Horizontal To Vertical Spectral Ratio Analysis Research Articles

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.

A Seismic Monitoring Tool for Tidal-Forced Aquifer Level Changes in the Río de la Plata Coastal Plain, Argentina

Ambient seismic noise has gained extensive applications in seismology and plays a pivotal role in environmental seismic studies. This study focuses on the Río de la Plata Coastal Plain, employing the horizontal-to-vertical spectral ratio (HVSR) method on ambient seismic noise records to analyze subsurface dynamics. The region’s hydrogeology is complex, featuring partially interconnected coastal aquifers. The HVSR analysis reveals two peaks, with P0 associated with the sediment-basement interface and P1 linked to a shallower stratigraphic discontinuity. Temporal analysis of P1 highlights cyclical patterns correlated with estuarine levels, suggesting a relationship between variations in seismic velocities and tidal dynamics. Comparisons with aquifer data support the hypothesis that tidal variations influence subsurface mechanical properties, impacting the HVSR function. The study hints at the potential of ambient seismic noise analysis as a non-invasive and cost-effective method for studying coastal aquifers and understanding groundwater dynamics. Ongoing research aims to further explore these relationships for enhanced groundwater resource management.

Assessing Shallow Soft Deposits through Near-Surface Geophysics and UAV-SfM: Application in Pocket Beaches Environments

This study employs a multimethod approach to investigate the sediment distribution in two pocket beaches, Ramla Beach and Mellieha S Beach, in Malta. Both study sites were digitally reconstructed using unmanned aerial vehicle (UAV) photogrammetry. For each case, an ERT and a dense network of ambient seismic noise measurements processed through a horizontal-to-vertical spectral ratio (HVSR) technique were acquired. Electrical resistivity tomography (ERT) analysis enables the estimation of sediment thickness in each beach. HVSR analysis revealed peaks related to beach sediments overlying limestone rocks in both sites and also indicated a deeper stratigraphic contact in Mellieha S Beach. Based on ERT measurements, sediment thickness is calculated for each HVSR measurement. Interpolation of results allows for bedrock surface modelling in each case study, and when combined with digital terrain models (DTMs) derived from photogrammetric models, sediment volumes are estimated for each site. The geometry of this surface is analyzed from a geological perspective, showing structural control of sediment distribution due to a normal fault in Mellieha S Beach and stratigraphic control facilitated by a highly erodible surface in Ramla Beach. The results emphasize the importance of adopting a three-dimensional perspective in coastal studies for precise sediment volume characterization and a deeper understanding of pocket beach dynamics. This practical multimethod approach presented here offers valuable tools for future coastal research and effective coastal management, facilitating informed decision making amidst the growing vulnerability of coastal zones to climate change impacts.

Application of HVSR Method on Microtremor Data for Analysis of Earthquake Potential in Candipuro District, Lumajang, Indonesia

The Earthquake of 6.1 magnitude occurred on April 10, 2021, in Lumajang district. One affected area suffered building losses, road damage, and casualties in Candipuro District, Lumajang Regency, East Java. Then the research was conducted to determine the strength of the soil against the potential earthquake by micro zoning the area in Candipuro District. This research uses the microtremor method with HVSR (Horizontal to Vertical Spectral Ratio) analysis. Analysis of earthquake potential includes parameters of Ground Amplification (A0), Dominant Frequency (f0), Ground Vulnerability Index (Kg), and Ground Shear Strain (GSS). The acquisition was completed with 16 measurement points in Supiturang Village with 500 meters between points using the Portable Seismograph TDS - 303. The results obtained in Supiturang Village have the results of the amplification value factor which is classified as medium-high (2.7 - 8.7), low dominant frequency value (0.5 - 1.4 Hz), medium-high soil susceptibility index (10 - 90), VS30 value which includes medium - hard soil (187.81 - 548.38) m/s, and GSS value which shows the response of cracks and soil subsidence to shaking (2 x 10-5 - 2 x 10-4).

