Horizontal To Vertical Spectral Ratio Research Articles

Frequency and Amplification for Assessing site Effects and PVS in the Padang City Railway

The train uses a special line in the form of railroad tracks that produce vibrations during movement. This study aims to determine the type of soil layer and the peak particle velocity due to the train around the Padang City train line. The acquisition was carried out at 8 points with a recording duration of 65 minutes using three geophone components. Microtremor data was analyzed using horizontal to vertical spectral ratio (HVSR) method. The results show that the research area has a type of surface layer of soil is sediment with a thickness of 30 meters or more. This is indicated by the dominant frequency value ranging from 0.64 Hz – 1.67 Hz. The research area has an amplification value ranging from 0.85 to 1.29. The train vibration has a dominant frequency ranging from 2.20 Hz – 13.54 Hz and an amplification of 1.11 – 1.82. The particle velocity values ​​obtained from the PVS values ​​ranged from 0.1605 mm/s – 0.7592 mm/s. The research area can be categorized as safe from train vibrations because of the low amplification value and the PVS value which is below the safe limit (<3 mm/s) according to SNI 7571:2010.

Shallow seismic structure around the Vrancea Seismic Zone from joint inversion of ambient noise H/V ratios and surface wave dispersion

The South-East Carpathians in Romania host the Vrancea Seismic Zone (VSZ), an intraplate seismic nest of frequent high magnitude earthquakes that cause unusually high damage over extended areas. Estimating the amplification of seismic signals strongly depends on the depth to the geophysical bedrock and the complex near surface geological structures. To improve seismic hazard estimates and ultimately mitigate seismic risk, we determine the frequency of resonance at 81 broadband seismic stations in southeast Romania using the horizontal-to-vertical spectral ratio (HVSR) of ambient seismic noise. We jointly invert HVSR functions with Rayleigh wave phase velocity dispersion curves using a Markov-chain Monte Carlo algorithm under the assumption of a diffuse field. Our results show excellent correlation between the fundamental frequency of resonance and local surface geology: stations located on foreland sedimentary basins show HVSR peaks in the range 0–1 Hz, while those located in orogenic nappes reach values as high as 10 Hz. Seismic bedrock topography mirrors the crystalline bedrock depth inferred from gravity modeling, but is consistently ∼2000 m shallower, corresponding to the interface between Paleozoic-Triassic to Jurassic-Cretaceous sedimentary levels on the Moesian Platform constrained by borehole data. The near surface seismic model shows anomalously low velocities in the Focsani forearc basin, adjacent to the VSZ, consistent with extremely low resonance frequencies and deep sedimentary layer successions (>5 km). In the back-arc of the Carpathian Orogen, at the contact with the Transylvanian Basin, average resonance frequencies are <2 Hz and inferred seismic bedrock depths vary laterally, likely affected by buried volcanic structures and diapiric salt domes. Our study presents the largest endeavor to study anomalous intensity patterns and local site effects in and around the VSZ and has fundamental implications for seismic risk evaluation and future civil engineering design planning.

Active and passive seismic methods for site characterization in Nuweiba, Gulf of Aqaba, Egypt

Nuweiba is located on the western coast of the Gulf of Aqaba (GoA), which represents the southern part of the Gulf of Aqaba - Dead Sea transform fault (GoA-DST). The GoA is characterized by high seismic activity and is the source of the November 22, 1995, Nuweiba earthquake, with a moment magnitude of Mw 7.3. This seismic unrest caused extensive damage in the city of Nuweiba due to the strong shaking caused by the earthquake. In addition, soil amplification triggered by the thick soft-sediments of the Wadi Watir delta beneath the city amplified the destructive power of the event. Therefore, an assessment of the effect of the Wadi Watir sediments on the ground motion at Nuweiba is very important for future construction, seismic hazard assessment, and seismic risk mitigation. Accordingly, the goal of the current study is to define the amplification characteristics of the Wadi Watir deposits using microtremor and surface wave data sets. The horizontal-to-vertical spectral ratio (HVSR) technique is applied to the microtremor data and Multichannel Analysis of Surface Waves (MASW) is applied to the surface wave data. The data are processed to obtain the distribution of the fundamental resonance frequency (F0), the depth of the geophysical bedrock using joint inversion of the Rayleigh wave dispersion curve and ellipticity peak, the shear wave velocity structure, the time-averaged shear-wave velocity in the topmost 30 m (Vs30), and the shear wave transfer function. The results of the HVSR show that F0 in the northern part of the city is 4.2–5.2 Hz and 0.5–0.8 Hz in the middle of the city. Bedrock depth is found to be in the range of 27 and 199 m. The one-dimensional velocity structure obtained from the MASW measurements has resolved two sedimentary layers within the upper 15 m. The Vs30 ranges between 207 and 556 m/s, according to the Vs30 the soil in the study area is categorized into soil classes B and C according to Eurocode 8. The obtained shear wave transfer functions show that the highest amplification factors are observed in the middle part of the study area.

