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Application of the MHVSR method for determining the location of landslide areas before geotechnical project proposal: a case study of Tortum Lake, Turkey
Abstract
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.
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Many developed and developing countries use rail systems in addition to road systems in public transportation.
One of the most frequently used types of these rail systems is the light rail systems (LRS). The routes of the LRS are
generally preferred on the existing road route. This situation creates some problems in terms of ground conditions.
Since the rail system vehicle weights and passenger capacities are much more than road vehicles, the loads they
apply to the ground are also higher. However, deformations occurring on the road surface generally affect vehicles
and passengers in terms of comfort, while small deformations on rail systems can bring major problems such as
accidents and deaths. For this purpose, it is very important to investigate the ground structure of the rail system
route and to analyze the suitability situation in order to prevent these problems. In this study, the suitability of the
LRS route planned to be built in the city center of Erzurum in terms of physical properties of soil was analyzed. In
the analysis, it was suggested that some points were problematic in the results obtained by using the basic rotary drilling, multichannel analysis of surface waves (MASW), and microtremor methods and ground improvement or
revision of the route was suggested for these points. Where the ground structure is not suitable, an alternative route
has been created by taking passenger demand into consideration.
Information regarding the shape and depth of a landslide sliding surface (LSS) is fundamental for the estimation of the volume of the unstable masses, which in turn is of primary importance for the assessment of landslide magnitude and risk scenarios as well as in refining stability analyses. To assess an LSS is not an easy task and is generally time-consuming and expensive. In this work, a method existing in the literature, based on the inclination of movement vectors along a cross-section to estimate the depth and geometry LSSs, is used for the first time while exploiting satellite interferometric data. Given the advent of satellite interferometric data and the related increasing availability of spatially dense and accurate measurements, we test the effectiveness of this method-here named the vector inclination method (VIM)-to four case landslides located in Italy characterized by different types of movement, kinematics and volume. Geotechnical and geophysical information of the LSS is used to validate the method. Our results show that each of the presented cases provides useful insight into the validity of VIM using satellite interferometric data. The main advantages of VIM applied to satellite interferometry are that it enables estimation of the LSS with a theoretical worldwide coverage, as well as with no need for onsite instrumentation or even direct access; however, a good density of measurement points in both ascending and descending geometry is necessary. The combined use of VIM and traditional investigations can provide a more accurate LSS model.
Landslides recurrently impact the Italian territory, producing huge economic losses and casualties. Because of this, there is a large demand for monitoring tools to support landslide management strategies. Among the variety of remote sensing techniques, Interferometric Synthetic Aperture Radar (InSAR) has become one of the most widely applied for landslide studies. This work reviews a variety of InSAR-related applications for landslide studies in Italy. More than 250 papers were analyzed in this review. The first application dates back to 1999. The average production of InSAR-related papers for landslide studies is around 12 per year, with a peak of 37 papers in 2015. Almost 70% of the papers are written by authors in academia. InSAR is used (i) for landslide back analysis (3% of the papers); (ii) for landslide characterization (40% of the papers); (iii) as input for landslide models (7% of the papers); (iv) to update landslide inventories (15% of the papers); (v) for landslide mapping (32% of the papers), and (vi) for monitoring (3% of the papers). Sixty-eight percent of the authors validated the satellite results with ground information or other remote sensing data. Although well-known limitations exist, this bibliographic overview confirms that InSAR is a consolidated tool for many landslide-related applications.
Early warning systems (EWSs) to detect and monitor landslides are a great challenge. They are important due to the high cost of catastrophic landslides and are challenging because of the difficulty in identifying a diverse range of landslide-triggering factors. While there has been a very limited number of successes, recent advances in Earth observation (EO) from the ground, aircraft, and space have dramatically improved our ability to detect and monitor active landslides. A growing body of geotechnical theory suggests that prefailure behavior can offer clues to the location and timing of impending catastrophic failures. In this article, we use two recent landslides in China as case studies to demonstrate that satellite radar observations can be used to detect deformation precursors to catastrophic landslides and that early warnings can be achieved with real-time, in situ observations. We propose a novel and exciting framework that employs EO technologies to build an operational landslide EWS.
Intensification of agriculture leads to stress on the environment and subsequently can have strong societal and ecological impacts. In deserts, areas of very high sensitivity to land-use changes, these local-scale impacts are not well documented. On the arid southwestern coast of Peru, several vast irrigation programmes were developed in the 1950s on the flat detritic plateau surrounding narrow valleys to supply new farming areas. We document the long-term effects of irrigation on the erosion of arid deserts in the Vitor and Siguas valleys, south Peru, using 40 yr of satellite data. We demonstrate that irrigation initiated very large slow-moving landslides, affecting one-third of the valleys. Their kinematics present periods of quiescence and short periods of rapid activity, corresponding to landslide destabilization by their headscarp retrogression. This analysis suggests that the landslide motion continues long after their initiation by irrigations. Those landslides affect the fertile valley floors, leading to the destruction of villages and agricultural areas. We conclude that modern intensive farming can strongly impact traditional agriculture in desert areas where water management is particularly critical. Slow-moving landslides in two valleys in Peru were initiated by irrigation programmes in the region, suggest analyses of 40 years of satellite data.
Capacitively-Coupled Resistivity (CCR), Electrical Resistivity Tomography (ERT), and
seismic surface wave testing using Love and Rayleigh waves were performed on Kinion Lake
Dam, an embankment dam that has historically experienced significant seepage and internal
erosion issues. The goal of this study is to detect seepage locations in the embankment dam for
remediation purposes and map bedrock location across the area using nondestructive geophysical
measurements. Surveys were completed along the crest and downstream toe of the dam to map
subsurface conditions with a focus on seepage detection. The seismic surface wave data were
analyzed using two different methods: Multichannel Analysis of Surface Waves (MASW) and
Full Waveform Inversion (FWI). Both MASW and FWI methods were shown to be capable of
resolving the bedrock layer below the dam, but the FWI method, provided a higher resolution
image of the subsurface conditions compared to MASW method. However, the MASW and FWI
provided little guidance regarding the seepage path through the dam. The CCR and ERT surveys,
on the other hand, were shown to be very valuable for seepage detection. The seepage paths of
the embankment dam were detected by comparing the resistivity measurements acquired in a wet
winter and a dry summer. These seepage regions correspond well with previous sinkholes
observed in the pool area following a drought. Additionally, new drilling in the two regions
which showed differences in resistivity indicated the areas contained highly fractured limestone
with evidence of solution cavities. It seems that the highly fractured limestone becomes fully
saturated during the wet season causing very low resistivity values for these layers in wet
seasons, but it quickly loses the water during the dry season leading to a highly resistive material
in dry seasons. Therefore, seasonal resistivity measurements can be considered as a useful
method for locating potential weak zones of earthen hydraulic structures in a cost-effective and
timely manner.