Preliminary result of dominant frequency and seismic amplification in Penajam Paser Utara and its surrounding regions using the HVSR method

East Kalimantan has been designated as the new capital city of Indonesia, precisely in Penajam Paser Utara and Kutai Kartanegara Regency. According to the historical records of seismic activity, there were several destructive earthquakes in East Kalimantan. Therefore, it is necessary to study the characteristics of the local site to assess the seismic hazard potential in the new capital city area and its surroundings. In this research, the Horizontal to Vertical Spectral Ratio (HVSR) method was applied by utilizing microtremor data from 21 seismometers installed in the research area. The results of the HVSR analysis obtained the dominant frequency (f0) and amplification factor (A0) from the H/V curve. Based on the H/V calculation, the highest f0 is 13.04 Hz and the lowest value is 0.58 Hz. Meanwhile, the highest value for A0 is 6.41, and the lowest value is 1.41. The results of the study show that the research area has a low to medium level of seismic hazard.

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

<p class="AbstractText">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.</p>

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

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.

An Early-Warning System to Validate the Soil Profile during TBM Tunnelling

Identification of soil condition at the working face of a tunnel boring machine (TBM) is a key factor for the efficiency and safety of TBM tunneling. The paper presents the first application of the Horizontal-to-Vertical Spectral Ratio (HVSR) method on microtremors induced by a TBM during tunnelling. The innovative application is based on the development of an easy-to-use and economical early-warning system, which aims to confirm, or otherwise, the soil profile established in the design phase of tunnels by comparing the soil natural frequencies obtained from the soil profile carried out during the design phase and the soil natural frequencies coming from the HVSR analysis of the microtremors induced by the TBM during tunnelling. Just one or two geophones are necessary to use the proposed procedure. It can be applied to an area up to about 20 m ahead of the TBM excavation front and constitutes a powerful early warning system. Due to the great heterogeneity of the subsoil, dual-mode TBMs are often used, frequently changing from Open-Face (OF) mode for rock formations to Earth Pressure Balance (EPB) mode for cohesive and incoherent soil. Any “additional” information on the soil, which will be dug in the next days or hours is extremely useful for subsoil with great heterogeneity. The new procedure offers a reasonable time interval in which to modify the excavation method. This, in turn, can avoid damage to the TBM and existing structures and infrastructures. It allows us also to have a valuable geotechnical database for future works on the infrastructural networks. The proposed procedure has been successfully applied during the construction of the new underground lines in Catania (Italy).

Site characterizations of BMKG seismic network, Indonesia, based on HVSR analysis

In the last few years BMKG (Agency for Meteorology Climatology and Geophysics) has increased the number of seismic stations significantly. Until mid-2020, when this study was conducted, the number of BMKG stations has reached 339 units. Development of the network is aimed to improve the data quality, speed of earthquake data processing and information dissemination to the public, accuracy of the hypocenter, as well as the magnitude. The objective of this study is to identify the site characteristics of the network. We analysed the noise recorded at BMKG stations throughout Indonesia using the HVSR (Horizontal to Vertical Spectral Ratio) method. The results of HVSR analysis were used to classify the site conditions of each station. We got the number of stations with the classification of hard rock, rock, hard soil, medium soil, and soft soil, 47, 57, 52, 30, and 84, respectively. This site condition represents the stations characteristics and affects the quality of seismic waveform data.

Estimation of near-surface shear-wave velocity by joint inversion of dispersion and HVSR curves of Rayleigh waves from multicomponent active-seismic records

The multichannel analysis of surface waves (MASW) method has been widely used for near-surface geophysical and geotechnical studies by using the dispersive characteristics of Rayleigh waves to determine subsurface shear-wave velocity. Commonly, Rayleigh waves are obtained only from the vertical component of P-SV waves. However, the multimodal Rayleigh waves estimated from the horizontal (radial) component data have a different energy distribution compared with those estimated from the vertical component data. The horizontal-to-vertical spectral ratio (HVSR) of Rayleigh waves can be calculated from multicomponent active-seismic records after mode separation. The HVSR peak and trough frequencies are very sensitive to model parameters. We propose to estimate near-surface shear-wave velocity by joint inversion of dispersion and HVSR curves of Rayleigh waves from multicomponent active-seismic records. The proposed joint inversion method can combine the strengths of MASW and HVSR analysis methods. A synthetic example demonstrates that our proposed joint inversion method can provide better constrained and more reliable S-wave velocity in the shallow subsurface.