A geophysical multidisciplinary approach to investigate the shallow subsoil structures in volcanic environment: The case of Ischia Island

The ability to image the underground structures of volcanoes is limited by the precision, resolution and penetration depth of each single geophysical method. In order to improve the knowledge of specific volcanic edifices and to better understand the general behavior of structures, the use of a combination of methods is strongly recommended to exploit and maximize their complementary capabilities of resolution and penetration depths.In this work a large dataset of seismic and electromagnetic measurements has been used to provide a more detailed and improved geophysical image of the shallower portion of the northern sector of Ischia Island (Campania region, Italy), severely hit by the August 21, 2017 earthquake (Mw 3.9). We analysed data by using different methodologies: Horizontal-to-Vertical Spectral Ratio (HVSR), seismic array technique (f-k), polarization analysis and Time Domain ElectroMagnetic (TDEM) survey. These methods are sensitive in a different way to tectonic features, lithologies, layer geometry and fluid distribution. Thus, their combination is useful for studying sites with complex crustal structures such as Ischia island, which is characterized by a well-developed geothermal system linked to the presence of a shallow magmatic body. Results of our study provides detailed information of the physical properties of the subsoil through: 1) the spatial distribution of the amplification parameters of ground motion, showing frequency peaks below 1 Hz and/or between 1 Hz and 5 Hz; 2) the definition of the velocity models up to 600 m depth, with shear wave velocities ranging from 150 m/s for the shallower layers to 2500 m/s for the half space; 3) the recognition of the correlation between the principal fault structures and polarization directions of the noise wavefield, mostly oriented along EW and NE-SW directions; 4) the resistivity models of the first 80 m depth with high resistivity values of the shallow layers in the range 50–100 Ω.m and low resistivity values of the bottom layers in the range 1–10 Ω.m.

EESD special issue: AI and data‐driven methods in earthquake engineering – (Part 1)

EESD special issue: AI and data‐driven methods in earthquake engineering – (Part 1)

Identification of Peak Ground Acceleration and hazard potency of earthquake disaster on the mainland surface of Wangi-Wangi Island, Wakatobi Regency, Southeast Sulawesi, Indonesia

This study combines microtremor data processed using the Horizontal to Vertical Spectral Ratio (HVSR) method and earthquake data. Microtremor data were obtained by filtering the waveform of ground vibration data obtained through measurements on the mainland surface of Wangi-Wangi Island in the frequency range of 0.5 to 25 Hz using a Band Pass Filter. The ground vibration data were obtained by recording signals at 47 different measurement points in a time range of 29.25 to 48.16 minutes. While the earthquake data used are sourced from 1920 to 2020. The earthquake data processed are those that meet the requirements for moment magnitude (M w) ≥ 5.0 SR and epicenter depth (h) ≤ 45 km. The results of calculations using the Kanai’s formula show that the PGA values are in the range of 0.0463 to 0.3665g. If zoning mapping is carried out, then the hazard potency of disaster due to earthquakes based on the PGA values on the Wangi-Wangi Island area is mostly Low potential (PGA < 0.26g), only a few areas have Medium potential (0.26g ≤ PGA ≤ 0.70g), and there are no areas with High potential for earthquake threat (PGA > 0.70g).

