Horizontal To Vertical Spectral Ratio Research Articles

Imaging buried anticlines in the Po Plain, northern Italy, based on HVSR frequency and amplitude analyses

AbstractThe use of the HVSR (Horizontal-to-Vertical Spectral Ratio) method on single-station microtremor measurements is well documented in small alluvial plains for bedrock mapping. In large sedimentary basins, like the Po Plain, its application is still debated. To shed some light on this issue, we investigated two seismogenic structures buried below the Po Plain Quaternary deposits: the Mirandola and Casaglia anticlines. We acquired and analysed a dense distribution of HVSR data covering the two areas and mapped the frequency and amplitude values of the observed resonance peaks. The top of both anticlines is highlighted by high amplitude peaks picturing E-W elongated sectors with high-impedance contrast, where Quaternary deposits are reduced in thickness to about 60–130 m and directly overlay the Pliocene (Mirandola) and Miocene (Casaglia) marine units. In Mirandola, the high-amplitude peaks also correspond to higher resonance frequencies, while in Casaglia, the distribution of resonance frequencies is relatively uniform suggesting a flatter crestal region and the lateral continuity of the resonance surface. The combination of peak frequency and amplitude information on a dense grid of measurement points is thus confirmed to be useful for identifying and mapping buried geological structures such as structural highs. Further modelling is being carried out to estimate the depth of the surface responsible for the observed resonances, through calibration with borehole information. Graphical Abstract

Shear wave velocity model using HVSR inversion beneath Bandar Lampung City

The horizontal-to-vertical spectral ratio (HVSR) method has been used to characterize site-effect parameters that are indispensable in seismic hazard and risk-reduction studies in urban areas and rapid land-use planning. This method is widely used because it is the cheapest and simplest geophysical method for the acquisition and processing stages. In subsequent developments, the HVSR method has been widely used to determine elastic rock parameters, particularly shear wave velocity (vS), through the HVSR curve inversion process. Furthermore, the vS structural model can be used to delineate the presence of complex geological structures, particularly faults and sedimentary basins. Bandar Lampung is a city in Lampung Province with many fault structures and groundwater basins to the south. There are 83 HVSR measurement points around Bandar Lampung for delineating the presence of fault structures and groundwater basins. We produced the HVSR curve from the measurement results and then performed an inversion process using the particle swarm optimization algorithm to obtain vS for the depth profile. Subsequently, from this profile, we produced a two-dimensional (2D) lateral and vertical model. The mean vS value was calculated from all the measurement points, and we found stiff soil layers reaching depths of approximately 5 m, with a value of vS < 330 m/s. A bedrock layer with a velocity exceeding 1250 m/s was visible at a depth of 100 m. Based on the 2D model, the vS structure shows that the city of Bandar Lampung is divided into two zones, with a NW-SE boundary. The north-middle-eastern part of the city consists of harder rocks. This harder rock is characterized by extremely high vS values, starting from a depth of 50 m. In contrast, the south-middle-west exhibits a low-moderate vS anomaly associated with groundwater basins SW of the city. From the 2D vS structural model, fault structures can be found along the city, characterized by a contrast of vS values from low to medium and from medium to high.

Microtremor Analysis to Identify Fissure Vulnerable Zones in Demak, Central Java, Indonesia

Demak Regency, located in Central Java, Indonesia stands out among the regencies for its rapid development and regional growth, leading to a significant reliance on groundwater. This is thought to be caused by low precipitation and the geological characteristics of Demak Regency, which is mostly located on alluvial plains. To address these concerns, this study aimed to pinpoint regions vulnerable to ground fractures resulting from excessive groundwater extraction compounded by seismic activity. The research encompassed 191 locations, including 177 single station microtremor measurement points and 14 array microtremor measurement points. Single station microtremor measurements utilized the Horizontal to Vertical Spectral Ratio (HVSR) method, while array measurements employed the Spatial Autocorrelation (SPAC) method. Ground shear strain values spanning from 32.12 × 10-6 - 96.39 × 10-6. Notably, areas exhibiting heightened values are concentrated in Wonosalam Subdistrict and the Southwest of Karangtengah Subdistrict. The morphology of the bedrock within the study area tends to mirror the sedimentary layer's thickness, reflecting relatively minimal surface height discrepancies at the measurement points. Vulnerability to fissures resulting from excessive groundwater extraction and seismic activity is concentrated on the north side of Wonosalam District, the eastern sector of Demak District, and a small section of Mijen District.

