Microtremor Observations Research Articles

Shallow subsurface structure estimated from dense aftershock records and microtremor observations in Furukawa district, Miyagi, Japan

We conducted single-site and array observations of microtremors in order to revise the shallow subsurface structure of the Furukawa district, Miyagi, Japan, where severe residential damage was reported during the Great Eastern Japan Earthquake of 2011, off the Pacific coast of Tohoku. The phase velocities of Rayleigh waves are estimated from array observations at three sites, and S-wave velocity models are established. The spatial distribution of predominant periods is estimated for the surface layer, on the basis of the spectral ratio of horizontal and vertical components (H/V) of microtremors obtained from single-site observations. We then compared ground motion records from a dense seismometer network with results of microtremor observations, and revised a model of the shallow (~100 m) subsurface structure in the Furukawa district. The model implies that slower near-surface S-wave velocity and deeper basement are to be found in the southern and eastern areas. It was found that the damage in residential structures was concentrated in an area where the average value for the transfer functions in the frequency range of 2 to 4 Hz was large.

Characteristics of long period microtremor and validation of microtremor array measurements in inland areas of China

To study the characteristics of long period microtremor and applicability of microtremor survey, we have made microtremor observations using long period seismometers of the China’s mainland from coastal cities like Shanghai and Tianjin through Beijing, Xi’an, to the far inland cities of Lanzhou and Tianshui. The observation shows that the level of power spectrum of long period microtremors reduced rapidly from the coast to the inland area. However, the energy of long period microtremors in Beijing, Xi’an, Lanzhou and Tianshui area are only approximately ten-thousandth to thousandth of that in Shanghai. Aiming at the complexity of the underground structure in the far inland, a series of underground structure models with different distributions were assessed using three-dimensional, dynamic finite element method (FEM) analyses. The results were used to evaluate microtremor survey methods and their limitations with regard to aggregate variability and thickness determinations. Multiple-wave reflections between layers with the change of underground structure distribution occurred, which have significant effect on the performance of the different field approaches. Information over a broad spectrum from which velocity-depth profiles were produced via inversion approaches. Neither the thickness nor the shear wave Velocity V of the subsurface layer inversion results appeared over a large evaluation with increasing slope angle. In particular, when the angle of the model reached 45°, it could not be accurately evaluated using one-dimensional inversion methods. Finally, the array microtremor survey (AMS) was carried out in Shanghai City, China. Although AMS techniques do not have the layer sensitivity or accuracy (velocity and layer thickness) of borehole techniques, the obtained shear wave velocity structure is especially useful for earthquake disaster prevention and seismic analysis.

Building damage survey and microtremor measurements for the source region of the 2015 Gorkha, Nepal, earthquake

We performed a damage survey of buildings and carried out microtremor observations in the source region of the 2015 Gorkha earthquake. Our survey area spans the Kathmandu valley and areas to the east and north of the valley. Damage of buildings in the Kathmandu valley was localized, and the percentage of the totally collapsed buildings was less than 5 %. East of the Kathmandu valley, especially in Sindhupalchok district, damage of buildings was more severe. In the center of Chautara and Bahrabise, towns in Sindhupalchok district, the percentage of the totally collapsed houses exceeded 40 %. North of the Kathmandu valley, the damage was moderate, and 20–30 % of the buildings were totally collapsed in Dhunche. Based on the past studies and our microtremor observations near the strong motion station, the H/V spectrum in Kathmandu has a peak at around 0.3 Hz, which reflects the velocity contrast of the deep sedimentary basin. The H/V spectra in Bahrabise, Chautara, and Dhunche do not show clear peaks, which suggests that the sites have stiff soil conditions. Therefore, the more severe damage outside the Kathmandu valley compared with the relatively light damage levels in the valley is probably due to the source characteristics of the earthquake and/or the seismic performance of buildings, rather than the local site conditions.

