Microtremor Observations Research Articles

Analysis of the Liquefaction Potential at the Base of the San Marcos Dam (Cayambe, Ecuador)—A Validation in the Use of the Horizontal-to-Vertical Spectral Ratio

Ground liquefaction potential analysis is a fundamental characterization in areas with continuous seismic activity, such as Ecuador. Geotechnical liquefaction studies are usually approached from dynamic penetration tests, which pose problems both in their correct execution and in their evaluation. Our research involves analyzing dynamic penetration tests and microtremor geophysical surveys (horizontal-to-vertical spectral ratio technique, HVSR) for analyzing the liquefaction potential at the base of the San Marcos dam, a reservoir located in Cayambe canton (Ecuador). Based on the investigations performed at the time of construction of the dam (drilling and geophysical refraction profiles) and the application of 20 microtremor observation stations via the HVSR technique, an analysis of the safety factor of liquefaction (SFliq) was conducted using the 2001 Youd and Idriss formulation and the values of the standard penetration test (SPT) applied in granular materials (sands). In addition, the vulnerability index (Kg) proposed by Nakamura in 1989 was analyzed through the HVSR records related to the ground shear strain (GSS). The results obtained in the HVSR analysis indicate the presence of a zone of about 100 m length in the central part of the foot of the dam, whose GSS values identified a condition of susceptibility to liquefaction. In the same area, the SPT essays analysis in the P-8A drill hole also shows a potential susceptibility to liquefaction in earthquake conditions greater than a moment magnitude (Mw) of 4.5. That seismic event could occur in the area, for example, with a new activity condition of the nearby Cayambe volcano or even from an earthquake from the vicinity of the fractured zone.

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 > 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 > 1.0 are found in West Aceh and the Geureudong volcano area.

The Impact of Ground Irregular Sedimentary Structure on the Seismic Motion Amplification Characteristics: A case study in Tottori, Japan

The amplification characteristics of seismic motion are determined by the ground structure. In design practice, the ground is assumed to be horizontally stratified. However, the actual ground forms irregular sedimentary structures and the propagation direction of the seismic motion in the soil changes in a complicated way. Thus, the actual amplification factor of seismic motion is dramatically different from the value assumed in design practice. This phenomenon is called the multidimensional effect. The present study targeted at a seismic observation point in Tottori Prefecture, Japan, and estimated the irregular ground structures based on the microtremor observation results. With this ground structure model, Finite Element Analysis (FEA) was conducted and the amplification factors were compared with those determined assuming horizontal stratification. When there is no ground nonlinearity, the multidimensional effect of the ground was more notable in points with thick sedimentary strata, where the peak amplification factor according to the FEA was 1.9 to 3.6 times larger than when horizontal stratification was assumed. In points with thin sedimentary strata, the peak amplification factor ratio was 2.1–2.4. First-order peak frequency was different between cases with irregular ground structures and with horizontal stratification. Furthermore, when the nonlinearity of the ground was evident, the multidimensional effect on the peak amplification factor was not as noticeable as when the ground behaved linearly. The peak magnification ratio due to the multidimensional effect was found to be 2.3. The results of this study show that the amplification characteristics of the seismic motion considered in design practice are likely to be on the dangerous side when the ground is not horizontally stratified.

Estimation of phase velocity using array observation of microtremors with arbitrary shape

To estimate the phase velocity using the array observations of microtremors, some algorithms for the estimation include constraints on the array shape, such as equilateral triangles or the placement of receivers on a circle, in order to reduce the estimation error of the phase velocity. In the present study, a direct estimation technique is introduced for the phase velocity using records obtained through an array with an arbitrary shape based on a complex coherency function (CCF), where CCF is defined as the normalized cross spectrum of the microtremor records observed simultaneously by two receivers. The particle swarm optimization (PSO) method, one of metaheuristic optimization methods, is applied and optimal values are provided for the phase velocity and other unknown parameters. Approximate representations of the stochastic properties for the unknown variables are analytically derived based on the discrete representation of the CCF, for a case where the arrival directions of microtremors are treated as random variables following a uniform distribution. Furthermore, the validity of the proposed method is confirmed using numerical simulations and actual observation records.Graphical