A Local Seismic Project near the Dalk Glacier Area, Larsemann Hills, East Antarctica: Toward Subice Imaging and Icequake Monitoring

Abstract During the 2019–2020 field season of the 36th Chinese National Antarctic Research Expedition, two seismic arrays were deployed in the Dalk Glacier area of Larsemann Hills, East Antarctica. The arrays consisted of 100 short-period nodal stations and were intended to investigate the physical properties and seismic events in the region, with the goal of enhancing our understanding of the glacier’s structure and dynamics. With these data, we use horizontal-to-vertical spectral ratio (HVSR) analysis to estimate the ice thickness. Noise cross-correlation functions and multimode dispersion curves of Rayleigh waves were extracted from the vertical-component ambient noise data to illuminate near-surface glacial structures. Teleseismic events with Mw &amp;gt;5.5 and two typical kinds of icequakes were observed via visual inspection. These initial results improve the understanding of the physical properties of the ice sheet as well as the glacial seismicity in Dalk Glacier.

Mapping the soil thickness in the Main Ethiopian Rift using passive seismic data and HVSR, case for Northern water divided between the Ziway-Shala lakes basin and the Awash basin

A passive seismic survey wasconducted in the northern water dividebetween the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift (CMER), and Awash basin. Previous geophysical studies in the same areahad mainly focused on mapping intermediate to deeper Earth geological structures, and little attention given to shallowest layer.In contrast, the study aimed to map these shallow geologic structures of soil layer undulations at this specific location of CMER. A rapid and non-invasive technique was employed, which involve recording three component measurements of ambient seismic noise data. The collected seismic noise data was processed and analyzed using the average of Horizontal to Vertical Spectral Ratio (HVSR) which helpsto determine resonance frequencies of the survey stations. After frequency estimatesweremadefor each station, corresponding depth estimates were obtained using the method of Nakamura.The point depth to the shallowest layer was determined for each siteand these depth valueswere gridded and mapped to show subsurface topographic undulations or thickness of topsoil cover.The result showed that the shallowest bedrock layer wasthickening northwards, toward the Awash basin.

Identifikasi Daerah Rawan Longsor secara Mikrozonasi di Jalan Alternatif Provinsi menggunakan Metode Simple Additive Weighting (SAW)

&lt;p class="AbstractText"&gt;The alternative provincial in Bengkulu Tengah Regency is located in a location bordering the Musi Fault segment that is part of the Great Sumatran Fault Zone. This condition makes cross-provincial roads in Central Bengkulu Regency vulnerable to natural disasters such as landslides. The purpose of the study was to determine areas that have the potential for landslides in earthquake-prone areas of Central Bengkulu Regency, Bengkulu Province, Indonesia. We performed the HVSR (horizontal to vertical spectral ratio) analysis and the simple additive weighting (SAW) method to achieve the study purpose. Field data acquisition using the PASI Gemini2 broadband seismometer (triaxial geophone). Next, we analyzed the tremor data recorded in the field using a wave spectrum. From the results of this study, we knew that the possibility of landslides in the survey area classified into three categories that is low, medium and high. Landslide risk should be paid more attention in areas with high potential, especially in densely populated areas. The main threat in the study area is an earthquake along the active Sumatran fault. Earthquakes in areas that are very steep and prone to landslides can increase the risk and cause landslides.&lt;/p&gt;