Near-surface structure from ambient-noise tomography and horizontal-to-vertical spectral ratio beneath the Nankou-Sunhe fault*

Active faults pose a great threat to urban security. As the largest NW-trend active fault in Beijing area, the Nankou-Sunhe fault plays an important role in earthquake disaster and city construction. In this study, we collect continuous ambient noise data recorded by 43 temporary short-period seismograph between September 21th to October 12th 2019 to investigate the near-surface structure beneath the Nankou-Sunhe fault by using ambient noise tomography (ANT) and horizontal-to-vertical spectral ratio (HVSR) method. From ambient noise processing, fundamental-mode Rayleigh wave signals are clearly observed in the frequency band of 0.4–2.5 Hz. Then direct surface-wave inversion algorithm is applied to calculate the 3D shear-wave velocity model. Our results show that there is a sharp velocity contrast across the Nankou-Sunhe fault, with low velocities down to about 2 km on the hanging wall and high velocity on the footwall of the fault. According to the geological investigation, the low velocities are related to thicker sediments and Jurassic volcanic rock below which are the cap rock of the hydrothermal system. From the HVSR analysis, the HVSR curves of the sites near the fault shows double-peak, one less than 1 Hz and the other centered 7 Hz. After converting frequency to depth by the empirical equation, the results show that the thickness of sediments is thinned from southwest to northeast, which generally agrees well to field survey. Our results provide high-resolution near-surface structure for future study on disaster risk reduction and urban planning.

Assessment of seismic vulnerability using the horizontal-to-vertical spectral ratio (HVSR) method in Haenam, Korea

The town of Haenam in southwest Korea lies partially on reclaimed coastal land and experienced an unprecedented earthquake swarm during April and May 2020. Construction of a new town in the area means that there is demand to evaluate the seismic hazard caused by site-specific amplification of seismic ground motions by artificial unconsolidated cover. We used a microtremor horizontal-to-vertical spectral ratio (HVSR) method to identify resonance frequencies, image depths to bedrock, and assess seismic ground vulnerability across the epicentral area of the recent earthquake swarm. Microtremor measurements were taken at 144 sites across an 18.5 km × 12.5 km study area. The HVSR analysis shows resonance frequencies as low as 1.1 Hz. Using mean shear-wave velocities obtained by Multi-Channel Analyses of Surface Waves (MASW), we find that resonance frequencies correspond to depths to bedrock of between 3 m and 56 m. There are clear relationships between the distribution of resonance frequencies, depths to bedrock, surface geology, and areas of development in recent land reclamation projects. Higher resonance frequencies characterize areas of thin sedimentary cover or hard rock sites, while relatively low resonance frequencies characterize reclaimed land or greater depths to bedrock. We quantify the seismic hazard potential across the study area using the seismic vulnerability of ground index (Kg), and observe a clear association between zones of high Kg and areas of reclaimed land. This study demonstrates the value of the HVSR technique in characterizing the subsurface and assessing seismic vulnerability in areas of reclaimed land, where it can be used as a viable and low-cost alternative to large-scale active seismic refraction or MASW measurements. The results can also be used to inform and guide urban planning projects.

A study of horizontal-to-vertical component spectral ratio as a proxy for site classification in central Asia

SUMMARYTien Shan of central Asia is known as one of the world's largest, youngest and most active intracontinental orogens. In this study, we implemented the horizontal-to-vertical spectral ratio (HVSR) technique as a widely used first-order approximation of the site effect parameters (i.e. fundamental frequency and site amplification). A set of data including 2119 strong-motion recordings from 468 earthquakes with hypocentral distances up to 500 km and small to moderate moment magnitudes ($ {M_{\rm{w}}}\sim $3.0–5.5) recorded by 24 broad-band stations from five different networks, located in Afghanistan, Tajikistan and Kyrgyzstan was deployed to investigate site-specific characteristics. We fitted a Gaussian-shape pulse function to evaluate fundamental frequencies and site amplifications. The HVSRs analysis revealed that although the majority of the stations (16 out of 24) show flat amplification functions, there are few stations with single sharp amplification functions. Then, we classified the stations based on the predominant frequency. Furthermore, we approximated the time-averaged shear wave velocity in the uppermost 30 m (${V_{{\rm{S}}30}}$) using the fundamental frequency and its corresponding amplitude. Moreover, we compared the HVSRs obtained from P waves, S waves, coda and pre-event noise. All peak frequencies including the fundamental frequency estimated from different seismic phases are in good agreement; whereas generally, the amplitude of the P-wave window is the lowest, the amplitudes of the S wave and noise windows are similar to the whole record and the amplitudes of early and late coda windows are the highest. We also observed that the HVSRs of noise using a 5 s window may have anomalous high amplitudes and peaks. These anomalous high amplitudes and peaks in the noise HVSRs indicate the existence of some unnatural sources or artefacts such as traffic and wind with specific resonance frequencies, suggesting 5 s ambient noise window is insufficient to capture site characteristics. Finally, to assess the reliability of the determined geotechnical results, we implemented a blind theoretical HVSR inversion to obtain representative shear wave velocity profiles as well as ${V_{{\rm{S}}30}}$ along with associated uncertainties for stations characterized by a single-peak HVSR curve using a Bayesian statistical framework.

