values and average shear wave velocity (Vs30). The SPT-N value in Hawassa town ranges from 5-50. Whilst the Vs30 varies from 245-365m/s. The site class was C and D based on the NEHRP (2015) whereas B and C based on the EC8 (2003). The effects of the liquefaction are predicted using equivalent linear response analysis and liquefaction hazard evaluation methods at the five typical sites. Equivalent linear response analysis was used to determine local site effect in terms of PGA (g), SA (g) and maximum strain (%).Whereas the liquefaction hazard evaluations were done by liquefaction susceptibility criteria, factor of safety (FS), liquefaction potential index (LPI), probability of liquefaction (PL) and probability of liquefaction induced ground failure (PG). The predicted ground motion parameter shown: (i) PGA (g) values at surface ranges from 0.16g to 0.27g, (ii) the maximum strain (%) is less than 0.01 and (iii) the SA (g) vary from 0.7g to 1.2g with in the period interval of 0.1se and 1sec. The FS was less than unity for the typical one site and greater than unity for the other typical 4 sites. As a result, the liquefaction will be occurred only at the typical one site than other typical four sites for the given input motion. It is also evident that the typical one site has very high LPI, very likely to the PL and extremely high to PG than the other typical four sites. The findings from the present study will be helpful in retrofitting, analysis, design of structures and solving practical challenges in earthquake engineering.
... The MHVSR technique, which was first introduced by Nogoshi and Igarashi in 1971 and then popularized by Nakamura in 1989, is a passive geophysical method that has been widely utilized for seismic microzonation and estimation of the fundamental frequency of a site (Eker et al., 2015;Wood et al., 2019b). According to Nakamura's studies (Nakamura, 1989(Nakamura, , 2000(Nakamura, , 2019, the peak frequency from the MHVSR is approximately equal to the fundamental frequency of a site. ...
... The MHVSR method is a common passive geophysical method that has been widely utilized for seismic microzonation and estimation of fundamental site frequency (fr) (Eker et al., 2015;Wood et al., 2019b). This method is based on the analysis of the ratio between the amplitude of horizontal and vertical components (H/V) of microtremors or environmental noise. ...
This dissertation is aimed at understanding two aspects of active surface wave methods to improve the accuracy and reliability of this method. These include (1) the performance of four common wavefield transformation methods for the multichannel analysis of surface wave (MASW) data processing and (2) near-field effects. Toward this end, extensive field measurements were conducted considering different factors affecting these two topics. The MASW and microtremor horizontal to vertical spectral ratio (MHVSR) were then employed to examine their efficiency for infrastructure health monitoring.
Regarding the performance of the four common transformation techniques, it was observed that for sites with a very shallow and highly variable bedrock topography with a high-frequency point of curvature (>20 Hz), the Phase Shift (PS) method leads to a very poor-resolution dispersion image compared to other transformation methods. For sites with a velocity reversal, the Slant Stack (p) method fails to resolve the dispersion image for frequencies associated with layers located below the velocity reversal layer. Overall, the cylindrical frequency domain beamformer (FDBF-cylindrical) method was determined to be the best method under most site conditions. This method allows for a stable, high-resolution dispersion image for different sites and noise conditions over a wide range of frequencies, and it mitigates the near-field effects by modeling a cylindrical wavefield. However, the FDBF-cylindrical was observed to be dominated by higher modes at complex sites. Therefore, the best practice is to use more than one transformation method (FDBF-cylindrical and FK methods) to enhance the data quality.
Regarding the near-field effects for active surface wave methods, it was observed that near-field effects are independent of surface wave type (Rayleigh and Love) and depth to impedance contrast. For sites with a very shallow impedance contrast, the FDBF-cylindrical transformation technique outperforms others in terms of dispersion resolution by significantly mitigating near-field effects. It is also revealed that source type is an important parameter, influencing the normalized array center distance criteria required to mitigate near-field effects. The best practical criteria for near-field mitigation include a normalized array center distance of 1.0 or greater for low-output impulsive sources such as a sledgehammer and a normalized array center distance of 0.5 for high-output harmonic sources such as a vibroseis. These criteria should not be violated when using a limited number of source offsets (≤2). But, if the multiple source offset approach (≥3 source offsets) is used where some of the source offsets meet the criteria, the near-field criteria can be violated for other source offsets. Additionally, it is recommended to use the multiple source offset approach along with the FDBF-cylindrical for data processing to mitigate near-field effects.
For health monitoring of earthen hydraulic infrastructures, MASW was determined to be effective for detecting weak zones of such structures. In this regard, it is very important to use the reference shear wave velocity profiles to avoid misinterpretation of the results. Additionally, the grid pattern MHVSR method was determined to very effective for landslide evaluations for sites with shallow and complex bedrock topography, where bedrock is a key feature in the slope stability model.