Separating broad-band site response from single-station seismograms

SUMMARY In this paper, we explore the use of seismicity data on a single-station basis in site response characterization. We train a supervised deep-learning model, SeismAmp, to recognize and separate seismic site response with reference to seismological bedrock (VS = 3.45 km s−1) in a broad frequency range (0.2–20 Hz) directly from single-station earthquake recordings (features) in Japan. Ground-truth data are homogeneously created using a classical multistation approach—generalized spectral inversion at a total number of 1725 sites. We demonstrate that site response can be reliably separated from single-station seismograms in an end-to-end approach. When SeismAmp is tested at new sites in both Japan (in-domain) and Europe (cross-domain), it achieves the lowest standard deviation among all tested single-station techniques. We also find that horizontal-to-vertical spectral ratio (HVSR) is not the optimal use of single-station recordings. The individual components of each record carry salient information on site response, especially at high frequencies. However, part of the information is lost in HVSR. SeismAmp could lead to improved site-specific earthquake hazard prediction in cases where recordings are available or can be collected at target sites. It is also a convenient tool to remove repeatable site effects from ground motions, which may benefit other applications, for example, improving the retrieval of seismic source parameters. Finally, SeismAmp is trained on data from Japan, future studies could explore transfer learning for practical applications in other regions.

A Comparative Analysis for Defining the Sliding Surface and Internal Structure in an Active Landslide Using the HVSR Passive Geophysical Technique in Pujilí (Cotopaxi), Ecuador

Geophysical techniques were employed to analyze one of the landslides that affected the main access road to Pujilí (Ecuador). A passive seismic technique was utilized to test a total of 70 horizontal to vertical spectral ratio (HVSR) points, complemented by an active seismic-refraction profile and a multichannel analysis of surface waves (MASW) survey. The results from the active geophysical surveys facilitated the determination of the shear-wave-velocity value for the surface materials that were in motion. However, the HVSR provided the fundamental frequency fo and amplification Ao values of the ground. The Nakamura (1989) relationship was applied to obtain the thickness of the sediments over a compact material from the fundamental frequency of the terrain in a two-layer model. Additionally, constrained models of the shear-wave velocity (Vs) distribution in the landslide area, obtained from the active seismic surveys, were used to invert the ellipticity curves. The results from this inversion were compared with those obtained by applying the Nakamura equation. The landslide-rupture surface was delineated for each type of analysis, which verified the correlation and minimal differences between the results of the three proposed studies, thus validating the procedure. The directivity of the microtremor HVSR signals was also analyzed, demonstrating a relationship with the internal structure of the sliding material. Furthermore, the ability to slide concerning the Kg parameter (vulnerability index, Nakamura, 1989) was studied. The usefulness of the directivity analysis in defining the internal structures in landslide materials and in determining the areas with the most significant instability was demonstrated. Overall, the HVSR is considered valuable when conducting early landslide studies and is helpful in determining the rupture plane while remaining a simple, fast, and economical technique.

The effect of saturation and salts on the rigidity of fine-grained material

Repeated multi-channel analysis of surface waves (MASW) and horizontal to vertical spectral ratio (HVSR) experiments were conducted on a hummock within a large landslide in east-central Utah adjacent to a small wetland. Stakes were placed into the ground to ensure identical array geometries and geophone locations. The landslide is an admixture of fine-grained material (clay, quartz and Na-plagioclase) and intermediate-composition volcanic clasts. A prominent crust of Na-salts covered the hummock surface, produced by evapotranspiration of shallow groundwater associated with the adjacent fen.The experiments produced three primary observations. First, on any given day, the Rayleigh wave frequency spectra were highly reproducible between individual experiments. Second, from May to October the spectra exhibited high variability between experiments conducted weeks apart, especially at frequencies <20 Hz. Third, a persistent subsurface boundary exists across which Vs increases from ~175 to ~300 m/s. However, that boundary migrated over time from a depth of ~5 to ~4 m.The low velocity of the upper ~5 m subsurface layer represents poorly-compacted landslide material, perhaps weakened from high Na+ concentrations. However, the variability in frequency spectra and upward migration of the boundary must have arisen from changing moisture and salt content over time. The fen was instrumented with a piezometer that showed an overall ~1 m decline in water levels from May to October with small, superimposed recharge events resulting from summer monsoon rains.This study leads to two important conclusions. Specific to this site and perhaps others, variable moisture content can affect capillary forces and the proportion of evaporite cements in the shallow subsurface that are manifested in strikingly different frequency spectra and migrating stiffness (Vs) boundaries. More generally, moisture and salt content can affect estimates of subsurface stiffness, including the position of apparent boundaries within unconsolidated materials.