Soil Vulnerability Analysis Using the Microtremor Method for Mapping the Landslide Susceptibility Area in Batu City, Indonesia

Based on Regional Agency for Disaster Management (BPBD) Batu City data, Indonesia, 36 landslides have been recorded in the last three years around the Payung area, where this area connects several regions around Batu City. So, it is necessary to map areas with potential ground movement as an initial step for the local government in formulating a landslide disaster management strategy. This research aims to map landslide susceptibility areas around Payung, Batu City, identified based on the soil vulnerability index value range in the peak ground acceleration (PGA) mapping results. The data was measured using the microtremor method. There were 22 measurement points in 3 zones with different heights, and the microtremor signal was recorded for 35-40 minutes at each point. The microtremor signal from seismic recording is composed of three components (NS, EW, and Up-Down), which are used to develop the Horizontal to Vertical Spectral Ratio (HVSR) technique, where the ratio value is obtained by comparing horizontal (H) components (HNS and HEW) to the vertical component (Up-Down (V)). The research results show that landslide susceptibility areas are located at points M10, M13, and M16, with a soil vulnerability index range of 21-40.1. When a landslide occurs in that area with high vulnerability, it will impact points along Payung Street at points M3–M9. The points with high soil vulnerability index values need special treatment to reduce the impact of landslides.

Micro Zonation to Earthquake Vulnerability in West Semarang Coastal Indonesia Area Based on HVSR (Horizontal to Vertical Spectral Ratio) Microtremor Method

Micro Zonation to Earthquake Vulnerability in West Semarang Coastal Indonesia Area Based on HVSR (Horizontal to Vertical Spectral Ratio) Microtremor Method

Directional HVSR at the Chalco lakebed zone of the Valley of Mexico: Analysis and interpretation

This study delves into Horizontal-to-Vertical Spectral Ratio (HVSR) and directional HVSR (D-HVSR) using continuous 30-min ambient seismic noise (ASN) signals recorded at 68 sites in the Chalco lakebed zone of the Valley of Mexico (VM). Our aim is to explore the shallow stratigraphy and subsurface structures in that zone. The isoperiod map using the HVSR, and the ensuing velocity structure inversion reveals a basin of irregular shape and shallow depth in the surroundings. Additionally, we explore azimuthal effects by computing the D-HVSR by rotating the records to obtain the time series for a given direction. We found different polarizations at the measurement points, and focused on the frequencies in which the effect is more pronounced. The cases with significant polarizations at the dominant frequencies suggest an irregularity at the basement level. This is supported by preliminary synthetic results that uses the three-dimensional (3D) Indirect Boundary Element Method (IBEM) to assess the effects of interface irregularities like a channel or dike. In the present stage of development, the D-HVSR is a powerful tool to reveal the existence of lateral variations. The complexity of the lakebed zone response in the VM and its irregular characteristics show the potential of the D-HVSR approach as well as the need to deeply explore into the modeling for its better understanding and interpretation.

Evaluation of Site Terms for Ground-Motion Models for South America

ABSTRACT The time-averaged shear-wave velocity in the top 30 m, VS30, is used to represent the site condition in many ground-motion models (GMMs). Regionalized GMMs account for regional differences in the ln(VS30) scaling by including region-specific coefficients for the site term. For example, recent GMMs developed for subduction zone earthquakes as part of the Next Generation Attenuation-Subduction (NGA-Sub) project include region-specific VS30 scaling for seven regions: South America, Central America, Japan, New Zealand, Taiwan, Cascadia (Pacific Northwest America), and Alaska. Of these seven regions, South America has the largest within-event standard deviation. One cause for the larger within-event standard deviation is a weaker relation between VS30 and the site term in South America compared to other regions. A cause for this weak correlation is that most of the VS30 values for the South American region in the NGA-Sub data set are based on proxies. The relation between VS30 and the site term improves significantly for periods less than 1 s if only stations with measured VS30 are considered, but the correlation is weak for periods greater than 1 s even for stations with measured VS30, indicating that, for the South America region, VS30 is not well correlated with the deeper shear-wave velocity profile that controls the long-period amplification. To improve the site terms for South America, we develop a model based on the horizontal-to-vertical spectral ratio (HVSR) from microtremors, similar to the approach used by Pinilla-Ramos et al. (2022) for California. The data set includes 660 recordings from 51 earthquakes recorded at 274 sites. The site-term model includes the period-dependent HVSR amplitude and the geometric mean of the average HVSR amplitude over the frequency band of 0.25 to 15 Hz. Including these two parameters reduces the standard deviation of site-specific site terms over the period range 0.3–3 s, with the largest reduction in the period range 1–2 s. This site-term model can be incorporated into subduction GMMs and implemented in probabilistic seismic hazard analyses for South America.