... The MHVSR technique, which was first introduced by Nogoshi and Igarashi in 1971 and then popularized by Nakamura in 1989, is a passive geophysical method that has been widely utilized for seismic microzonation and estimation of the fundamental frequency of a site (Eker et al., 2015;Wood et al., 2019b). According to Nakamura's studies (Nakamura, 1989(Nakamura, , 2000(Nakamura, , 2019, the peak frequency from the MHVSR is approximately equal to the fundamental frequency of a site. ...
... The MHVSR method is a common passive geophysical method that has been widely utilized for seismic microzonation and estimation of fundamental site frequency (fr) (Eker et al., 2015;Wood et al., 2019b). This method is based on the analysis of the ratio between the amplitude of horizontal and vertical components (H/V) of microtremors or environmental noise. ...
This dissertation is aimed at understanding two aspects of active surface wave methods to improve the accuracy and reliability of this method. These include (1) the performance of four common wavefield transformation methods for the multichannel analysis of surface wave (MASW) data processing and (2) near-field effects. Toward this end, extensive field measurements were conducted considering different factors affecting these two topics. The MASW and microtremor horizontal to vertical spectral ratio (MHVSR) were then employed to examine their efficiency for infrastructure health monitoring.
Regarding the performance of the four common transformation techniques, it was observed that for sites with a very shallow and highly variable bedrock topography with a high-frequency point of curvature (>20 Hz), the Phase Shift (PS) method leads to a very poor-resolution dispersion image compared to other transformation methods. For sites with a velocity reversal, the Slant Stack (p) method fails to resolve the dispersion image for frequencies associated with layers located below the velocity reversal layer. Overall, the cylindrical frequency domain beamformer (FDBF-cylindrical) method was determined to be the best method under most site conditions. This method allows for a stable, high-resolution dispersion image for different sites and noise conditions over a wide range of frequencies, and it mitigates the near-field effects by modeling a cylindrical wavefield. However, the FDBF-cylindrical was observed to be dominated by higher modes at complex sites. Therefore, the best practice is to use more than one transformation method (FDBF-cylindrical and FK methods) to enhance the data quality.
Regarding the near-field effects for active surface wave methods, it was observed that near-field effects are independent of surface wave type (Rayleigh and Love) and depth to impedance contrast. For sites with a very shallow impedance contrast, the FDBF-cylindrical transformation technique outperforms others in terms of dispersion resolution by significantly mitigating near-field effects. It is also revealed that source type is an important parameter, influencing the normalized array center distance criteria required to mitigate near-field effects. The best practical criteria for near-field mitigation include a normalized array center distance of 1.0 or greater for low-output impulsive sources such as a sledgehammer and a normalized array center distance of 0.5 for high-output harmonic sources such as a vibroseis. These criteria should not be violated when using a limited number of source offsets (≤2). But, if the multiple source offset approach (≥3 source offsets) is used where some of the source offsets meet the criteria, the near-field criteria can be violated for other source offsets. Additionally, it is recommended to use the multiple source offset approach along with the FDBF-cylindrical for data processing to mitigate near-field effects.
For health monitoring of earthen hydraulic infrastructures, MASW was determined to be effective for detecting weak zones of such structures. In this regard, it is very important to use the reference shear wave velocity profiles to avoid misinterpretation of the results. Additionally, the grid pattern MHVSR method was determined to very effective for landslide evaluations for sites with shallow and complex bedrock topography, where bedrock is a key feature in the slope stability model.
... In this respect, the single station microtremor horizontal to vertical spectral ratio (MHVSR) (Nakamura 1989) method can be considered as a suitable complement or alternative to the resistivity and seismic methods for bedrock mapping. The MHVSR method is a common passive geophysical method that has been widely utilized for seismic microzonation and estimation of fundamental site frequency (fr) (Eker et al. 2015;Wood et al. 2019). This method is based on the analysis of the ratio between the amplitude of horizontal and vertical components (H/V) of microtremors or environmental noise. ...
Landslides with a shallow and complex bedrock layer, where bedrock topography affects the stability of the slide, are a widespread phenomenon. The current methods for evaluating such landslides include conventional in situ methods and array-based geophysical methods. However, these methods are not capable of characterizing the complete spatial extent of the bedrock layer cost-effectively and are difficult to conduct for steep slopes. Therefore, in this study, microtremor horizontal to vertical spectral ratio (MHVSR) is proposed as an effective tool when used in conjunction with other methods to improve our understanding of the landslide. In this study, the method is used to make a tight grid of MHVSR measurements within the landslide. Using this method, a 3D image of bedrock topography can be created over a larger spatial extent to reveal the potential critical landslide zones. This method is employed for two active landslides that have recently experienced considerable movements. Using the MHVSR, several bowl-shaped features were detected in the bedrock layer, which were not detected using conventional invasive in situ methods. These features play a key role in landslide behavior as they can trap water and create a fully saturated soft zone within the critical slide. Missing such key features in the geologic model of the landslide can lead to errors in the slope stability models and cost overruns in rehabilitation efforts. The grid pattern MHVSR method used in this study offers a simple, rapid, and cost-effective tool for landslide site characterization for sites with shallow and complex bedrock topography.