The results of the continuous monitoring of ground deformation throughout the Tuscany region using radar images acquired by the Sentinel-1 satellite constellation of the European Space Agency (ESA) are presented here. This new monitoring approach, based on systematic imagery processing and analysis of deformation time series, is discussed at regional (for the entire Tuscany) and at local scale in the context of a case study of the Carpineta landslide, which is a large, active earth slide in the Northern Apennines (Pistoia province). The landslide registered an acceleration during the winter and spring of 2018 as a direct consequence of rainfall and snow melt. The increase in the deformation rate of the landslide, which led to the damage of several buildings, was promptly detected and monitored due to the enhanced temporal repetitiveness offered by the Sentinel-1 constellation. The results demonstrate that advances in satellite sensors, increases in computing capacity and the refinement of processing approaches and data screening tools can contribute to the development of new paradigms in satellite-based monitoring systems. Sentinel-1 data, which are systematically acquired with short revisiting times and then promptly processed, can now be used as a tool for the systematic tracking of ground deformation at the regional scale and for the continuous monitoring of slow and very slow landslides.
Landslide deformations involve approximately all geological materials (natural rocks, soil, artificial fill, or combinations of these materials) and can occur and develop in a large variety of volumes and shapes. The characterization of the material inhomogeneities and their properties, the study of the deformation processes, and the delimitation of boundaries and potential slip surfaces are not simple goals. Since the ‘70s, the international community (mainly geophysicists and lower geologists and geological engineers) has begun to employ, together with other techniques, geophysical methods to characterize and monitor landslides. Both the associated advantages and limitations have been highlighted over the years, and some drawbacks are still open. This review is focused on works of the last twelve years (2007-2018), and the main goal is to analyse the geophysical community efforts toward overcoming the geophysical technique limitations highlighted in the 2007 geophysics and landslide review. To achieve this aim, contrary to previous reviews that analysed the advantages and limitations of each technique using a “technique approach,” the analysis was carried out using a “material landslide approach” on the basis of the more recent landslides classification.
The investigation of large landslides in high alpine environments is often hindered by the difficult accessibility of the mountainous terrain. Efforts are typically concentrated on the remote measurement of the surface displacements, in order to define the general slope dynamics and identify phases of increasing activity. The characterization of such phenomena is challenging, due to their complex nature as well as the limitations of monitoring techniques. Appropriately integrating monitoring data from different sources can help reduce uncertainties, yet it is seldom done. In this paper, the outcomes of GNSS, satellite InSAR, and GBInSAR campaigns performed at the Bosmatto landslide (Northwestern Alps, Italy) are presented. The joint analysis provided a comprehensive view of the deformation field of the landslide, which revealed a gradually decreasing dip angle of the calculated movement vectors from head to toe. The instability was interpreted as a 2.5 – 3.5 ∗ 10 ⁶ m ³ rockslide, moving at peak velocities >50 mm/y according to a broadly roto-translational mechanism. The impact of the seasonal snow cover on the reliability of the interferometric acquisitions was also evaluated. Advantages and implications offered by the combination of multiple monitoring techniques are highlighted.
Landslides are one of the most relevant geohazards worldwide, causing direct and indirect costs and fatalities. Italy is one of the countries most affected by mass movements, and the Molise region, southern Italy, is known to be susceptible to erosional processes and landslides. In January 2003, a landslide in the municipality of Agnone, in the Colle Lapponi-Piano Ovetta (CL-PO) territory, occurred causing substantial damage to both structures and civil infrastructure. To investigate the evolution of the landslide-affected catchment over approximately one century, different data were taken into account: (i) literature information at the beginning of the twentieth century; (ii) historical sets of aerial optical photographs to analyse the geomorphological evolution from 1945 to 2003; (iii) SAR (Synthetic Aperture Radar) data from the ERS1/2, ENVISAT and COSMO-SkyMed satellites to monitor the displacement from 1992 to 2015; (iv) traditional measurements carried out through geological and geomorphological surveys, inclinometers and GPS campaigns to characterize the geological setting of the area; and (v) recent optical photographs of the catchment area to determine the enlargement of the landslide. Using the structure from motion technique, a 3D reconstruction of each set of historical aerial photographs was made to investigate the geomorphological evolution and to trace the boundary of the mass movements. As a result, the combination of multitemporal and multitechnique analysis of the evolution of the CL-PO landslide enabled an assessment of the landslide expansion, which resulted in a maximum length of up to approximately 1500 m. A complete investigation of the past and present deformational sequences of the area was performed to potentially plan further mitigation and prevention strategies to avoid possible reactivations.
The combined application of continuous GPS data (high temporal resolution) with spaceborne interferometric synthetic aperture radar (InSAR) data (high spatial resolution) can reveal much more about the complexity of large landslide movement than is possible with geodetic measurements tied to only a few specific measurement sites. This approach is applied to a ~4 km2 reactivated translational landslide in the Columbia River Gorge (Washington State), which moves mainly during the winter rainy season. Results reveal the complex three-dimensional shape of the landslide mass, how onset of sliding relates to cumulative rainfall, how surface velocity during sliding varies with location on the topographically complex landslide surface, and how the ground surface subsides slightly in weeks prior to downslope sliding.
A variety of methods (detailed geomorphological surveys, geotechnical investigations, local instrumentation, satellite data, and radar interferometry) along with geophysical techniques may be used to investigate slope instabilities and to detect the inhomogeneities of materials as well as their properties, boundaries, and sliding surfaces. Of these techniques, the method based on seismic noise measurements allows abrupt changes in seismic impedance at landslide boundaries resulting from varying levels of seismic velocity and material density to be detected. Peaks of the Horizontal to Vertical Spectral Ratio have proven to serve as effective indicators of the resonance frequency of low-impedance surface layers. In this work, horizontal to vertical spectral ratio surveys of the Castagnola (La Spezia, Italy) and Roccalbegna (Grosseto, Italy) landslides were carried out. From roughly 100 single-station measurements made inside and outside the landslides at each site, we define a threshold number of single-station seismic noise measures beyond which information is redundant because the variation in reconstructed impedance contrast surfaces is not significant. This approach allows one to reliably retrieve the geometry of a landslide body, ultimately generating useful information for determining whether further measurements are needed to improve landslide body reconstruction.