Microtremor HVSR analysis of heterogeneous shallow sedimentary structures at Pohang, South Korea

AbstractWe apply the single-station microtremor horizontal-to-vertical spectral ratio (HVSR) method to image the geometry of Pohang Basin, South Korea, which experienced the greatest earthquake damage in Korea during a series of anthropogenic earthquakes between November 2017 and February 2018. We collected and analysed the ambient seismic noise at 124 temporary stations. The resonance frequencies, which vary significantly across the area (0.35–19.86 Hz), were inverted to constrain the depth of the major impedance contrast, which is interpreted to be the sediment–bedrock interface beneath each station. The sedimentary layer thickness is generally thin in the north and thickens to the southern and central parts of the basin, where the depth to bedrock is up to 340 m. We compare the HVSR results with six borehole observations in the area, whose depth to bedrock ranges from 189 to 359 m. The sediment thicknesses obtained via the direct borehole measurements and HVSR method are comparable with each other. The resultant three-dimensional shape of the sedimentary basin provides crucial information for the microzonation of the Pohang area for seismic risk mitigation. It also provides a realistic initial velocity model for three-dimensional tomographic inversions to elucidate the detailed subsurface structure of the region.

Non-stability of the noise HVSR at sites near or on topographic heights

SUMMARYThe horizontal to vertical spectral ratio (HVSR) of seismic noise is often used to investigate site effects, and it is usually assumed to be a stable feature of the site considered. Here we show that such an assumption is not always justified, and may lead to incorrect conclusions. The HVSR analysis was performed on ambient seismic noise recordings lasting from weeks to months at many sites in Calabria, Italy. Results show a variety of site effects, from the resonance of a shallow sedimentary layer to the polarized amplification of horizontal ground motion associated with topographic effects. We describe the results of seven sites whose HVSR is characterized by dual content: one that is persistent, and another appearing only occasionally. Two sites very near the coast of the Tyrrhenian sea and five sites in the Calabrian Arc mountains show the most remarkable results. The shape of the HVSR changes significantly at these sites when the amplitude of background noise increases in a broad frequency band during periods of bad weather. The occasional contribution to the HVSR consists of one or more peaks, depending on the site, that appear only when the amplitude of ambient noise is higher than usual. The seven sites where we observe the HVSR variability are all located in complex geological environments, on mountains, ridges or foothills. A variation of the HVSR correlated with the day–night cycle is also observed at some of these sites.

Integrated Analysis of Microtremor Horizontal to Vertical Spectral Ratio, Surface Waves Dispersion Curve, and Seismic Refraction Tomography to Estimate Weathered Layer Thickness and Seismic Vulnerability: Case Study Kalirejo Village, Kokap Sub-District, Kulon Progo Regency