常時微動観測と臨時余震観測による福島県浪江町市街地の地盤震動特性評価

2011年東北地方太平洋沖地震の本震では、福島県浪江町の市街地において地震動による建物被害が発生した。周辺の市町村と比較して建物被害が顕著であるのみならず、市街地においても地区毎の被害率に差が見られた。本研究では、被害率の差の原因を調べるため、地区毎の表層地盤構造の違いを明らかにすることを目的とした常時微動観測、および臨時余震観測を実施した。微動H/Vスペクトルによる卓越周波数の分布、および微動アレー観測により求められる表層のS波速度モデルによると、市街地では概ね一様な表層地盤が示唆される。また、臨時余震観測の記録から地区毎の震動特性の差を比較したところ、PGV比およびスペクトル比において、差がばらつきの範囲内であった。震動特性の差の小さいサイト間を評価する場合にはばらつきの影響を考慮する必要があり、本研究のように狭い範囲を対象とした評価では特にその問題が顕在化するため、ばらつきの程度に応じて評価の対象とする空間解像度を定めることが重要である。

DETERMINED SOIL CHARACTERISTIC OF PALU IN INDONESIA BY USING MICROTREMOR OBSERVATION

The Palu city, which is located in the active seismic zone of the Palu-Koro fault. Several powerful earthquakes have struck Palu city at previous years. One of the greatest earthquake event occurred in year 2006 which 6.2 Mw, about more a hundred buildings and houses were collapsed due to this earthquake. Palu-Koro fault also produced several tsunami events in year of 1927, 1968 and 1996. The latest earthquake struck Palu on 2012 (7.2 Mw) and it produced local tsunami. Since no soil profile map of Palu region available, we performed single observations of microtremors at 122 sites in Palu. Its results enabled us to estimate the site dependent amplification characteristics of earthquake ground-motion. We also conducted a 8-site microtremor array investigation to gain a representative determination of the soil condition of subsurface structures in Palu. From the dispersion curve of array observations, the central business district of Palu corresponds to relatively from soft soil to medium condition. Predominant periods due to horizontal vertical ratios (HVSRs) in Palu are in the range of 1.2 to 2.5 s

Influence of wind disturbance on smart stiffness identification of building structure using limited micro-tremor observation

While most of researches on system identification of building structures are aimed at finding modal parameters first and identifying the corresponding physical parameters by using the transformation in terms of transfer functions and cross spectra, etc., direct physical parameter system identification methods have been proposed recently. Due to the problem of signal/noise (SN) ratios, the previous methods are restricted mostly to earthquake records or forced vibration data. In this paper, a theoretical investigation is performed on the influence of wind disturbances on stiffness identification of building structures using micro-tremor at limited floors. It is concluded that the influence of wind disturbances on stiffness identification of building structures using micro-tremor at limited floors is restricted in case of using time-series data for low-rise buildings and does not cause serious problems.

Ground failures on reclaimed land during the 2011 Tohoku earthquake: A case study in Urayasu City, Japan

The 2011 Tohoku earthquake caused ground failures over a wide area around Tokyo Bay, especially soil liquefaction in areas of reclaimed land (umetate-chi in Japanese). These areas, which were constructed largely by using sediment dredged from the seafloor of Tokyo Bay, suffered the greatest damage during the earthquake; no comparable damage has been reported in Holocene lowlands within our survey area. To investigate the relationship of the near-surface sediment profile in Urayasu City to ground failures caused there by the earthquake, we used aerial photographs and historical records of reclamation work, collected and analyzed hand-auger samples, and conducted dynamic cone penetration tests and micro-tremor observations. Two large cracks that formed in the grounds of Urayasu High School during the earthquake coincided with rows of piles used during reclamation work. Hand-auger samples and dynamic cone penetration tests revealed distinct differences in the lithology of the sediments on either side of both cracks; these differences allowed us to define three areas with different geological and seismological characteristics. The surface traces of the two cracks were coincident with the boundaries between these three areas. Micro-tremor observations in the three areas and in a nearby area of Holocene lowland revealed that the major ground failures of the 2011 Tohoku earthquake were closely related to subsurface interfaces between different lithologies that were created by land reclamation. This case study shows that understanding both Holocene geological processes and knowledge of the history and methods of land reclamation are important components of planning for the prevention or mitigation of earthquake disasters in reclaimed coastal areas.

GROUND MOTION IN YOGYAKARTA CITY, YOGYAKARTA SPECIAL PROVINCE, INDONESIA ON DENSELY MICROTREMOR OBSERVATIONS AND SHEAR WAVE VELOCITY