東京低地の埋没谷形状と地盤震動特性：上野-小岩測線での検討結果

To better understand how ground motion in the Tokyo Lowland is affected by the geological conditions in the shallow subsurface, we analyzed borehole logs and conducted microtremor observations along the Ueno-Koiwa survey line, which crosses the buried valley beneath the Tokyo Lowland. Remarkable peaks in the H/V spectra at ~1 Hz were found in the area of the lower buried terrace (buried flat surface 2), whereas the peaks at ~1 Hz are small in the area of the buried valley bottom with the thickest valley-filling post-Last Glacial Maximum (LGM) deposits. The sharp peaks around buried terrace 2 are generated by the large contrast in the physical properties of the soft, muddy post-LGM deposits and the underlying gravel bed, which suggests larger ground-motion amplification during an earthquake. The variation of the ground motion characteristics along the survey line obtained by microtremor observation is consistent with the distribution of seismic intensity during the 1923 Kanto Earthquake. It is, therefore, important to consider not only the thickness and physical properties of the post-LGM deposits during seismic hazard assessment, but also the total geological composition, including the Pleistocene strata below the post-LGM deposits.

Analysis of Soil Dynamics and Seismic Vulnerability in Kalibening District, Banjarnegara Using the HVSR Method

On April 18, 2018, an earthquake with a magnitude of 4.4 Mw struck the Kalibening district, Banjarnegara. The observations on the damage distribution caused by the earthquake showed something unique: the areas that suffered severe damage were concentrated in villages on hills. This observation may indicate a site-effect phenomenon due to local geological conditions. In this study, we carry out microtremor measurements at 30 stations. The measurement area includes areas that have suffered damage due to the 2018 earthquake and also with minimum damage. We analyze the microtremor observation to estimate the site’s characteristics and map the seismic vulnerability based on predominant frequency (f0 ) and amplification factor (A0 ) using the Horizontal to vertical spectrum ratio (HVSR) method. The results show that the predominant frequency in the Kalibening district area is 0.4 – 3.7 Hz, while the amplification factor is 2.5 – 10.7. The Kalibening Basin has a low f0 (<2.5 Hz) and A0 (<4), which indicates that the Kalibening Basin has a thick and soft soil type but low-density contrast. Areas with a high degree of damage have dominant frequency values in the range of 1.5 – 2.3 Hz and amplification of 5.1 – 9.2. These results suggest that the location has a high-density contrast and thick sediment, causing significant amplification. The seismic vulnerability index (Kg) values are in the range of 3.6 - 46.74. The area around the Kalibening Basin has a low Kg value, While the damaged areas have a moderate to high seismic vulnerability index.

The depth of alluvial sediments and subsurface profiling along the Eskişehir Basin in Central Turkey using microtremor observations

The depth of alluvial sediments and subsurface profiling along the Eskişehir Basin in Central Turkey using microtremor observations

An assessment of uncertainties in VS profiles obtained from microtremor observations in the phased 2018 COSMOS blind trials

Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 (VS10), 30 (VS30), 100 (VS100), and 300 (VS300) meters and show its usefulness in quantitative comparisons of VS profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects.

New method to estimate bedrock shape of small-scale basin using modal properties of sediment

2-D resonances are known to occur in sedimentary basins, and it has been reported that the fundamental mode shapes appear to reflect the bedrock shape. This suggests that the bedrock shape of a valley can be estimated by evaluating the relationship between the mode shapes and the bedrock shape, and by identifying the resonance modes from the observation records of the ground motions. In the present study, the fundamental mode shapes of a small-scale valley were identified by applying the frequency domain decomposition (FDD) technique to microtremor observation records. 2-D finite difference models were used to investigate the relationship between the mode shapes and the bedrock shape. A new method was proposed for estimating the bedrock shape by comparing the mode shapes obtained from observation records and a numerical analysis. Furthermore, the applicability of the new method was confirmed using synthetic and observed data.