The Daguangbao mega-landslide (China), induced by the 2008 Wenchuan earthquake (Mw = 7.9), with an area of approximately 8 km2, is one of the largest landslides in the world. Experts predicted that the potential risk and instability of the landslide might remain for many decades, or even longer. Monitoring the activity of such a large landslide is hence critical. Terrain Observation by Progressive Scans (TOPS) mode from the Sentinel-1 satellite provides us with up-to-date high-quality Synthetic Aperture Radar (SAR) images over a wide ground coverage (250 × 250 km), enabling full exploitation of various InSAR applications. However, the TOPS mode introduces azimuth-dependent Doppler variations to radar signals, which requires an additional processing step especially for SAR interferometry. Sentinel-1 TOPS data have been widely applied to earthquakes, but the performance of TOPS data-based time series analysis requires further exploitation. In this study, Sentinel-1 TOPS data were employed to investigate landslide post-seismic activities for the first time. To deal with the azimuth-dependent Doppler variations, a processing chain of TOPS time series interferometry approach was developed. Since the Daguangbao landslide is as a result of the collapse of a whole mountain caused by the 2008 Mw 7.9 Wenchuan earthquake, the existing Digital Elevation Models (DEMs, e.g. SRTM and ASTER) exhibit height differences of up to approximately 500 m. Tandem-X images acquired after the earthquake were used to generate a high resolution post-seismic DEM. The high gradient topographic errors of the SRTM DEM (i.e. the differences between the pre-seismic SRTM and the actual post-seismic elevation), together with low coherence in mountainous areas make it difficult to derive a precise DEM using the traditional InSAR processing procedure. A re-flattening iterative method was hence developed to generate a precise TanDEM-X DEM in this study. The volume of the coseismic Daguangbao landslide was estimated to be of 1.189 ± 0.110 × 109 m3 by comparing the postseismic Tandem-X DEM with the preseismic SRTM DEM, which is consistent with the engineering geological survey result. The time-series results from Sentinel-1 show that some sectors of the Daguangbao landslide are still active (and displaying four sliding zones) and exhibiting a maximum displacement rate of 8 cm/year, even eight years after the Wenchuan earthquake. The good performance of TOPS in this time series analysis indicates that up-to-date high-quality TOPS data with spatiotemporal baselines offer significant potential in terms of future InSAR applications.
The paper reviews the applicability of microtremor measurments to evaluate site response of soft soils. Three different techniques commonly used to estimate site effects from microtremor measurements were evaluated: interpretation of Fourier amplitude spectra, computation of spectral ratios relative to a firm reference station, and computation of spectral ratios of horizontal components relative to the vertical component of ground motion (Nakamura's technique). These techniques are applied to microtremor records obtained in three cities in Mexico: Mexico City, Oaxaca, and Acapulco. The results strongly suggest that the techique by Nakamura effectively compensates for source effects in microtremor measurements, which eliminates a major limitation to their application in earthquake engineering. -from Authors
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 microtremor survey was implemented to identify the local site response under a possible seismic event. The results were compared and correlated by using passive and active surface wave measurements, engineering geological and geotechnical deep borehole logs along with local geology and geological tectonic setting. Regarding the microtremor results, larger fundamental periods were acquired than expected over the area and the effect of the tectonic deformation on the stiffness of the soils was also observed at either the H/V curves or the shear wave velocity profiles. The results have demonstrated that geotechnical information down to a depth of 30 m may not be compared with the H/V parameters. The H/V amplitudes were not always accompanied with the higher periods in the Quaternary sediments. This could be due to the basin and basin edge effects observed particularly at the center and boundary of the basin, respectively. Additionally, the microtremor survey showed that the spectral ratio amplitudes derived by the H/V ratio is debatable and it is not a direct indicator of local soil behavior under a seismic excitation.
Early Warning Systems (EWS) are efficient tools for preventing and mitigating the risks associated to landslides occurrence. In this paper, an integrated methodology for landslides’ analysis is presented and described. Such methodology is aimed at the creation of early warning systems and is based on the integration between a modern monitoring technique, such as the Ground-Based Interferometric Synthetic Aperture Radar (GBInSAR), along with advanced numerical modelling. The paper also shows the application of the proposed methodology to the case study of a rockslide in central Italy. The integration between monitoring data, thanks to a GBInSAR survey and advanced numerical simulations with the combined Finite-Discrete Elements Method (FDEM), allowed for the definition of a set of surface velocity thresholds to be adopted for the long-term monitoring of the landslide and for the creation of an effective EWS.
This paper introduces a novel modular approach for the monitoring of desiccation-induced deterioration in earthen embankments (levees), which are typically used as flood-defense structures. The approach is based on the use of a combination of geotechnical and noninvasive geophysical probes for the continuous monitoring of the water content in the ground. The level of accuracy of the monitoring is adaptable to the available financial resources. The proposed methodology was used and validated on a recently built, 2-km-long river embankment in Galston (Scotland, United Kingdom). A suite of geotechnical probes was installed to monitor the seasonal variation of water content over a 2-year period. Most devices were calibrated in situ. A novel procedure to extrapolate the value of water content from the geotechnical and geophysical probes at any point of the embankment is shown. Desiccation fissuring degrades the resistance of embankments against several failure mechanisms. An index of susceptibility is proposed here. The index is a useful tool to assess the health state of the structure and prioritize remedial interventions. (C) 2014 American Society of Civil Engineers.