Microtremor is currently considered the foremost tool in site effect studies. The ground motion is estimated with microtremor observations, meaning that subsoil mechanical properties and geometry are evaluated and from them an estimate of local amplification is computed. Here, the ground motion is studied by the site effects of seismic hazard zonation of urban areas in Yogyakarta City. The main purpose of this paper is zoning the geological engineering features and assessing seismic of the research urban area. In this regard, the microtremors are measured at 274 sites by single station sampling method and Nakamura technique. The microtremors of all over the city are processed by a model of Mitutoyo-GPL-6A3P. The amplification factor generally ranges between 0.70 and 5.56 and the natural frequency normally varies between 0.40 and 3.30 Hz. The information layers are prepared in GMT used for detecting the zonation of potential seismic hazard. The shear wave velocity is calculated in 12 existing drilling sites based on the geotechnical approach of SPT for soil condition. To study the ground motion, geological engineering condition is investigated using amplification factor, natural frequency, shear wave velocity maps which are analyzed using densely single microtremor observation and SPT from existing drilling sites. Keywords: Ground motion, amplification factors, natural frequency; H/V spectral ratio, microtremor observations, Yogyakarta Urban

ESTIMATION OF SUBSURFACE STRUCTURE BASED ON MICROTREMOR, BORE HOLE OBSERVATIONS AND STOCHASTIC STRONG GROUND MOTION SIMULATIONS IN PALU CITY, CENTRAL SULAWESI, INDONESIA: A VALIDATION AND SENSITIVITY STUDY ON THE 23 JANUARY 2005 (PALU) EARTHQUAKE

In this study, we investigated the subsurface structure and strong ground motion parameters for Palu City. One of the major structures in Central Sulawesi is the Palu-Koro Fault system. Several powerful earthquakes have struck along the Palu-Koro Fault during recent years, one of the largest of which was an M 6.3 event that occurred on January 23, 2005 and caused several casualties. Following the event, we conducted a microtremor survey to estimate the shaking intensity distribution during the earthquake. From this survey we produced a map of the peak ground acceleration, velocity and ground shear strain in Palu City. We performed single observations of microtremors at 151 sites in Palu City. The results enabled us to estimate the site-dependent shaking characteristics of earthquake ground motion. We also conducted 8-site microtremor array investigation to gain a representative determination of the soil condition of subsurface structures in Palu. From the dispersion curve of array observations, the central business district of Palu corresponds to relatively soil condition with Vs ≤ 300 m/s, the predominant periods due to horizontal vertical ratios (HVSRs) are in the range of 0.4 to 1.8 s and the resonant frequency are in the range of 0.7 to 3.3 Hz. Three boreholes were throughout the basin especially in Palu area to evaluate the geotechnical properties of subsurface soil layers. The depths are varying from 1 m to 30 m. Strong ground motions of the Palu area were predicted based on the empirical stochastic green’s function method. Peak ground acceleration and peak ground velocity becomes more than 0.04 g and 30 kine in some areas, which causes severe damage for buildings in high probability. Keywords: Palu-Koro fault, microtremor, bore holes, peak ground acceleration and velocity.

Estimation of surface-wave phase velocity from microtremor observation using an array with a reference station

A procedure for estimation of Rayleigh wave phase velocities from microtremor observations, using an array with a reference station, is investigated in this study. Simultaneous observation of microtremors at a reference station and at a strong motion observation array in the Kanto Basin, Japan, was carried out. We first calculated cross correlations between records at the reference station and those at stations in the array using a seismic interferometric processing method on a 4300-h data series. After identifying dispersive Rayleigh waves from results of multiple filtering analysis of the cross correlations, semblance analysis of the cross correlations for different segments was carried out to estimate phase velocities for fundamental and higher-mode Rayleigh waves. The phase velocities from the proposed method are more appropriate than those from conventional methods at long periods as they avoid contamination by higher mode Rayleigh waves. The fundamental Rayleigh wave phase velocities were inverted to an S-wave velocity profile for deep sedimentary layers.We also examined the variations in the phase velocity with decreasing data duration. The phase velocities at periods less than 3 s from 6-h records are similar to those from 4300-h records, suggesting that our method is possibly applicable in microtremor exploration.