Seismic Performance Evaluation of Existing Building in Earthquake Prone Area Based on Seismic Index and Seismic Demand Method

Padang is the capital of West Sumatra Province, close to three earthquake sources such as the Megathrust Zone, the Mentawai Fault, and the Great Sumatra Fault on the mainland. Due to the fact that the impact of the earthquake can create a tsunami event in the future, the government needs to prepare shelters for vertical evacuation. About 65 existing buildings have been selected as vertical evacuation buildings, but there is no clear information on their seismic performance. This research proposes a complete assessment of the existing RC building by considering soil characteristics such as the predominant period and Vs30 from microtremor observations. A selected 3 stories of a school building located in the earthquake red zone (Vs30 < 150m/s and predominant period T > 1.5s), less than 1 Km far from the shoreline, was considered in this paper. Based on the seismic index evaluation of the building, the X direction is smaller than the seismic demand (0.8) for the first and second floors (unsatisfactory). For the first, second and third floor, the Y direction has a larger seismic index than the seismic demand. Therefore, the performance is satisfactory for the first to the third story. Based on the required inspection, the building is unsatisfactory for the X-direction. Therefore, it needs to be strengthened. We propose a re-size of the RC for the building column to improve the ductility. The proposed size for the column is 50 x 60 cm. From the re-seismic evaluation, a satisfactory seismic index for the X and Y direction can be obtained.

ESTIMATION OF SUBSURFACE STRUCTURES AND GROUND MOTION CHARACTERISTICS IN THE AREAS DAMAGED IN THE 2016 CENTRAL TOTTORI PREFECTURE EARTHQUAKE IN JAPAN

On 21 October 2016, an earthquake with a magnitude of 6.6 (determined using the Japan Meteorological Agency’s [JMA] seismic network) occurred in the center of Japan’s Tottori Prefecture. In the city of Kurayoshi and in the towns of Yurihama and Hokuei, JMA seismic intensities of lower 6.0 were observed, and structural damage to housing was concentrated in a limited number of areas. This research conducted microtremor observations around damaged areas and used previous studies to estimate subsurface structures and ground motion characteristics. Low-velocity layers with an S-wave velocity of between 80 and 200 m/s were estimated at all observation sites and a tendency is present for the thickness to increase towards the coastal plains and decrease inland. In an evaluation of the relationship between ground motion characteristics and earthquake damage, researchers found that the occurrence of damage correlated to significant changes in layer thickness. Researchers believed that the damage correlates to the 2-D or 3-D effects of subsurface structures

Assessment of seismic vulnerability index of RAJUK area in Bangladesh using microtremor observations

Microtremor Horizontal to Vertical spectral ratio technique, also known as the Nakamura’s method is growing in status for site response analysis. 500 locations in RAJUK area (1530 km2) have been selected for microtremor observations. Microtremor data have been compiled and studied to estimate the predominant resonance frequency and H/V peak amplitude following the SESAME (2004) guideline. Finally, seismic vulnerability index of site soil using Nakamura’s technique has been determined from predominant resonance frequency and H/V peak amplitude parameter. The calculated seismic vulnerability index for the studied 500 locations varies between 0.16 and 7.28. The low seismic vulnerability index (Kg) value means that the areas are relatively stiff and underlain by substantial deposit of sediments. The relatively higher Kg values are spread in the soft alluvial deposit areas. The areas with high Kg values are considered as fragile zones that may initiate significant damage to infrastructure situated in those areas during an earthquake.

Relationship between Shear Velocities Recorded by Microtremor Observations and Seismic Cone Penetration Test Results

This research proposes a relationship between two methods such as a numerical approach by conducting a microtremor array observation and field survey by using the seismic cone penetration test unit (SCPTu). A database of shear-wave velocity (Vs) measurements was established using the microtremor array technique and seismic cone penetration test unit (SCPTu) on high-quality samples of rock and soft soil in Padang city, Indonesia. The study also demonstrates that the Vs values obtained from the different methods are consistent with the microtremor array technique. This technique may thus be deemed a valuable tool, as it can be used in engineering practice with confidence. Comparison of the Vs for different soils at the first layer between the microtremor array observation results and the SCPTu results exhibited the microtremor array method is unable to determine the Vs at the layer where its Vs changes dramatically, such as at the same layer as station UNP at 2 to 3.5m deep.