Site Characterization and Shallow Shear Velocity through HVSR Measurements around the Hyderabad Metropolitan Region, India

ABSTRACT We investigate the site characterization and shallow shear velocity profiles from the analysis of the Horizontal to Vertical Spectral ratio (HVSR) around the Hyderabad metropolitan region (HMR), which falls under the southern Indian shield. This work uses both the ambient noise and microearthquake data to compute the HVSR, and additionally the Random Decrement technique to compute the HVSR of extracted Rayleigh waves. This study indicates comparable HVSR curves at each station with the three different datasets, from which we obtain the average dominant frequency (f0) and amplification value (A0). They are further used to calculate the seismic vulnerability index value (Kg). We observe that the value of f0 around the HMR is not fixed, but is varying in the range of 3.4 to 18 Hz, whereas the value of A0 is in the range of 1.7 to 12 approximately and Kg in the range of 0.16 to 1.68 approximately, with an exception of ∼ 33 at VKB (Vikarabad) station, which may be due to a local unconsolidated sub-surface structure. Based on the Diffused Field Assumption (DFA), we invert the average HVSR curves and average dispersion curves of Rayleigh waves, and provide the shallow shear velocity profiles up to 300 m, along with an approximate estimate of VS30 (in upper 30 m depth). The estimated VS30 values vary between 911 to 3143 m/s, falling under the classifications A and B (mostly Hard Rock and Rock type) of National Earthquake Hazards Reduction Program (NEHRP) (BSSC, 2001). However, our study shows some stations with shear velocity inversions at shallow depths within 300 m, indicating layers of low velocity, needing further study. In the absence of detailed near-surface findings, these findings are valuable inputs for geotechnical engineering studies and urban-city planning around the HMR, and emphasizes the effectiveness the HVSR method to determine sub-surface topography and/or unknown soil structures as an economical investigation viability.

Site database for national strong motion stations in mainland China

Ground motion amplification and spectral content are significantly influenced by local soil and geological conditions. This article summarizes the development of the China Site Database (CNSDB) in the China Ground Motion Flatfile project. CNSDB contains site information for 1450 strong-motion recording stations within the National Strong Motion Observation Network System (NSMONS) in China. The primary site information in CNSDB includes the time-averaged shear-wave velocity in the upper 30 m (VS30), and site class according to the seismic design code in China. These site data are derived using different methods from various data sources. Priority is given to approaches that minimize bias and dispersion relative to measurement-based results. Recommended VS30 values and site class are derived from reliable velocity profiles or field survey results whenever available. A China-specific VS30 extrapolation model is developed using 6179 engineering boreholes and is then utilized to estimate VS30 for profiles <30 m. VS30 values and site class are estimated from horizontal-to-vertical spectral ratio (HVSR) curves of earthquake recordings using machine-learning techniques. For the remaining sites without velocity profiles or sufficient ground-motion recordings for HVSR computation, VS30 values are determined as the weighted average of proxy-based estimates using geological attributes, topographic slope, and terrain categories. We also discuss the potential application of CNSDB in the development of China seismic hazard map considering site amplification factors.

Mapping bedrock topography and detecting blind faults using the fundamental resonance of microtremor: a case study of the Pohang Basin, southeastern Korea

