Microtremor Observations Research Articles

Microtremor observations of deep sediment resonance in metropolitan Memphis, Tennessee

We measured site resonance periods throughout metropolitan Memphis, Tennessee. The site-resonance was derived from recordings of ambient ‘microtremor’ and is a direct measurement of seismic ground shaking. In the Memphis metropolitan area, the period, T0 (in s) is predicted by the empirical relation T0=−0.39899+7.8922×10−3H−3.0634×10−6H2, [350<H<1100m], where H is the thickness (in m) of the unconsolidated sediments that overly hard Paleozoic rocks. Assuming that a quarter-wavelength relationship governs the relationship between resonant period, sediment thickness, and average shear wave speed, we estimate the average shear wave speed of unconsolidated sediments as a function of sediment thicknesses: Vs=521.15+0.37459H [350<H<1100m]. The scatter in the observations, and discordance with earlier work suggests that caution is warranted in applying these values of wave speed indiscriminately to sites in the absence of further detailed work.

名古屋市域の地盤・強震動・微動データのコンパイルと地盤震動性状区分

In this paper, various soil data, observed seismic ground motions and microtremor data have been collected and compiled in Tokai area and Nagoya city. Now we have over 500 seismic observation sites in Aichi, Gifu and Mie prefectures, and over 300 points in and around Nagoya city have been examined by microtremor observations. Database are totally organaized and is opend to public by use of WWW. Such data are important for the newly developed performance based seismic design and the seismic damage estimation for urban disaster prevention.

Broadband microtremor observation of basin resonance in the Mississippi Embayment, central US

We report systematic variations in the fundamental period of seismic site resonance within the Mississippi embayment. The resonance periods are revealed by peaks in horizontal to vertical (H/V) spectral ratios of microtremor measurements, and increase with the depth to the sediment/basement interface. The sediment depth varies slowly enough spatially that at any site it can be modeled as a one‐dimensional problem, revealing structural details of the sedimentary column. The strong impedance contrast and gentle dip of the base of the embayment sediments may trap seismic energy within the soft sediments, increasing durations and amplitudes of strong ground motions. Broadband recordings of microtremor within the Mississippi embayment provide a simple method to study the seismic response of the basin.

Soil-structure interaction analysis of the Hualien containment model

This paper presents results from forced vibration tests, microtremor observations and earthquake response analysis of a nuclear reactor containment model constructed on stiff soil in Hualien, Taiwan. The dynamic behavior of the soil-structure system is simulated successfully with two numerical models: a sway-rocking model, whose soil parameters are evaluated on the basis of the continuum formulation method, and a finite element model, using the program SASSI with the flexible volume substructuring approach. The dependences of the soil parameters of both models on the amplitudes of the different dynamic excitations are investigated in detail. An original numerical simulation of microtremor is performed. Comparison with results of a previous study involving a rigid tower on a soft soil site in Chiba, Japan is offered.

Soil amplification based on seismometer array and microtremor observations in Chiba, Japan : Lu, L; Yamazaki, F; Katayama, T Earthq Engng Struct Dynam V21, N2, Feb 1992, P95–108

Soil amplification based on seismometer array and microtremor observations in Chiba, Japan : Lu, L; Yamazaki, F; Katayama, T Earthq Engng Struct Dynam V21, N2, Feb 1992, P95–108

Soil amplification based on seismometer array and microtremor observations in Chiba, Japan

AbstractSoil amplification characteristics of earthquake ground motion were investigated in terms of peak ground acceleration and transfer function based on the Chiba array observation records. The amplification of peak ground acceleration occurred mostly at the top soft layer and was similar for the three components. The effects of non‐linear response of soil deposits on the transfer function were examined. Transfer functions calculated by ensemble average were close for the two horizontal components while those obtained from a smoothing operation were generally different. Both the transfer functions from the ensemble average and the smoothing operation underestimated the gain factor around the natural frequencies. A two‐step smoothing procedure was proposed and a rotary spectrum was used to improve the estimation of the transfer function. Microtremors were observed at the locations of the boreholes where seismometers are buried. The power spectrum and spatial coherency of the microtremors were compared with those of the earthquake ground motion. Emphasis was placed on the wavetypes which dominated the peaks in the power spectra.

A STUDY BY FIELD OBSERVATION ON THE SLIGHTLY-LONG-PERIOD MICROTREMORS OF GROUND

In order to investigate the slightly-long-period microtremors, the measurement of microtremors at the seaside region and the periodical observations of microtremors over two years at the basin of inland have been carried out. It is conjectured that the slightly-long-period tnicrotremors are due to natural disturbances such as the swell of the sea.Microtremors are affected by sea waves even at the region 60km away from the seashore, but the microtremors on the deposit change in the same manner as ones on the hill. The ratio of mean amplitude on the deposit to one on the hill is almost constant for each site. The ratio corresponds to the amplification degree of earthquake movements by each deposit. This ratio is useful for geophysical exploration and seismic hazard analysis.