Quantitative Evaluation of the Seismic Reinforcement Effect Based on Observed Microtremors

To validate the seismic retrofitting of buildings quantitatively, we observed microtremors in the Uji campus buildings at Kyoto University. We compared the dynamic characteristics derived from the resonant frequency of microtremors in a five-story steel-frame structure and a three-story reinforced-concrete (RC) building before and after retrofitting. We calculated the weight of the building from the retrofitting design plan and constructed a numerical model of the structure by estimating the building stiffness that fits the resonant frequency. Assuming the characteristics of the restoring force for each story of the numerical model from previous studies, we calculated the non-linear response of the building to the strong motions predicted for earthquakes occurring at the Obaku fault system that runs right in front of the Uji campus. We compared the response of the numerical models constructed before and after retrofitting to verify the effects of the seismic retrofitting. We found that the proposed method of dynamic response model construction using microtremor observations is effective when the reinforcement members are rigidly fixed to the existing structure, as is the case for RC buildings.

Effects and dynamic characteristics of the core‐suspended isolation system assessed by long‐term structural health monitoring

AbstractThe seismic isolation mechanism of the core‐suspended isolation (CSI) system comprises a double layer of inclined rubber bearings installed on top of a reinforced core structure. A multilevel structure is then suspended from a hat truss or umbrella girder constructed on the seismic isolation mechanism. The first building to use the CSI system was the Safety and Security Center in Tokyo, Japan, whose structural health monitoring system has detected and recorded 231 earthquake motions since 2006, including the 2011 Tohoku Pacific Earthquake (2011‐TPE). The present study estimates the dynamic characteristics of this CSI‐equipped building in earthquakes, and the effects of the CSI system are revealed via the observed earthquake records. The temporal changes of the fundamental period and damping factor are estimated from the 2011‐TPE; the fundamental period increases with the deformation of the isolation level, whereas the fundamental damping factor is only related weakly to that deformation. The 231 observed earthquake records reveal that the CSI system performs seismic isolation by reducing the response acceleration of the suspended structure by roughly a half for peak ground accelerations exceeding 30 cm/s2. Daily microtremor observations are used to diagnose earthquake damage; the fundamental frequencies in each direction remain almost constant and were not changed by the 2011‐TPE.

Natural Frequency Measurements of the Fluid-Elastic-Coupled Shell Plate Vibration in a Large-Sized Cylindrical Steel Tank by Microtremor Observations

AbstractNaturally occurring microtremor observations measured the natural frequencies of the fluid-elastic-coupled shell plate vibration (bulging) in a large flat-bottomed cylindrical steel tank with a capacity of 125,000-m3. Five peaks appear in the observed microtremor spectral ratios of the top or midheight of the shell plate to the bottom on the tank foundation. Comparing the spectral ratios to the solutions obtained by FEM eigenvalue analysis assuming a fixed base suggests that the five peaks are the bulging modes of (m, n)=(1, 1–5), where m and n denote the vertical order and the circumferential wavenumber, respectively. The measured non-soil-coupled natural frequencies from the spectral ratio agree fairly well with those obtained from FEM analysis. The measured natural frequencies of the fundamental mode (m = n = 1) also agree well with those projected by a simplified equation developed under the assumption of a fixed base, which is adopted in the seismic codes of the Japanese Fire Service Act. This equation should provide a reliable soil-coupled natural frequency of the fundamental mode for a tank situated on firm ground in which the storage-soil-coupled effects are presumed weak. Additionally, a simple method is presented to determine the non-soil-coupled natural frequency of the fundamental mode from the observed microtremor spectral ratios without referencing the FEM eigenvalue solutions. This simple method works very well for the tank examined.