MICROZONING OF HAZARD VULNERABLE USING MICROSEISMIC METHOD WITH HVSR (HORIZONTAL TO VERTICAL SPECTRAL RATIO) ANALYSIS IN BAYAT DISTRICT, KLATEN, INDONESIA

The Yogyakarta earthquake in 2006 caused damage in several areas, especially in the area of Klaten Regency. As a result, many infrastructures, houses, health facilities and casualties have been damaged. The research was conducted with the target of knowing the level of earth's hazard by microzoning soil vulnerability areas in the Bayat District, Klaten, Central Java. This study used the microseismic method with the HVSR (Horizontal to Vertical Spectral Ratio) analysis method. Where are the natural/dominant frequency parameters, natural amplification, and earthquake vulnerability index as measured parameters? This study aims to carry out microzonation of disaster vulnerability risks which are interpreted from the results obtained, namely amplification maps (A0), dominant frequency (f0), earthquake vulnerability (Kg), and shear wave velocity maps at a depth of 30 meters (Vs30). Data acquisition was carried out at 19 measurement points with a spacing between points having a distance of 1-2 km using a portable TDS seismograph. The acquisition results obtained varied values with A0 values 0.9 – 8.8, f0 values 1.0 – 13.8 Hz, Kg values 0.4 – 38.5, and Vs30 values 228.1 – 1635.4 m/s. From these results it can be categorized as zoning regarding the value of the level of disaster vulnerability based on the value of earthquake vulnerability (Kg) obtained to minimize damage and casualties

Microzonation Site Effects and Shear Strain during Earthquake Induced Landslide Using HVSR Measurement in Ulu Mana Sub-District, South Bengkulu Regency Indonesia

The Ulu Manna area is classified as an area with high landslide potential because of its location and geological structure, which is hilly. The risk of landslides in the Ulu Manna area due to earthquakes in weak areas can be studied using ground shear strain (GSS). This study aimed to provide information on the potential of landslides in the Ulu Manna area, South Bengkulu Regency Indonesia. The data was collected using the Horizontal-to-Vertical Spectral Ratio (HVSR) method. The study area is 195.8 km2 and consists of 32 data collection points. The data processing was performed using WinMASW 5.2 HVSR and ArcGIS Desktop 10.8.2 software to obtain dominant frequency values, amplification values, subsurface soil vulnerability index values, maximum soil acceleration values, and soil shear strain values. The soil shear strain values obtained are on the order of 10-4 to 10-3, meaning that the dynamic characteristics of the soil in the study area are elastic-plastic. This plastic-elastic nature characterizes the area as an area with high landslide potential

Combination of passive and active methods towards site characterization of accelerometer stations in Greece

AbstractThe strong correlation between earthquake damage and the local geological conditions in an area has long been established, and site characterization studies are common practice for realistic estimation of seismic hazard and mitigation of the respective seismic risk. Estimation of the time‐averaged S‐waves velocity of the topmost 30 m (VS30) is recognized as a basic proxy for site characterization and has been incorporated in several building codes. In our work, we examine the inversion of single‐station ambient noise horizontal to vertical spectral ratio (HVSR) curves to estimate the local one‐dimensional (1D) VS(Z) structure. The inversion is performed based on the diffuse field assumption, which allows for the inversion of HVSR, including the contributions of both body and surface waves. Although ambient noise data acquisition is immensely cost‐effective, HVSR curve inversion is subject to the solution non‐uniqueness issue, offering ambiguous results. To provide a possible solution for this matter, additional near‐surface geophysical methods such as the multichannel analysis of surface waves and the electrical resistivity tomography (ERT) were applied to derive information on the shallow subsurface structure. The acquired 1D seismic velocity profile was implemented as initial model in the HVSR inversion to constrain the velocities of the upper meters. Additionally, the inferred stratigraphy from the ERT electrical profile was utilized to constrain the thicknesses of the layers. Tests were conducted with and without a priori information from complementary techniques to explore how the HVSR inversion procedure is facilitated. The proposed methodology was applied at the locations of six accelerometers in the city of Thessaloniki, northern Greece, under different geological conditions. The reliability of the inverted VS(Z) models was checked by performing 1D numerical simulations of ambient noise waveforms generated by distributed sources and by direct comparison between synthetic ambient noise HVSR and single‐station earthquake HVSR curves.