The Huangtupo landslide is one of the largest in the Three Gorges region, China. The county-seat town of Badong, located on the south shore between the Xiling and Wu gorges of the Yangtze River, was moved to this unstable slope prior to the construction of the Three Gorges Project, since the new Three Gorges reservoir completely submerged the location of the old city. The instability of the slope is affecting the new town by causing residential safety problems. The Huangtupo landslide provides scientists an opportunity to understand landslide response to fluctuating river water level and heavy rainfall episodes, which is essential to decide upon appropriate remediation measures. Interferometric Synthetic Aperture Radar (InSAR) techniques provide a very useful tool for the study of superficial and spatially variable displacement phenomena. In this paper, three sets of radar data have been processed to investigate the Huangtupo landslide. Results show that maximum displacements are affecting the northwest zone of the slope corresponding to Riverside slumping mass I#. The other main landslide bodies (i.e. Riverside slumping mass II#, Substation landslide and Garden Spot landslide) exhibit a stable behaviour in agreement with in situ data, although some active areas have been recognized in the foot of the Substation landslide and Garden Spot landslide. InSAR has allowed us to study the kinematic behaviour of the landslide and to identify its active boundaries. Furthermore, the analysis of the InSAR displacement time-series has helped recognize the different displacement patterns on the slope and their relationships with various triggering factors. For those persistent scatterers, which exhibit long-term displacements, they can be decomposed into a creep model (controlled by geological conditions) and a superimposed recoverable term (dependent on external factors), which appears closely correlated with reservoir water level changes close to the river's edge. These results, combined with in situ data, provide a comprehensive analysis of the Huangtupo landslide, which is essential for its management.
Rockslides in alpine areas can reach large volumes and, owing to their position along slopes, can either undergo large and rapid evolution originating large rock avalanches or can decelerate and stabilize. As a consequence, in particular when located within large deep-seated deformations, this type of instability requires accurate observation and monitoring. In this paper, the case study of the La Saxe rockslide (ca. 8 × 106 m3), located within a deep-seated deformation, undergoing a major phase of acceleration in the last decade and exposing the valley bottom to a high risk, is discussed. To reach a more complete understanding of the process, in the last 3 years, an intense investigation program has been developed. Boreholes have been drilled, logged, and instrumented (open-pipe piezometers, borehole wire extensometers, inclinometric casings) to assess the landslide volume, the rate of displacement at depth, and the water pressure. Displacement monitoring has been undertaken with optical targets, a GPS network, a ground-based interferometer, and four differential multi-parametric borehole probes. A clear seasonal acceleration is observed related to snow melting periods. Deep displacements are clearly localized at specific depths. The analysis of the piezometric and snowmelt data and the calibration of a 1D block model allows the forecast of the expected displacements. To this purpose, a 1D pseudo-dynamic visco-plastic approach, based on Perzyna’s theory, has been developed. The viscous nucleus has been assumed to be bi-linear: in one case, irreversible deformations develop uniquely for positive yield function values; in a more general case, visco-plastic deformations develop even for negative values. The model has been calibrated and subsequently validated on a long temporal series of monitoring data, and it seems reliable for simulating the in situ data. A 3D simplified approach is suggested by subdividing the landslide mass into distinct interacting blocks.
Abstract The dimensions of landslides are subject to geometric, material and external controls. Geometric controls include: slopes and orientation of the ground surface and bedding planes, valley width and geometry, depth of failure, plus the presence and incision depth of bounding streams. Material controls include undrained and remoulded shear strength, sensitivity, stability number, remoulding index, rapidity, and the mobility of the depleted mass. Dense networks of trees can influence mobility. The position of layers prone to landsliding in the slope is important.
Landslide phenomena involve the northern coast of Malta, affecting in particular the urban area of Xemxija.
Limestones overlying a clayey formation represent the shallower lithotypes that characterize the surficial geology of this area, where lateral spreading phenomena and rockfalls take place. Ambient noise records, processed through spectral ratio techniques, were analysed in order to characterize the dynamic behavior of the rock masses affected by the presence of fractures linked to the landslide body existing in the area. Experimental spectral ratios were also calculated after rotating the horizontal components of the seismic signal, and a direct estimate of the polarization angle was also performed in order to investigate the existence of directional effects in the ground motion. The results of the morphologic survey confirmed the existence of large cliff-parallel fractures that cause cliff-edge and unstable boulder collapses. Such phenomena appear connected to the presence, inside the clay formation, of a sliding surface that was identified through the interpretation of the noise measurement data. The boundaries of the landslide area
appear quite well defined by the pronounced polarization effects, trending in the northeastern direction, observed in the fractured zone and in the landslide body in particular.
The frequency-dependent properties of Rayleigh-type surface waves can be utilized for imaging and characterizing the shallow subsurface, Most surface-wave analysis relies on the accurate calculation of phase velocities for the horizontally traveling fundamental-mode Rayleigh wave acquired by stepping out a pair of receivers at intervals based on calculated ground roll wavelengths. Interference by coherent source-generated noise inhibits the reliability of shear-wave velocities determined through inversion of the whole wave field. Among these nonplanar, nonfundamental-mode Rayleigh waves (noise) are body waves, scattered and nonsource-generated surface waves, and higher-mode surface waves. The degree to which each of these types of noise contaminates the dispersion curve and, ultimately, the inverted shear-wave velocity profile is dependent on frequency as well as distance from the source. Multichannel recording permits effective identification and isolation of noise according to distinctive trace-to-trace coherency in arrival time and amplitude. An added advantage is the speed and redundancy of the measurement process. Decomposition of a multichannel record into a time variable-frequency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency component in a unique and continuous format. Coherent noise contamination can then be examined and its effects appraised in both frequency and offset space. Separation of frequency components permits real-time maximization of the SM ratio during acquisition and subsequent processing steps. Linear separation of each ground roll frequency component allows calculation of phase velocities by simply measuring the linear slope of each frequency component. Breaks in coherent surface-wave arrivals, observable on the decomposed record, can be compensated fur during acquisition and processing. Multichannel recording permits single-measurement surveying of a broad depth range, high levels of redundancy with a single field configuration, and the ability to adjust the offset, effectively reducing random or nonlinear noise introduced during recording. A multichannel shot gather decomposed into a swept-frequency record allows the fast generation of an accurate dispersion curve. The accuracy of dispersion curves determined using this method is proven through field comparisons of the inverted shear-wave velocity (v(s)) profile with a downhole v(s) profile.
1] The Portuguese Bend landslide, located on the Palos Verdes peninsula in southern California, is a slow‐moving landslide whose activity is closely tied to infiltration from rainfall. We use radar interferometry to constrain the seasonal kinematics of the landslide from 1995 to 2000. Traditional interferometric synthetic aperture radar methods estimate an average downslope displacement rate of 1.00 m/yr with an uncertainty range of 0.39–2.57 m/yr during the summer months. Interferograms show a consistent loss in phase coherence over the landslide for epochs greater than 6 months, especially in the winter. Interferograms become incoherent as a result of large displacement, and decorrelation is used to map the spatial extent of slide movement through time. The displacement rate increases sharply several weeks after the beginning of the rainy season as rainwater percolating into the slide elevates pore pressures. Phase coherence is lost when the slide velocity surpasses ∼2.6 m/yr for interferograms with durations of a few months. A physically based model based on rainfall‐driven landslide movement incorporates these observations to constrain hydraulic diffusivity to between 0.9 and 3.5 × 10 −6 m 2 /yr.