宮崎平野における常時微動計測結果に基づく地盤構造の評価

宮崎平野における常時微動計測結果に基づく地盤構造の評価

The importance of the geophysical methods for the determination of the urban subsurface structures

In this work, it is intended to highlight the indispensable significance of the geophysical methods at the level of primary decision-making at urban planning studies and to help resolving a range of problems connected to risk assessment.To this purpose, a research approach that integrated different geophysical methods for local subsoil evaluation using for seismic microzonation studies was carried out in the city of Leonessa. A detailed gravity survey and microtremor observations were made to map and model the thickness of Quaternary deposits (gravel, sand and clay) overlaying the Meso-Cenozoic pelagic basin deposits. The Leonessa basin, the test area of this study, is one of the major intra-mountain tectonic depressions of Central Apennines boarded on the southern side by the great normal fault, here referred to as the Leonessa fault.The study involved a test area of 50 km2 covered by 300 gravity stations and by 30 sites of 3-component single-site observations. The gravity measurements were performed using a LaCoste & Romberg gravimeter mod.D60. Instead, the acquisition of the ambient seismic noise was performed using the REF TEK (Third Generation Broadband Seismic Recorder) model 13 0S-01 equipped with three components 1 Hz sensors. Stations were accurately located with differential GPS (Ashtech Z-Xtreme dual-frequency GPS) that provided centimetric accuracy in elevation. The microtremor data were analyzed by means of the horizontal-to-vertical spectral ratio (H/V). A realistic density of the unconsolidated Quaternary deposits (1.75-2.00 g/cm3) determined on core samples and a density of 2.60 g/cm3 for the basement limestone were used to constrain the 2D gravimetric models. The gravity and microtremor data were compared with a collection of 69 boreholes, 5 well logs, 130 geotechnical field testing and 61 geophysical surveys (MASW and Down Hole) made available by the the municipality of the Leonessa. The 2D models match quite well with all these data.Finally, the data obtained with these two different geophysical techniques allowed us to evaluate the thickness and the fundamental resonant frequency of the Quaternary sedimentary infilling, and to reconstruct the 3D bedrock configuration.We think that, in the process of creating earthquake resistant cities, geological, geophysical and geotechnical investigations have an indispensable significance in the evaluation of suitability for settlement and land use decisions.

Identification of Engineering Bedrock in Jakarta by Using Array Observations of Microtremors

The bedrock depth is one of the most important parameters in seismic hazard analysis. This parameter has not been identified well in Jakarta. This study was conducted to determine the bedrock depth in Jakarta based on S-wave velocity parameters. Microtremors array method was applied in this study to obtain 1D and 2D S-wave velocity profiles. The spatial autocorrelation (SPAC) method was used to estimate dispersion curves, while S-wave velocity structure was derived by genetic algorithm. Reffering to the geological condition in the study area, microtremors array measurements were conducted for two East-West lines in the northern and southern parts of Jakarta. The result of 2D construction of S-wave velocity structure shows stratigraphy cross sections that consists of four layers, where the bedrock depths in northern Jakarta can be depicted in the range from 519 to 662 m and in the southern part in the range from 353 m to 399 m. It has a positive correlation with the local geological condition i.e the sediment thickness increases to the north.

Dynamic analysis of earth dam damaged by the 2011 Off the Pacific Coast of Tohoku Earthquake

This study examines possible failure mechanisms and dynamic behavior of the Fujinuma dam, which failed following the 2011 Off the Pacific Coast of Tohoku Earthquake. The study comprises two parts: an investigation through field and laboratory experiments, and a numerical simulation of the dam. Field measurements and laboratory experiments were conducted to acquire necessary information. Microtremor observations of the surviving portion of the failed dam were performed to extract data from its dynamic characteristics. Triaxial and other laboratory tests provided information required for the analysis. For the seismic analysis, a coupled solid–fluid finite element method was applied and observed and simulated motions of the 2011 Tohoku Earthquake used as input. Mechanical behavior of the dam material was described using the Mohr–Coulomb failure criterion. Frequency and dynamic analyses were performed and dam behavior and possible failure phenomena presented. Furthermore, a comparison between the simulation results and existing facts is discussed.

Estimation of Subsurface Soil Layers using H/V Spectrum of Densely Measured Microtremor Observations (Case Study: Yogyakarta City, Central Java-Indonesia)

Yogyakarta City was seriously damaged by the Yogyakarta earthquake which was an Mw 6.3 event that occurred on May 27, 2006 and caused more than 5,700 fatalities and over 37,000 injuries. Seismic damage is closely related to the ground structure. It is very important to clarify the geological bedrock of the region, when designing earthquake-resistance buildings and structures and in drawing up an earthquake disaster prevention plan for the region. Microtremor measurement is effective as a method for conveniently and cheaply understanding the ground properties. The single observations of microtremors were densely performed at 274 sites in Yogyakarta City. The predominant periods due to

Ambient noise as the new source for urban engineering seismology and earthquake engineering: a case study from Beijing metropolitan area