Shear wave velocity estimation in the Bengal Basin, Bangladesh by HVSR analysis: implications for engineering bedrock depth

The S-wave velocity (VS) in the Bengal Basin, Bangladesh has not been resolved from the ground surface to an intermediate depth in a regional context despite its importance for seismic hazard and risk evaluation. For this reason, we estimated VS profiles beneath 19 seismic stations in Bangladesh to a depth approximately 2800 m by employing full horizontal to vertical spectral ratio (HVSR) curve inversion under the diffuse field theory for the noise wavefield. The seismic stations are concentrated in three tectonic zones within the basin: the Surma basin (SB, Zone 1), Bengal Foredeep (BF, Zone 2), and Chittagong Tripura Fold Belt (CTFB, Zone 3). Full HVSR analysis (from 0.2 to 10 Hz) allowed us to obtain deep profiles with combined insights from shallow geotechnical boreholes and deep P-wave velocity (VP) information from active seismic surveys. From the resultant VS profiles, engineering bedrock (VS > 760 m/s) depths were also identified throughout the study area for the first time. The VS profiles within the Holocene to Miocene sedimentary sequences showed rapid variations from location to location. This is due to the highly variable near-surface geology caused by the dense and complex river network and tectonic deformation in Bangladesh. Except for three stations, the engineering bedrock depth exceeded 30 m at all stations. These results indicate the existence of deep soft soil in the study area, where VS30 based site characterization is inappropriate. Furthermore, seismic site response was estimated at a station (DHAK) by simulating a subduction zone earthquake. The resulting response spectrum (RS) exhibited ground motion amplification over a longer period, suggesting that multistory buildings at the site may be at risk if subjected to large earthquakes. The outcomes of this study can serve as useful guidelines for seismic risk reduction planning in Bangladesh.

Horizontal to Vertical Spectral Ratio (HVSR) Analysis of the Passive Seismic Data on Alluvial Lithology: An Example from Bandung Basin Rim

Bandung basin is surrounded by active volcanoes at the center of Western Java region. Soil type of this area consists mostly of volcanic alluvial, formed by the past volcanic activities. At this location, we performed passive seismic surveys to reveal the sedimentary characteristics using a triaxial seismometer, to study the properties which have potential implications on environmental aspects, such as natural disaster mitigation and the city planning. In this work we reported the analysis of three component vibration signals by means of the Horizontal to Vertical Spectral Ratio (HVSR) calculation to identify the sediment thickness through the dominant frequency of the earth vibration. The method signal processing successfully revealed the amplification factor of the area, which is very important to determine the potential ground motion induced disasters. The data was collected using a laboratory developed seismic logging device with sensitivity of 0.28 ± 5% V⋅s/cm. The signal preconditioning was carried out to shape the acquired signals with high noise level, prior to the HVSR calculation. From the measurements and computation, we obtained the thickness variation at the chosen survey area of around 32 to 64 meters, corresponding to the measured dominant frequency of around 2.5 to 5.0 Hz. The results agree very well with the reference sediment thickness measured by the microtremor surveys. The vibration frequencies are also consistent with the range suggested by the literature

Spatial variation of strong ground motions in a heterogeneous soil site based on observation records from a dense array