SUMMARY The Pohang Basin sustained the most extensive seismic damage in the history of instrumental recording in Korea due to the 2017 Mw 5.5 earthquake. The pattern of damage shows marked differences from a radial distribution, suggesting important contributions by local site effects. Our understanding of these site effects and their role in generating seismic damage within the study area remains incomplete, which indicates the need for a thorough exploration of subsurface information, including the thickness of soil to bedrock and basin geometry, in the Pohang Basin. We measured the depth to bedrock in the Pohang Basin using dense ambient noise measurements conducted at 698 sites. We propose a model of basin geometry based on depths and dominant frequencies derived from the horizontal-to-vertical spectral ratio (HVSR) of microtremor at 698 sites. Most microseismic measurements exhibit one or more clear HVSR peak(s), implying one or more strong impedance contrast(s), which are presumed to represent the interface between the basement and overlying basin-fill sediments at each measurement site. The ambient seismic noise induces resonance at frequencies as low as 0.32 Hz. The relationship between resonance frequency and bedrock depth was derived using data from 27 boreholes to convert the dominant frequencies measured at stations adjacent to the boreholes into corresponding depths to the strong impedance contrast. The relationship was then applied to the dominant frequencies to estimate the depth to bedrock over the whole study area. Maps of resonance frequency and the corresponding depth to bedrock for the study area show that the greatest depths to bedrock are in the coastal area. The maps also reveal lower fundamental frequencies in the area west of the Gokgang Fault. The results indicate a more complex basin structure than previously proposed based on a limited number of direct borehole observations and surface geology. The maps and associated profiles across different parts of the study area show pronounced changes in bedrock depth near inferred blind faults proposed in previous studies, suggesting that maps of bedrock depth based on the HVSR method can be used to infer previously unknown features, including concealed or blind faults that are not observed at the surface.

Seismic vulnerability distribution of the earthquake prone area in Central Aceh

Abstract Central Aceh is known as an earthquake-prone area due to the presence of the Sumatra Fault and secondary fault crossing the area resulting the M 6.1 earthquake in 2013. The objective of the research is to investigate seismic vulnerability of Central Aceh. Microtremor observations were conducted at 25 locations in Central Aceh analysed using the Horizontal to Vertical Spectral Ratio (HVSR). Seismic amplification is represented by the maximum values of H/V. The dominant frequency f0 representing the sediment thickness was obtained when the H/V curve is maximum. The frequency values ranged from 14.9 to 25.9 Hz, and the seismic vulnerability values varied from 0.02 to 3.1. The top soil consists of thin layers of sedimentary material dominated by tertiary rocks. The rocks are classified as Type IV and Class I based on the dominant frequency values. This result is consistent with the geology, consisting of sedimentary and volcanic rocks. Areas with a seismic vulnerability value &gt; 1.0 is considered to have a high risk of earthquake damage because they are likely to experience earthquake amplification. Areas with seismic vulnerability value &gt; 1.0 are found in West Aceh and the Geureudong volcano area.

3D shear wave velocity imaging of the subsurface structure of granite rocks in the arid climate of Pan de Azúcar, Chile, revealed by Bayesian inversion of HVSR curves

Abstract. Seismic methods are emerging as efficient tools for imaging the subsurface to investigate the weathering zone. The structure of the weathering zone can be identified by differing shear wave velocities as various weathering processes will alter the properties of rocks. Currently, 3D subsurface modelling of the weathering zone is gaining increasing importance as results allow the identification of the weathering imprint in the subsurface not only from top to bottom but also in three dimensions. We investigated the 3D weathering structure of monzogranite bedrock near the Pan de Azúcar National Park (Atacama Desert, northern Chile), where the weathering is weak due to the arid climate conditions. We set up an array measurement that records seismic ambient noise, which we used to extract the horizontal-to-vertical spectral ratio (HVSR) curves. The curves were then used to invert for 1D shear wave velocity (Vs) models, which we then used to compile a pseudo-3D model of the subsurface structure in our study area. To invert the 1D Vs model, we applied a transdimensional hierarchical Bayesian inversion scheme, allowing us to invert the HVSR curve with minimal prior information. The resulting 3D model allowed us to image the granite gradient from the surface down to ca. 50 m depth and confirmed the presence of dikes of mafic composition intruding the granite. We identified three main zones of fractured granite, altered granite, and the granite bedrock in addition to the mafic dikes with relatively higher Vs. The fractured granite layer was identified with Vs of 1.4 km s−1 at 30–40 m depth, while the granite bedrock was delineated with Vs of 2.5 km s−1 and a depth range between 10 and 50 m depth. We compared the resulting subsurface structure to other sites in the Chilean coastal cordillera located in various climatic conditions and found that the weathering depth and structure at a given location depend on a complex interaction between surface processes such as precipitation rate, tectonic uplift and fracturing, and erosion. Moreover, these local geological features such as the intrusion of mafic dikes can create significant spatial variations to the weathering structure and therefore emphasize the importance of 3D imaging of the weathering structure. The imaged structure of the subsurface in Pan de Azúcar provides the unique opportunity to image the heterogeneities of a rock preconditioned for weathering but one that has never experienced extensive weathering given the absence of precipitation.