In highly populated urban centers, traditional seismic survey sources can no longer be properly applied due to restrictions in modern civilian life styles. The ambient vibration noise, including both microseisms and microtremor, though are generally weak but available anywhere and anytime, can be an ideal supplementary source for conducting seismic surveys for engineering seismology and earthquake engineering. This is fundamentally supported by advanced digital signal processing techniques for effectively extracting the useful information out from the noise. Thus, it can be essentially regarded as a passive seismic method. In this paper we first make a brief survey of the ambient vibration noise, followed by a quick summary of digital signal processing for passive seismic surveys. Then the applications of ambient noise in engineering seismology and earthquake engineering for urban settings are illustrated with examples from Beijing metropolitan area. For engineering seismology the example is the assessment of site effect in a large area via microtremor observations. For earthquake engineering the example is for structural characterization of a typical reinforced concrete high-rise building using background vibration noise. By showing these examples we argue that the ambient noise can be treated as a new source that is economical, practical, and particularly valuable to engineering seismology and earthquake engineering projects for seismic hazard mitigation in urban areas.

常時微動観測に基づく大野盆地のS波速度構造の推定

常時微動観測に基づく大野盆地のS波速度構造の推定

常時微動計測・振動実験・強震観測に基づく超高層建物の振動特性とその変化

Vibration characteristics of a high-rise building has examined by microtremor observation, free vibration test and earthquake observation. Natural frequencies and damping ratios for lower modes are estimated by microtremor tests, free vibration tests with AMD and man-powered excitations. Natural frequencies are shown to be lowered approximately 2% comparing to the situation soon after the construction. Relatively low damping ratios less than 1% are stably obtained. Amplitude dependent characteristics are slightly observed in estimated natural frequencies. Vibration characteristics during low-level earthquake response are estimated. Obtained properties during several years are effective for structural health evaluation after the large earthquake response.

Estimation of Long Period Ground Motion Earthquake Effects to High-rise Buildings at Hanoi and HoChiMinh Cities, Vietnam

Estimation of dynamic characteristics of subsurface such as predominant period, amplification factor, shear wave velocity play an important role in researchs of the effects of long-period ground motion earthquakes on high-rise buildings in big cities. However, in Vietnam, due to the lack of observation devices, there is a shortage of dynamic ground information. Consequently, in this paper, at first, the results of applying the Nakamura Method to data processing of microtremor observations in Hanoi and HoChiMinh City of Vietnam are presented. By using this method, the predominant period of the ground at the sites could be estimated from the horizontal to vertical (H/V) spectral ratios of microtremors. The obtained results show that the grounds of the mentioned two cities have considerable possibility of resonating with long-period ground motions. Then, the spectral accelerations in bedrock of Hanoi and HoChiMinh city were estimated by attenuation relationship of Uchiyama and Midorikawa 2006 with the far-source rupture faults that were Sumatra earthquake 2004 and Tha-Khaek fault line in Thailand. Next, the accelerations on ground surface of the above two Vietnamese cities were calculated by using spectral ratio of microtremor and inverse Fourier transform. Finally, those obtained accelerations were used to do the time history analysis applied for 8 high-rise buildings in Hanoi and HoChiMinh city. The results of this final step show that far-source rupture faults have considerable possibility to generate long-period ground motions that cause certain affects to the behaviors of high-rise buildings in Vietnamese cities.

Site amplification investigation in Dhaka, Bangladesh, using H/V ratio of microtremor

The degree of damage during earthquakes strongly depends on dynamic characteristics of buildings as well as amplification of seismic waves in soils. Among the other approaches, microtremor is, perhaps, the easiest and cheapest way to understand the dynamic characteristics of soil. Non-reference microtremor measurements have been carried out in 45 locations in and around the capital Dhaka city of Bangladesh. Subsoil investigations (Standard Penetration Test and Shear Wave Velocity) have also been executed in those locations. Soil model has been developed for those locations for site response analysis by means of the program SHAKE. Among those 45 locations, predominant frequency of microtremor observation varies from 0.48 to 3.65 Hz. Out of those 45, for 35 locations Transfer function obtained from the program SHAKE have higher frequency compared to microtremor H/V ratio and for one location it has lower predominant frequency. For six locations, frequencies obtained from two methods are identical. For three other locations, there are no similarities between predominant frequency obtained from microtremor and transfer function. The seismic Vulnerability Index (Kg) for 45 sites varies between 0.45 and 31.85. Ten sites have been identified as having moderate vulnerability of soil layers to deform.