Rail systems have an important place among the types of transportation. Although it was preferred for intercity transportation in the past, but nowadays it is frequently preferred in urban roads. Some rail system structures move from the ground surface, while others move under the ground. Various geotechnical researches have been carried out for rail
systems moving under the ground. However, for the rail vehicles moving on the ground surface are generally placed on the highway route, ground parameters are not taken into consideration. This situation can cause serious rail system accidents.
This study has been conducted in Turkey's Erzurum drilling planned light rail system in terms of soil properties, it was evaluated by survey results of drilling borehole, microtremor, and multichannel analysis of surface waves (MASW).
According to the results of this study, a part of the light rail system (LRS) route was found to be insufficient in terms of ground safety. For this reason, improvement in the ground or revision of the route has been suggested.
On 25 and 29 January 2019, a large landslide destroyed an important part of the town of Pomarico (Basilicata Region, Southern Italy). Geological and geomorphological investigations provided a detailed description of the landslide features. Several geophysical surveys were carried out to deepen knowledge of the landslide and the residual risk assessment. Detailed electrical resistivity tomography (ERT), multichannel analysis of surface waves (MASW), and seismic refraction tomography (SRT) have been used to analyze geomorphological evidences of the failure and the potential kinematic evolution of the landslide scarp, a crucial factor to assess landslide residual risk. The joint analyses of the geophysical results, compared with geological and geomorphological data, allowed to obtain detailed information about the stratigraphic contact between clayey and sandy deposits in the crown area of the landslide, and to identify the post-failure stability condition changes in sands. The geophysical analyses confirmed the presence of multiple old degraded scarps developed over time and provided information on the decompression state of the different areas of the landslide crown. The results highlighted a subparallel stratification consisting of an anthropic surface carry-over material, which covers a layer of sands with silty intercalations, overlying clayey material that represents the bedrock of the investigated area. Furthermore, natural and anthropogenic caves, mainly developed in well-cemented layers of sands, were identified. This study emphasized how the integration of different geophysical methods constitutes a capable tool for characterizing landslides, contributing to assessing the landslide residual risk of the slope mass and evaluating the suitability of the methods in relation to the investigated landslide conditions.
This study examines the influence of soil aging on Standard Penetration Test (SPT) N-Vs correlations, and seismic site classification. Energy-corrected SPT-Vs correlations are developed for two regions where subsurface layering consists of soil deposits with very young geotechnical ages according to the last critical soil disturbance that occurred in the study areas. It is observed that the uncorrected correlations of the current study fall into the lower bound of the uncorrected SPT-Vs correlations existing in the literature due to soil aging effects. To better illustrate the effects of soil aging on SPT-Vs correlations, the uncorrected correlations from the literature and this study are divided into two age categories of Holocene and Pleistocene according to their geological age. Using the two age categories, the boundaries proposed in the NEHRP for different seismic site classes are divided into three zones according to their uncertainty level regarding seismic site classification. The results clearly indicate the need to consider the age of soil deposits for seismic site classification when using the SPT method to avoid incorrect seismic site classification. Additionally, it is found that the use of raw (uncorrected) SPT N values can lead to incorrect seismic site classification, with either conservative or unconservative results depending on the Energy Transfer Ratio (ETR) of the SPT hammer used for the seismic site class determination.
Heavy precipitation accompanied by a typhoon and a seasonal rain front fell over the main island of Japan on 9–11 September 2015, causing severe damage due to landslides and river flooding. In the Kanto and Tohoku regions, some automated meteorological data acquisition system observatories recorded high precipitation rates; rainfall on the first 2 days of the disaster (9–10 September) was more than twice the average monthly rainfall in September 1981–2010. The rainfall, known as the Kanto-Tohoku heavy rain, triggered shallow landslides on a relatively gentle hillslope behind some residences in Kanuma city, Tochigi Prefecture. The source areas of the landslides appeared neither along a clear longitudinal hollow nor around a transverse concave. To reveal the geophysical features affecting the induction of shallow landslides, seismic and electrical resistivity surveys were conducted. Portable cone penetration tests and soil sampling were also carried out to determine the physical properties of the soil. For surface geophysical surveys, a longitudinal survey line was situated on the left bank of a shallow landslide to measure the surface geophysical properties of the landslide area, based on the assumption that the slope next to the landslide has the same properties as the landslide area. Also, two transverse survey lines were situated just upslope of two shallow landslides to capture lateral variations in surface geophysical properties between the landslide and the area around the landslide. Topsoil in the source areas exhibited lower S-wave velocity, higher electrical resistivity, greater soil depth, and lower dry bulk density than soil along other parts of the survey line. These features indicate that the topsoil in the source area was composed of loose relatively deep sand and could be considered to be potentially susceptible to rainfall-induced landslides. Thus, combining surface geophysical surveys with other methods, such as airborne surveys and satellite observations, to detect landslide-prone zones in vast areas may effectively locate areas that do not initially appear different from their surroundings but are high risk of landslides.
Dynamic site characterization was conducted using the microtremor and earthquake horizontal to vertical spectral ratio methods (MHVSR and EHVSR) and surface wave methods at locations affected by the 2017 Mw 7.1 Puebla-Mexico City Earthquake. These data were compared to published results to understand the accuracy of currently available dynamic site information including site period maps and Vs profiles. Results indicate the current Mexican design code site period map, NTC [9], provides superior estimates of microtremor site periods in Mexico City compared to maps published by Lermo and Chávez-García [7] and Arroyo et al. [11]. However, the NTC [9] map still had significant errors of 15–30% between the estimated and measured site periods, but only had an average bias of 5%. Moreover, the changes in site period within the Mexico basin due to groundwater withdrawal and consolidation of the lacustrine clay deposits were shown to be progressing faster than originally estimated by Arroyo et al. [11].