Identification of Ground Vulnerability Based on Kg, PGA, and GSS Assessment by Single Station Microtremor in Bendosari Hamlet, Kalasan Village, Yogyakarta

Bendosari Hamlet, Kalasan, Yogyakarta is a research area that is one of the areas affected by the earthquake that occurred on May 27, 2006. The earthquake with a magnitude of 6.4 caused quite severe damage to buildings around the research area. Based on the phenomenon of earthquakes that have occurred, there needs to be further study related to vulnerable areas in the event of another earthquake. Research data is primary data taken using a single statiun microtremor tool with an overall acquisition point of 50 measurement points. This study aims to find out the value of seismic vulnerability index (Kg), Peak Ground Acceleration (PGA), and Ground Shear Strain (GSS). Using the Horizontal to Vertical Spectral Ratio (HVSR) method, the dominant frequency result (f0) and amplification factor value (A0) from data that has been recorded through the microtremor tool. Based on research that has been done, the results are obtained in the f0rm of seismic vulnerability index values that are in the range of 0.49 - 30.18, the value of the acceleration of ground movement is in the range of 0.21 - 0.61 Gal, and the ground shear strain value is in the range of 4 x 10-6 to 38 x 10-5. Point K26 which is the original documentation of buildings affected by the earthquake, analyzed that the value of Vs of 858.112 m / s on the vs ground profile indicates that there is hard soil at a depth after 27.2 m. This shows that the thickness of subsurface sedimentary rocks at point K26 is quite thick. Keywords: Bendosari Hamlet; Mikrotremor; GSS; HVSR; PGA; Seismic Vulnerability

Seismic characterisation of the subsoil under a historic building: Cathedral Church of Saint Mary in Murcia case study

This research focuses on the Cathedral Church of Saint Mary in Murcia, Spain, which is located in a moderate-to-high seismic risk zone in the Spanish context. The study uses geophysical techniques and geotechnical investigation to characterise seismic ground models at the building's scale, aiming to understand the real amplification of the ground under seismic effect in historic buildings. Three soil layers (silt, sand with gravel, and gravel) were identified through core borings. Multichannel Analysis of Surface Waves (MASW) profiles and Mini-array profiles revealed Vs values of 305 ± 32 m/s, 296 ± 62 m/s, and 440 ± 38.5 m/s, respectively, for these materials. Seismic Refraction Tomography (SRT) showed Vp values of 586 ± 73 m/s, 700 m/s, and 1466 ± 489 m/s for the corresponding layers. The horizontal-to-vertical spectral ratio (HVSR) approach identified a ground predominant period ranging from 0.37 to 0.38 s. Another significant peak at 2.3 s is observed, probably associated to the Triassic basement. Three seismic events with magnitudes Mw between 4.9 and 5.1 were used as inputs to determine the amplification factor (AF). The results indicate a PGA amplification factor between 1.7 and 2.1. These results contribute to the conservation and mitigation of seismic risk of this cultural heritage generating input data that enables precise computation of the soil-structure interaction.

Assessing soil vulnerability in Petobo post-liquefaction zone, Palu, Central Sulawesi: A microzonation study utilizing microtremor measurements

On September 28, 2018, a 7.5 Richter magnitude earthquake struck the Palu City neighborhood of Petobo. The tectonic activity along the Palu-Koro fault generated this earthquake, which resulted in soil liquefaction. The purpose of this study is to use microtremor measurements at 33 distinct places to investigate the properties of the soil layer after liquefaction. The obtained data was then evaluated utilizing Horizontal to Vertical Spectral Ratio (HVSR) methodologies such as Ground Shear Strength (GSS), amplification factor, and vulnerability index to determine the soil layer's properties and susceptibility. The dominant frequency ranges from 0.19 to 4.75 Hz, while the dominant period ranges from 0.21 to 5.17 seconds, according to the measurement results. According to these measurements, the silt layer varies between 5 and 30 m. GSS values in the 10-4 to 10-2 range indicate that soil cracking, subsidence, liquefaction, landslides, and compaction are likely. The soil vulnerability and amplification index values range from 2.36 to 4.37, respectively. These values show the potential level of danger, which might be low, medium, high, or extremely high. Peak ground acceleration varied from 299.52 to 301.52 gals, suggesting high to extremely high danger levels. The microzonation map created for this study is considered to be a useful resource for training disaster mitigation approaches and facilitating infrastructure development planning in the region.