The difference in local sediment thickness and soil properties has a significant impact on the spatial variation mechanism of seismic ground motion in the engineering scale. Due to the scarcity of observation data of dense arrays, the existing theoretical studies are mostly developed by numerical simulation methods, including human factors and a large number of assumptions. In view of this, based on the multistation observation records of the Luxian MS 6.0 earthquake and Yibin MS 5.1 earthquake obtained using a Zigong dense array, the study quantitatively analyzes the spatial characteristics of ground motion in heterogeneous soil sites by integrating a theoretical model with numerical analysis. In this study, many popular approaches including root-mean-square acceleration, horizontal-to-vertical spectral ratio (HVSR) of microtremor and strong motion records, and lagged coherency are comprehensively utilized to make the conclusion accurate and reliable. The results show that local soil conditions could affect the attenuation of coherence function with distance. The station-pairs with similar HVSR characteristics generally present a higher coherence level when the difference of the interstation distance is less than 100 m. In addition, the coherency function between stations will be greatly reduced when the H/V spectral ratio characteristics differ greatly, which is also obvious in the low-frequency part below 5 Hz. Finally, a lagged coherency model that considers the influence of heterogeneous soil is constructed in this study. The model has a definite physical meaning and can better represent the spatial variation of ground motion at nonbedrock sites.

Early Investigation of a Landslide Sliding Surface by HVSR and VES Geophysical Techniques Combined, a Case Study in Guarumales (Ecuador)

The access road to the powerhouse’s hydraulic system’s facilities in Guarumales (Azuay, Ecuador) presents a medium-sized landslide. Geophysical tests were conducted in the initial research stage, combining electrical and seismic methods. A vertical electrical sounding (VES) and horizontal to vertical spectral ratio (HVSR) survey campaign have been taken as a reference for the analysis of the landslide area. The distribution of these test points has been at three different levels along the landslide where the access road crosses it, trying to cover the area’s most extensive possible length and width. In the area, we find the geology dominated by the presence of schists, altered to different degrees and presenting blocks of material with a lower degree of alteration within colluvium formed by a clayey matrix and coarse material of the exact nature. There is also observed runoff water and groundwater in the area. The results obtained through SEV tests have allowed for defining the separation zone of the mobilized or sliding materials compared to the fixed or immobile ones (potentially, the sliding surface was marked). Using the HVSR technique, the natural vibration frequencies of the ground associated with the sliding mass (separation of seismic impedances between a two-layer model: mobile and fixed) have been determined. Previous authors proposed an empirical relationship establishing the exponential relationship, already proposed by previous authors, between sediment thickness and natural frequencies. It has been possible to determine the depth of the position of the loosely compacted sediment zone (and probably moving or mobilized) compared to that of compact materials (immobile) and thereby define the potential rupture surface.

Identification Protocols for Horizontal-to-Vertical Spectral Ratio Peaks

ABSTRACTPeaks in horizontal-to-vertical spectral ratios (HVSRs) of Fourier amplitudes from three-component recordings of ground vibrations without undue local anthropogenic influence are used to identify site resonances, which are an important component of site response. We address two topics: (1) how should HVSR peaks be identified and (2) are there appreciable differences in HVSR derived using different instruments recording microtremors and seismic strong ground motions? We propose identifying peaks by considering peak amplitudes relative to neighboring ordinates and peak width. The procedure incorporates a regression tree algorithm that can be tuned to conform with user preferences toward relatively “conservative” or “liberal” peak identification (producing few or many sites with peaks, respectively). We then investigate the consistency of microtremor-based HVSRs (mHVSRs) derived from seismometers and accelerometers, which show a high rate of false negatives (missed peaks) from accelerometers with a full scale of ± 2g or greater. In contrast, mHVSRs derived from collocated temporary and permanent seismometers (optimized to record teleseismic signals) have about 60%–80% consistency (with no apparent bias in peak attributes). This indicates that mHVSRs from accelerometers having a broad full scale are unreliable but that mHVSRs can be reliably obtained from temporary or permanent seismometers. Finally, we compare seismometer-based HVSR from microtremor and earthquake (eHVSRs) sources. Results are consistent for 60%–70% of sites (i.e., both either do or do not have significant peaks, and when peaks are present, they occur at similar frequencies, &amp;lt;20% change). For sites with an mHVSR peak, the rate of corresponding eHVSR peaks is nearly 50%, whereas for sites without an mHVSR peak the eHVSR peak rate is low (about 20%). The mismatch rate for mHVSR peak sites is sufficiently high that the use of eHVSR to derive site response models is likely too optimistic (overestimates model effectiveness); mHVSR is preferred for consistency with information available in forward applications.