In Peru landslides have been causing damages and casualties annually due to the high mountain relief and distinct seasonal precipitation distribution. Satellite Synthetic Aperture Radar (SAR) interferometry represents one possibility for mapping surface deformation at fine spatial resolution over large areas in order to characterize aspects of terrain motion and potentially hazardous processes. We present land surface motion maps derived from satellite SAR interferometry (InSAR) for a part of the Santa River Basin between the Cordilleras Blanca and Negra around the city of Carhuaz in Peru. Using both Persistent Scatterer Interferometry (PSI) and differential SAR Interferograms (DInSAR) from ALOS-1 PALSAR-1, ENVISAT ASAR, ALOS-2 PALSAR-2 and Sentinel-1 we mapped 42 landslides extending over 17,190,141m² within three classes of activity (i.e. 0–2 cm/a, 2–10 cm/a and>10 cm/a). A geomorphological inventory of landslides was prepared from optical satellite imagery and field experience and compared to the InSAR-based slope-instability inventory. The two approaches provide slightly different information about landslide spatial and temporal activity patterns, but altogether they can be combined for the assessment of the state of activity of landslides and possibly the development of hazard maps, which are not systematically available in this region. We conclude that ALOS PALSAR (1 and 2) and Sentinel-1 data have a high potential to derive high-quality surface deformation information of landslides in many mountainous regions worldwide due to their global and frequent acquisition strategies.
Large landslides and deep-seated gravitational slope deformations (DSGSD) represent an important geo-hazard in relation to the deformation of large structures and infrastructures and to the associated secondary landslides. DSGSD movements, although slow (from a few millimetres to several centimetres per year), can continue for very long periods, producing large cumulative displacements and undergoing partial or complete reactivation. Therefore, it is important to map the activity of such phenomena at a regional scale. Ground surface displacements at DSGSD typically range close to the detection limit of monitoring equipment but are suitable for synthetic aperture radar (SAR) interferometry. In this paper, permanent scatterers (PSInSAR™) and SqueeSAR™ techniques are used to analyse the activity of 133 DSGSD, in the Central Italian Alps. Statistical indicators for assigning a degree of activity to slope movements from displacement rates are discussed together with methods for analysing the movement and activity distribution within each landslide. In order to assess if a landslide is active or not, with a certain degree of reliability, three indicators are considered as optimal: the mean displacement rate, the activity index (ratio of active PS, displacement rate larger than standard deviation, overall PS) and the nearest neighbor ratio, which allows to describe the degree of clustering of the PS data. According to these criteria, 66% of the phenomena are classified as active in the monitored period 1992–2009. Finally, a new methodology for the use of SAR interferometry data to attain a classification of landslide kinematic behaviour is presented. This methodology is based on the interpretation of longitudinal ground surface displacement rate profiles in the light of numerical simulations of simplified failure geometries. The most common kinematic behaviour is rotational, amounting to 41 DSGSDs, corresponding to the 62.1% of the active phenomena.
In order to provide surface geodetic measurements with “landslide-wide” spatial coverage we develop and validate a method for the characterization of 3D surface deformation using the unique capabilities of the UAVSAR airborne repeat-pass radar interferometry system. We apply our method at the well-studied Slumgullion Landslide, which is 3.9 km long and moves persistently at rates up to ∼2 cm/day. A comparison with concurrent GPS measurements validates this method and shows that it provides reliable and accurate 3D surface deformation measurements. The UAVSAR-derived vector velocity field measurements accurately capture the sharp boundaries defining previously identified kinematic units and geomorphic domains within the landslide. We acquired data across the landslide during spring and summer and identify that the landslide moves more slowly during summer except at its head, presumably in response to spatiotemporal variations in snowmelt infiltration. In order to constrain the mechanics controlling landslide motion from surface velocity measurements, we present an inversion framework for the extraction of slide thickness and basal geometry from dense 3D surface velocity fields. We find that that the average depth of the Slumgullion Landslide is 7.5 meters, several meters less than previous depth estimates. We show that by considering a visco-plastic rheology we can derive tighter theoretical bounds on the rheological parameter relating mean horizontal flow rate to surface velocity. Using inclinometer data for slow-moving, clay-rich landslides across the globe we find a consistent value for the rheological parameter of 0.85 ± 0.08.
This paper relates landslide inventories to erosion rates and provides quantitative estimates of the landslide hazard associated with earthquakes. We do this by utilizing a three-parameter inverse-gamma distribution, which fits the frequency– area statistics of three substantially dcompleteT landslide-event inventories. A consequence of this general distribution is that a landslide-event magnitude m L =logN LT can be introduced, where N LT is the total number of landslides associated with the landslide event. Using this general distribution, landslide-event magnitudes m L can be obtained from incomplete landslide inventories, and the total area and volume of associated landslides, as well as the area and volume of the maximum landslides, can be directly related to the landslide-event magnitude. Using estimated recurrence intervals for three landslide events and the time span for two historical inventories, we estimate regional erosion rates associated with landslides as typically 0.1–2.5 mm year À1. We next give an empirical correlation between the earthquake magnitude, associated landslide-event magnitude, and the total volume of associated landslides. Using these correlations, we estimate that the minimum earthquake magnitudes that will generate landslides is M=4.3F0.4. Finally, using Gutenberg–Richter frequency-magnitude statistics for regional seismicity, we relate the intensity of seismicity in an area and the magnitude of the largest regional earthquakes to erosion rates. We find that typical seismically induced erosion rates in active subduction zones are 0.2–7 mm year À1 and adjacent to plate boundary strike-slip fault zones are 0.01–0.7 mm year À1 .
The spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity. The purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station. The technique is presented briefly and it is discussed why it may be applicable to study the intense S-wave part in earthquake records. Results are presented for three different cities in Mexico: Oaxaca, Oax., Acapulco, Gro., and Mexico City. -from Authors
The observations about the behavior of microtremor spectra presented here show that noise measurements can be used as a powerful tool to determine the thickness of soft cover layers. The most suitable method for this determination is Nakamura's technique, which is the ratio of the horizontal-component noise spectrum and that of the vertical component (H/V spectrum). The frequency of the main peak in these spectral ratios correlates well with the sediment thickness at the site. Using an extensive database of microtremor measurements carried out in the western Lower Rhine Embayment (Germany), it was possible to show that this correlation is clearly valid for a wide range of thickness, namely, from tens of meters to more than 1000 m. A simple formula was derived that, for the sediments to be found in the area investigated, directly calculates the cover thickness from the frequency of the main peak in the H/V spectrum. A comparison with calculated resonant frequencies suggests the relation derived from the noise measurements depending on the velocity depth function of the shear wave. Classical spectral ratios are shown to be strongly influenced by the noise level and are therefore less reliable in determining the resonant frequency of the subsoil. The practical relevance of the investigation is illustrated by means of cross sections, constructed from results of the microtremor analyses, which provide a convincing image of the surficial structure of the areas investigated.