Distribusi Kerentanan Seismik di Wilayah Pusat Kota Surabaya

Surabaya is the second major city of Indonesia and the economic capital of eastern Indonesia. The city’s central area is the governmental center of East Java Province. This area is traversed by the Surabaya section of the Kendeng Fault which could potentially generate a maximum M6.5 earthquake. The East Java megathrust zone also threatens this area with a potential maximum magnitude of M8.9 earthquake. The rock geology of this region is dominated by soft alluvial soil which could amplify earthquake shaking. This study aims to identify the distribution of seismic vulnerability index in Surabaya’s central area. Therefore, microtremor measurements were carried out at 61 points in this area. The results were then analyzed using the Horizontal to Vertical Spectral Ratio (HVSR) method to determine the amplification factor values and seismic vulnerability index. The results of the HVSR analysis show that the amplification factor value and seismic vulnerability index are in the low to medium category ranging from 0.8370 - 3.8298 and 0.6041 - 14.6268, respectively. The distribution of the results shows that the northern area is more vulnerable than the southern part. This is verified by the geological conditions of the northern part which is dominated by alluvial soil.

Determination of the VS profile at a “noisy” industrial site via active and passive data: The critical role of Love waves and the opportunities of multicomponent group velocity analysis

To define the procedures necessary to unambiguously define the subsurface model, a comprehensive set of active and passive seismic data was collected in an industrial area characterized by an extremely high level of background microtremors. Passive data are recorded to define three observables: the dispersion curve of the vertical component of Rayleigh waves via miniature array analysis of microtremors, the Love-wave dispersion curve via extended spatial autocorrelation, and the horizontal-to-vertical spectral ratio (HVSR). Active data used for the holistic analysis of surface waves are extracted from data recorded through a hybrid acquisition procedure accomplished with only two 3C geophones used to simultaneously define the HVSR at two points. Defined observables are combined according to three different approaches: the joint analysis of Rayleigh waves and HVSR, the joint analysis of Rayleigh and Love waves together with the HVSR, and the joint analysis of multicomponent group velocities together with the HVSR and Rayleigh-wave particle motion (RPM) curves. In agreement with the theory, data indicate that, in general, surface-wave modeling cannot be performed considering modal dispersion curves: dispersion obtained from passive data needs to be modeled considering the effective curve, whereas group velocity obtained from active data can be analyzed using the full velocity spectrum technique. Results indicate that joint inversion of Rayleigh-wave dispersion and HVSR does not necessarily ensure the correctness of the obtained S-wave velocity ([Formula: see text]) profile and that Love waves represent a key observable to fully constrain an unambiguous inversion procedure. However, the joint analysis of multicomponent group velocity spectra (from active multicomponent single-offset data) together with the HVSR and RPM curves is a further efficient way to obtain robust [Formula: see text] profiles through the active and passive data obtained by a single 3C geophone.

The Combination of Hvsr and Masw Methods with Landsat 8 Imagery to Assess the Changing Shoreline along the Coastal Area of Central Bengkulu

The Central Bengkulu Regency possesses a coastline measuring 21.8 km, situated in direct proximity to the Indian Ocean. The heightened wave and ocean current dynamics possess the capacity to induce detrimental effects in the form of coastal abrasion along the coastline. This study conducted an examination of the rate of shoreline changes resulting from coastal abrasion in Central Bengkulu Regency, Bengkulu, Indonesia. Horizontal-to-vertical spectral ratio (HVSR) analysis, multichannel analysis of surface waves (MASW), and Landsat-8 remote sensing were all used in the current investigation. The project gets started with conducting field investigations and measurements of geophysical techniques, including the collection of Landsat-8 satellite imagery spanning the period from 1990 to 2020. Additionally, an examination was conducted on the rate at which the shoreline undergoes changes in velocity. In order to determine the increase in change that occurs along coastlines over a period of several decades, a study was carried out during which Landsat-8 satellite images were analyzed. This study investigates the application of the HVSR technique for assessing the seismic vulnerability of the ground and, additionally, the MASW technique for measuring the shear wave velocity of the coastline’s soil. Both of these methodologies are compared and contrasted with one another in this research. The findings indicated that the mean pace of coastline change in Central Bengkulu Regency was 1.5 m/yr, with the maximum velocity recorded at 4.1 m/yr. This high velocity of shoreline change is correlated with the Vs30 value of MASW measurement and Kg of HVSR measurement, where the subsurface soil structure along the coast of Central Bengkulu Regency from Vs30 measurement shows that it is dominated by stiff soil structure that is susceptible to erosion. Outcomes from the study can inform decision-making processes about safeguards and preventative measures to prevent further coastal degradation. HIGHLIGHTS In this paper, we present a framework using application passive and active seismic method with Landsat 8 for knowing the changing of shoreline coastal area in Central Bengkulu Indonesia This study demonstrated that the high velocity of the shifting shoreline is correlated with Vs30 and Kg values The subsurface soil structure is predominantly composed of stiff soil, and the changing shoreline starts at 1.5 mm/yr, which makes the coast of Central Bengkulu vulnerable to erosion GRAPHICAL ABSTRACT