Subduction of Indo-Australia plate to Eurasia plate and locally active fault nearby Kulon Progo play as major source for earthquake events. After effect due to earthquake has different level of damage which depend on the magnitude and site characteristics. The horizontal-to-vertical spectral ratio (HVSR) passive seismic method is being used drastically to help in mapping the level of site vulnerability to earthquake event. HVSR analysis results help us acquire some physical values including weathered layer thickness where Vs 30 reference came from surface waves dispersion curve analysis of the MASW method as it is used as a parameter in calculating thickness value. Seismic refraction tomography is used to create subsurface model thus we may see the extent of underlying layer and its implication to earthquake event.Data measurements distribution are scattered in Kalirejo Village with the total of 63 passive seismic data, 33 MASW data, and 7 lines of seismic refraction acquisition. Some data show inadequate quality to be taken into further processing step, so data sorting activity should be carefully done. As a result, 21 of 63 passive seismic data are considered adequate to represent site physical values. Dominant frequency values ranging from 2 to 20 Hz, amplification factor varies between 1.5-12.5, and seismic vulnerability indices varies between 0.3-20. Surface waves dispersion curve inversion results are Vs 30 values ranging from 350 m/s to 980 m/s and seismic refraction tomography model shows Vp model with velocity values ranging from 0.2 to 3.2 km/s.

Site Characterization of Soil-rock Mixture Sedimentary Stratum Based on HVSR Analysis in the Chinese Loess Plateau

The soil-rock mixture sedimentary stratum is a compound with complex and loose topography, of which the structure is difficult to detect by the ordinary geophysical method. There is a need for a convenient, efficient and effective geophysical method to detect site effects in this area. This paper is an application of the S wave velocity profile inversion for the soil-rock mixture sedimentary stratum, using HVSR (Horizontal to Vertical Spectral Ratio) analysis of ambient noise by some three-component observations in the Chinese Loess Plateau. We carried out the measurement using three nested circular arrays and data recording systems with a spectrum expansion circuit. Inversion of the HVSR curves was performed by a three-layer model. Results of geological observation reveal that the upper part of the sedimentary stratum is Quaternary strata containing a large amount of humus and loess, the middle layer part is the stratum of the loose gravel and the under part is completely weathered granite with homogeneous lithology and fewer rocks. Interpretation results are consistent with previous drilling data, providing a valid geophysical basis for evaluating the stability of the soil erosion and designing a reasonable water and soil erosion control scheme.

Estimation of horizontal-to-vertical spectral ratios (ellipticity) of Rayleigh waves from multistation active-seismic records

The horizontal-to-vertical spectral-ratio (HVSR) analysis of ambient noise recordings is a popular reconnaissance tool used worldwide for seismic microzonation and earthquake site characterization. We have expanded this single-station passive HVSR technique to active multicomponent data. We focus on the calculation of the HVSR of Rayleigh waves from active-seismic records. We separate different modes of Rayleigh waves in seismic dispersion spectra and then estimate the HVSR for the fundamental mode. The mode separation is implemented in the frequency-phase velocity ([Formula: see text]-[Formula: see text]) domain through the high-resolution linear Radon transformation. The estimated Rayleigh-wave HVSR curve after mode separation is consistent with the theoretical HVSR curve, which is computed by solving the Rayleigh-wave eigenproblem in the laterally homogeneous layered medium. We find that the HVSR peak and trough frequencies are very sensitive to velocity contrast and interface depth and that HVSR curves contain information on lateral velocity variations. Using synthetic and field data, we determine the validity of estimating active Rayleigh-wave HVSR after mode separation. Our approach can be a viable and more accurate alternative to the empirical HVSR analysis method and brings a novel approach for the analysis of active multicomponent seismic data.

Local Site Effects Evaluation Using Microtremor Measurements at North Side of Pandan Mountain

Disaster mitigation has been undertaken to reduce the impact of loss or damage caused by the earthquake. For disaster mitigation purpose, it is necessary to conduct a local geological assessment. The horizontal to vertical spectral ratio (HVSR) of microtremor analysis is very popular in the context of seismic micro-zonation. Microtremor method is cheap, data acquisition is easy, and does not cause noise, so suitable for use in residential areas. This research aims to determine the distribution of the natural frequency value, the amplification factor value, and the soil vulnerability index value. This study was conducted geographically between 7°26'17.45" – 7°26'53.16" S and 111°47'14.76" – 111°48'7.06" E. Data processing using HVSR analysis method on Easy HVSR software. The results showed that the natural frequency (<em>f<sub>o</sub></em>) values range 0,95 Hz to 8,3 Hz and the amplification factor values range 2,05 to 8,67. From the result data of the natural frequency and the amplification factor, the soil vulnerability index can be calculated. The vulnerability index (Kg) value ranges from 1,09 to 68,33. The northeast side of the study area is indicated as a weak zone where have high potential to damage when the earthquake occurred.