Earthquake induced slope failures are responsible for a significant amount of life loss and damage, and their effective mitigation requires further advancements in our comprehension of slope behaviour under seismic shaking. One source of uncertainty in seismic landslide susceptibility assessment is the phenomenon of enhanced amplification of ground motion along down slope directions. This implies a strength demand beyond that estimated by standard slope stability analysis. An extensive accelerometer monitoring of slope dynamic response in areas exposed to seismic landslide hazard is unfeasible. An alternative approach can take advantage of recent development of reconnaissance techniques based on the analysis of ambient noise recorded by portable instruments. The most popular technique, known as Nakamura or HVNR method, consists in analysing H/V spectral ratios between Horizontal and Vertical components of Noise Recording, and allows the recognition of site resonance frequencies. The application of HVNR to complex site conditions typical of marginally stable slopes is often difficult and requires the development of “ad hoc” procedures both for acquisition and analysis of noise recording. Tests in different geologic and geomorphic settings show that an analysis of azimuthal variation of spectral ratios can reveal the presence and orientation of directional resonance, whereas the recognition of main resonance frequencies requires a proper selection of signals to be analysed. Efforts to evaluate amplification factors currently rely on numerical simulations, which in turn require S-wave velocity of slope materials. Ambient noise analysis in terms of velocity models can contribute through the inversion of H/V spectral ratios and surface wave velocity dispersion curves derived from the processing of multiple simultaneous noise recordings. However these applications require a correct identification of the nature of surface waves present in the noise recording.
1] Quantifying the velocity, volume, and rheology of deep, slow-moving landslides is essential for hazard prediction and understanding landscape evolution, but existing field-based methods are difficult or impossible to implement at remote sites. Here we present a novel and widely applicable method for constraining landslide 3-D deformation and thickness by inverting surface change data from repeat stereo imagery. Our analysis of La Clapière, an ~1 km 2 bedrock landslide, reveals a concave-up failure surface with considerable roughness over length scales of tens of meters. Calibrating the thickness model with independent, local thickness measurements, we find a maximum thickness of 163 m and a rheology consistent with distributed deformation of the highly fractured landslide material, rather than sliding of an intact, rigid block. The technique is generally applicable to any mass movements that can be monitored by active or historic remote sensing.
Landslides in sensitive clays represent a major hazard in the northern countries of the world such as Canada, Finland, Norway, Russia, Sweden and in the US state of Alaska. Examples of catastrophic landslides in sensitive clays that impacted populations are numerous: e.g., Saint-Jean-Vianney in 1971 (Tavenas et al. 1971; Potvin et al. 2001), Rissa in 1979 (Gregersen 1981; L’Heureux et al. 2012), Finneidfjord in 1996 (Longva et al. 2003), Kattmarka in 2009 (Nordal et al. 2009) and St-Jude in 2010 (Locat et al. 2012). In order to respond to the societal demands, the scientific community has to expand its knowledge of landslide mechanisms in sensitive clay to assist authorities with state-of-the-art investigation techniques, hazard assessment methods, risk management schemes, mitigation measures and planning.
Many studies show that certain geophysical methods, such as seismic and electrical-resistivity imaging, appear to be well adapted for investigating the internal structures of landslides and understanding the related hydro-mechanical mechanisms. These are methods that allow the direct and non-intrusive measurement of acoustic (P) and shear (S) wave velocities and electrical resistivity (ρ), which are three physical parameters considered as essential for estimating the mechanical properties of moving reworked material. We applied these techniques to the La Valette landslide (Southern French Alps), a typical example of an intra-material landslide, carrying out measurements simultaneously along two profiles, 400m and 300m long and respectively perpendicular to and along the slide direction. We then used suitable inversion algorithms to estimate both the P- and S-wave velocity fields and the electrical resistivity field from the recorded data. The results, aided by field surface observations, show that a correlation exists between the state of the material and the seismic-velocity and/or electrical-resistivity data, thus confirming that the simultaneous use of the two methods provides complementary information on the geomechanical behavior of the landslide. More particularly, the seismic data provide information on fissure density variations and the presence of shear-bent material, whereas the electrical resistivity data provide information on the groundwater content. To enable a more integrated petrophysical interpretation, we applied a data-fusion strategy based on fuzzy subsets to the geophysical datasets. Through combining the tomograms we identified a surface layer of soft material along the two profiles; the bottom of this layer was also recognized in a borehole. From a methodological point of view, the results show the applicability of adopting geomechanical hypotheses as inputs of geophysical data fusion for identifying areas where sediment mobilization could occur.
A field experiment with small aperture seismic arrays was performed on the unstable rock slope above the village of Randa in the southern Swiss Alps. The aim of this experiment was to constrain the seismic response of a potential future rockslide using ambient vibration recordings. Weak seismic events were identified on the recordings and site-to-reference spectral ratios were calculated using a reference site located on the stable part of the slope. Spectral ratios of up to 30 were observed at sites located within the unstable portion of the slope. A strong variation of spectral ratios with azimuth indicates a directional site effect. Neither amplification nor directionality were observed at sites located in the stable part of the slope. Furthermore, time-frequency polarization analysis of the ambient noise was performed to provide robust estimates of frequency dependent directions of the maximum polarization. It was found that the unstable part of the slope vibrates within a narrow range of directions (130 +/- 10°) for the frequency range centred around 5 Hz. The polarization directions estimated from ambient seismic vibrations are in good agreement with the deformation directions obtained by geodetic and in situ measurements. No directionality of ambient vibrations was observed at sites within the stable part of the slope.