Seismic microzonation and soil-structure resonance analysis in Suryabinayak Municipality, Bhaktapur, Nepal: insights from ambient vibration measurements

Bhaktapur, lying in the Kathmandu Basin, suffered damages during the 2015 Gorkha Earthquake, potentially exacerbated by local site effects. This research addresses the lack of site response study on Suryabinayak Municipality, located in the southern part of Bhaktapur district. Horizontal to Vertical Spectral Ratio (HVSR) and Floor Spectral Ratio (FSR) methods were employed to determine the fundamental frequencies of soil deposits at 241 free field stations and 20 Reinforced Concrete (RC) isolated buildings respectively. The fundamental frequency of soil deposits varies from 0.27 Hz to 10.00 Hz. Higher frequencies were noted near the basin edges, attributed to shallow sediment deposits, whereas a lower frequencies prevailed towards the basin centre due to an increase in sediment thickness. Out of 20 buildings studied, 7 are highly susceptible to soil-structure resonance as the frequency disparity between building and free-field is less than 15%. This research not only quantified the frequency distribution and soil-structure resonance likelihood but also established a correlation between building height and its fundamental frequency. A significant correlation is observed with a coefficient of determination (R2) value of 60.64% and 83.36% in the longitudinal and transverse directions respectively. The study’s results can be endorsed to mitigate seismic hazards, build seismic-resilient structures, and maintain historical monuments.

Microtremor Full-Wavefield Modeling of Effective Phase Velocity and Horizontal-to-Vertical Spectral Ratio at Kyoto Reference Borehole Site: Comparison with Surface-Wavefield Modeling Based on a Velocity Structure with a Cap Layer

ABSTRACT Two types of data commonly used for microtremor exploration are phase-velocity dispersion curves obtained through an array measurement and horizontal-to-vertical spectral ratios (HVSRs) obtainable by a single-station measurement. Phase-velocity dispersion curves obtained by applying the spatial autocorrelation method to the array waveforms have a characteristic peaked shape in some cases. This dispersion curve shape has traditionally been explained as a consequence of the predominance of higher modes over fundamental mode in the Rayleigh waves. In this study, the effects of body waves on phase velocities and HVSRs were investigated based on both field measurements and theoretical calculations of microtremors. We used vertical-component array waveforms and single-station three-component waveforms of microtremors, obtained at and around a site where combined P-wave–S-wave (PS) and density loggings were conducted in the Kyoto basin, Japan (site KD-1), to identify phase velocities and HVSRs at frequencies in the range 0.2–2 Hz. The corresponding theoretical phase velocities and HVSRs were identified using full-wavefield synthetic data, which were generated assuming excitation points randomly distributed over the surface of a horizontally stratified velocity structure model created based on the logging data. The following key results were obtained. The measured phase-velocity dispersion curve exhibits a peaked shape with the value exceeding the S-wave velocity of the Tamba Group (Tb-Group), which is the bedrock (half-space) of the velocity structure model. Theoretical calculations based on the surface-wavefield theory were unable to reproduce this peaked shape; however, theoretical calculations based on the full-wavefield theory reproduced it with extraordinary accuracy. To reproduce the peaked shape based on the surface-wavefield theory, it was necessary to construct a model containing a cap (i.e., high-velocity layer) connected under the Tb-Group. The theoretical calculation based on the full wavefield also accurately reproduced the peak value and peak frequency of the measured HVSRs.