Estimation of the subsurface structure in Georgetown, Penang Island using single point microtremor observation technique

In this paper, the subsurface structure was estimated using single point microtremor observation technique in Penang Island, Malaysia. Malaysia is located out of Pacific Ring of Fire hence no active tectonic activity happens. However, numerous earthquakes that happened at neighbouring countries act contrary to the myth that Malaysia is seismic free. Tremors from far field earthquake events in Sumatra can be felt at Penang Island. The local site condition is investigated in this study in order to predict the damages on buildings when the earthquake strikes. The study area focused on Georgetown as it is the heart of Penang Island where the economy and population concentrated on. The objective of this study is to determine V s,30 substructure of Georgetown area. Microtremor single point measurements were conducted to estimate the V s profile at Georgetown area. For estimating the ground structure, Rayleigh wave ellipticity method was adopted to single point measurements. From the results, it is found that Georgetown area has V s,30 of 216.4 m/s to 286.6 m/s and the predominant frequency ranging from 1.7 Hz to 2.43 Hz. Hence, the ground type of Georgetown area is classified as Class C in accordance to Eurocode 8.

Estimation of earthquake local site effects using microtremor observations for the Garhwal–Kumaun Himalaya, India

ABSTRACTThe Garhwal–Kumaun region of the Himalaya encompassing the state of Uttarakhand, India, has experienced several earthquakes in the past. Damage due to earthquakes is controlled by local site conditions, primarily resonance frequency and wave amplification from the ground. We present local site parameters with their site geology for 37 sites using ambient noise data. Horizontal to vertical spectral ratio technique is used to estimate the spectral ratio curves. Based on the type of curve, sites are classified into four classes, viz. clear peak, broad peak, double and multi‐peak, and flat H/V curve. Sites seen with clear or broad peaks are located on either soil or weathered rocks, thus indicating large impedance contrast and sharp discontinuity with large velocity contrast. Multiple peaks are observed in either soil or boulder bed and reveal large impedance contrast, probably representing shallow and thick strata. Sites with flat curves are found on weathered/phyllite/granite gneiss/granite schist rock types within highly dissected hilly areas. Fourteen sites have a peak frequency &gt;6 Hz with a dominance of broad and clear peaks in the Lesser and Higher Himalayan regions. On the contrary, foothills and part of Siwalik sites exhibited a peak frequency between 1.14 and 4.94 Hz. The results demonstrate that sites with thick soil cover and boulder bed areas, that is, Doon valley and foothills, show low‐frequency peaks and hard rock or shallow bedrock sites, that is, Lesser and Higher Himalaya exhibit a higher frequency range. The estimated H/V amplitude and peak frequency values have shown a good correlation with site geology and geomorphology.

Estimation of Subsurfacel Structure of Katsuyama Basin Based on Microtremor Observation

Estimation of Subsurfacel Structure of Katsuyama Basin Based on Microtremor Observation

Estimating the shear wave velocity structure above the fresh bedrock based on small scale microtremor observation array

S-wave velocity and thickness of weathered strata are the important parameters to engineering construction. In this study, array measurements of microtremors were carried out at 50 sites in Jimo, China. It revised the observation array that a small scale array was placed in the center of traditional regular triangle observation array. The field data of Jimo verified that it is effective to identify the different degrees of weathered strata above fresh bedrock. Based on the spatial autocorrelation (SPAC) method, the dispersion curves were achieved. While the S-wave velocity structure was estimated using the fork genetic algorithm. By using small scale observation array, it identifies different strata including weathered layer in detail. In order to test the accuracy of inversion results, 5 sites were chosen to drill. Comparison between inversion results and boreholes, it shows agreement with logs data. At last, the S-wave velocity structures in different depths above the fresh bedrock were plotted. We delineated the S-wave velocity structure for the first time in this research region using revised measurement of microtremors signal. The depth of fresh bedrock fluctuates strongly which is affected by weathered. This method provides another means to distinguish the degree of weathered, depth of fresh bedrock, and S-wave velocity structure of shallow strata.