ABSTRACT The frequency-dependent properties of Rayleigh-type surface waves can be utilized for imaging,and character- izing the shallow subsurface. Most surface-wave analysis relies on the accurate calculation of phase velocities for the horizontally traveling fundamental-mode,Rayleigh wave acquired,by stepping out a pair of receivers at inter- vals based on calculated ground,roll wavelengths. Inter- ference by coherent,source-generated,noise inhibits the reliability of shear-wave velocities determined,through inversion of the whole wave,field. Among,these nonpla- nar, nonfundamental-mode Rayleigh waves (noise) are body waves, scattered and nonsource-generated surface waves, and higher-mode surface waves. The degree to which each of these types of noise contaminates,the dis- persion curve and, ultimately, the inverted shear-wave velocity profile is dependent,on frequency,as well as dis- tance from the source. Multichannel,recording,permits,effective identifica- tion and isolation of noise according,to distinctive trace- to-trace coherency,in arrival time and,amplitude. An added,advantage,is the speed,and,redundancy,of the measurement,process. Decomposition,of a multichannel record into a time variable-frequency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency,component,in a unique,and continuous,format. Coherent,noise contamination,can then be examined,and its effects appraised,in both fre- quency,and offset space. Separation of frequency,com- ponents,permits real-time maximization,of the S/N ratio during acquisition and subsequent,processing steps. Linear separation of each ground,roll frequency,com- ponent,allows calculation of phase velocities by simply
As methods for dynamic characteristics estimation of surface layers, investigation of boreholes and a method which employs microtremors are well known. Borehole investigation, one of the most accurate methods, is costly and time-consuming and is not available all the time. The method that employs microtremors is handy but has not produced satisfactory results to this day. This paper describes a new processing method that employs microtremor observations yet produces accurate estimates of the characteristics of the ground motion. The method uses a vertical component and horizontal components. As a result, the spectrum ratio of the horizontal components and the vertical component of the microtremors bears a resemblance to the transfer function for the horizontal motion of the surface layers.
Landsliding in response to rainfall involves physical processes that operate on disparate timescales. Relationships between these timescales guide development of a mathematical model that uses reduced forms of Richards equation to evaluate effects of rainfall infiltration on landslide occurrence, timing, depth, and acceleration in diverse situations. The longest pertinent timescale is A/D 0 , where D 0 is the maximum hydraulic diffusivity of the soil and A is the catchment area that potentially affects groundwater pressures at a prospective landslide slip surface location with areal coordinates x, y and depth H. Times greater than A/D 0 are necessary for establishment of steady background water pressures that develop at (x, y, H) in response to rainfall averaged over periods that commonly range from days to many decades. These steady groundwater pressures influence the propensity for landsliding at (x, y, H), but they do not trigger slope failure. Failure results from rainfall over a typically shorter timescale H 2 /D 0 associated with transient pore pressure transmission during and following storms. Commonly, this timescale ranges from minutes to months. The shortest timescale affecting landslide responses to rainfall is H/g , where g is the magnitude of gravitational acceleration. Postfailure landslide motion occurs on this timescale, which indicates that the thinnest landslides accelerate most quickly if all other factors are constant. Effects of hydrologic processes on landslide processes across these diverse timescales are encapsulated by a response function, R(t*) t*/ exp (1/t*) erfc (1/ t*), which depends only on normalized time, t*. Use of R(t*) in conjunction with topographic data, rainfall intensity and duration information, an infinite-slope failure criterion, and Newton's second law predicts the timing, depth, and acceleration of rainfall-triggered landslides. Data from contrasting landslides that exhibit rapid, shallow motion and slow, deep-seated motion corroborate these predictions.
The mode summation method and a finite difference technique are applied to investigate the spectral ratio between the horizontal and vertical components (H/V ratio) of ambient vibrations and to explore the variation of the resonance frequency and the amplitude and shape of polarization as a function of the structure and the source positions. Layered structural models are used by assuming a large number of sources distributed around a receiver, with shallow source depths that are randomly assigned. We identify stable parts of the H/V ratios that are independent of the source distance and are dominated by the ellipticity of the fundamental-mode Rayleigh wave in the frequency band between the fundamental frequency of resonance of the unconsolidated sediments and the first minimum of the average H/V ratio. The ellipticity in this frequency band is determined by the layering of the sediments.
The numerical simulations are compared with observations at a site where the thickness and velocity structure of the unconsolidated sediments are known from S-wave and surface wave measurements. Two methods are applied to compute the H/V ratio, the classical method in the frequency domain and a method based on frequency–time analysis that allows us to locate P–SV wavelets in the time-series. The main problem in comparing synthetics with observations is the contribution of SH waves in the observed H/V ratios. We propose a method to minimize these effects and the effects of the superposition of different incoming P–SV waves. An inversion scheme is applied to the stable parts of the observed H/V ratio, based on a genetic algorithm, to retrieve the S-wave velocity structure from a single ambient vibration record.
This chapter deals with diverse problems related to measurements of the ambient noise and their application for determination
of soil or building properties. Although microtremors are studied since the first half of the 20th century, the number of
inves tigations rapidly increased following the article by Nakamura in 1989. In the last two decades a host of methods and
procedures emerged, some of which found their way into the everyday practice of seismologists and engineers dealing with the
problems of site affects and soil-structure interaction.
Even though the theoretical background of the H/V ratio technique was initially not completely clear (and controversies still
exist), its simplicity and low cost appealed to many. In his article in this chapter Y. Nakamura revisits the fundamental
assumptions behind the concept of the H/V ratio and presents examples of its use. He clearly advocates the interpretation
in terms of body waves, at least around the fundamental frequency of the soil.
We analyzed the phenomenology of microtremor H/V curves under inversions in the shear-wave velocity (Vs) profile in the subsoil. Under no Vs inversion the spectral signature of the H/V peaks is found to be ‘eye-shaped’ with the horizontal components higher than
the vertical. Conversely, under negative velocity gradients, numerous of differences emerge. I) A H/V ratio below 1 is observed
for a wide range of frequencies, due to the decrease of the horizontal components below the vertical one. II) In the presence
of persistent H/V <1, small bumps in the H/V ratio given by local minima in the vertical spectral component may represent
the relics of the peaks indicating resonances and stratigraphic discontinuities. As a consequence, in the presence of velocity
inversions the H/V>2 SESAME (2004) criterion fails but a stratigraphic interpretation may still be possible. III) The H/V
curves should always be interpreted together with the single component spectra. IV) Microtremor H/V measurements for stratigraphic/microzonation purposes on stiff artificial
soils, (asphalt, concrete, cement, pavements) should always be avoided since the latter often produce velocity inversions.
This may have consequences in the intermediate to high frequency domain (>1Hz) also in the application of reference site
methods, like Hsite/Hbedrock, to microtremor. Theoretical modeling confirms these experimental findings.