The Characteristics of Microtremors at the Site of SMART1 Array

In this study, three microtremor observations were conducted in theSMART1 area, on July 12,13 and 15,1982.The data, in each observation, were recorded by a small circular array of about 460m in diameter. The array consisted of 9receivers spaces from 100 to 400 meters apart. The origin and characteristics of microtremors were investigated by the technique of high-resolution frequency-wavenumber (f-k) analysis. Our results show that microtremors, in the frequency range of0.72 to 1.84 Hz, may be caused by the effects of the Pacific Ocean tide on the coast to the east of the Lanyang plain. The microtremors are composed of surface waves. Two-layer subsurface velocity structures are also estimated from the resulting dispersion curves. The top layer is about 112m thick with an S-wave velocity of 390-500m s^(-1). The second layer is about 188 m thick with an S-wave velocity of 690-850m s^(-1)

丘陵地における埋土地盤及びその上に建つ構造物の振動性状 : 境界要素法による理論解析的考察

The purpose of this paper is to clarify dynamic properties of a hilly area with topographic irregularity and a reinforced concrete structure (4-stories) located on that area, by observational and analytical approaches. In the observational approach, microtremores installed on a real fill ground with topographic irregularity, were observed along a measurement line. On the other hand, in the analytical approach the real fill ground is idealized into a visco-elastic body by boundary element method. This model ground is composed of a half space with a cavity, and the cavity represents the fill ground. The structure is also idealized as a visco-elastic body. For incident SH waves, frequency response analysis is carried out in this system, and frequency response properties of the model ground and structure and obtained. Results are summarized as follows: (1) The hilly area has a local amplification effect. Namely it has some uni-predominant frequency and the amplitude factor increases according to the thickness of a hilly area. (2) The stiffness of the hilly area is greater than that of the equivalent horizontal soil layer. (3) Frequency response properties obtained by boundary element method well correspond to results of microtremor observations. (4) In the case of the structure on the hilly area, stress concentrations are created in each part of the structure on boundaries of the fill ground by the local amplification effect.

Observation of 1- to 5-second microtremors and their application to earthquake engineering. Part II. Evaluation of site effect upon seismic wave amplification due to extremely deep soil deposits

abstractThe usefulness of long-period microtremor observation for earthquake engineering problems at extremely deep soil deposit site was examined in this study. Microtremor observations were made in the Niigata Plain, Japan, and in Los Angeles, California, where depths to the basement rock are several kilometers. These two locations were selected because strong-motion records obtained during the Niigata earthquake of 1964 and the San Fernando earthquake of 1971 contain large long-period amplitudes. Understanding why these predominate long-period motions were recorded is important for the evaluation of seismic input motions for large-scale structures.Long-period microtremor observations were carried out in both areas in order to evaluate the characteristics of deep soil. Observations were performed simultaneously at a number of stations to distinguish the nature of deep soil deposit. The result of Fourier analysis showed that amplitude of microtremors in long-period range increases systematically from the point of basement rock outcrop to deep deposit site, with the increase corresponding to the depth-to-basement rock. This relationship coincides with that observed in the strong-motion earthquake records. These analyses indicated that simultaneous observation of long-period microtremors at a number of stations can provide insight to the characteristic of deep soil amplification and, therefore, permit estimation of seismic input motions for large-scale structures.

濃尾平野の長・短周期微動特性 (3) : 洪積台地上における定点長期計測

Continued to the previous studies, observations of microseisms and microtremors were carried out as steady site observation on diluvium terrace in the NOHBI plain. The seismometer used in this study, the method of measurement and analysis were employed in much the same way as previous papers. Observation had been continued of every saturdays for one year since August in 1977. The purpose of this paper is to discuss the dynamical characteristics along with the change of each season. Analytical results are not only investigated every season but also discussed in view of atmospheric phenomena. Results obtained are as follows. (1) Dominant frequency of horizontal components in low frequency in summer is lower than that in the other seasons. (2) Spectral forms of each season are divided according to the characteristics of dominant frequency and amplitude. (3) The change of Fourier's amplitude is closely related to the change of wind velocity. (4) Mean spectra of abundant observations suggest a good approximation for dynamical properties.

やや長周期の微動観測と地震工学への適用 (3)

Continued to the previous papers observations of the long-period microtremors were carried out in Aomori and Miyako cities for a farther comparative research of their characteristics to the SMAC type accelerograph records in Tokachi-oki earthquake of 1968. The purpose we extended the observations from Hachinohe to these areas is following. The very large acceleration amplitudes in 1- to 5-sec in period were recorded more clearly in Aomori than in Hachinohe, but no significant indication was seen on the accelerograph record in Miyako. Still the differences were seemed too extreme to be explained by the epicentral distances. In Hachinohe we successfully gave an interpretation that the predominancy of the earthquake motions in the long period was considerably due to a large amplification effect by the deep soils. But we did not know if this was true in general.The observation and the analysis of the microtremors were performed in 1975 with a similar manner to those reported in the previous papers.The SMAC site in Aomori is situated on the ground of which depth to the bed rock is around 600m, and the predominant period of microtremors, obtained by a series of careful observations from bed rock to the SMAC site, was 3.0sec. This is nothing but a predominant period in the strong motions of the Tokachi-oki earthquake.On the contrary to this, the SMAC accelerograph in Miyako has been installed above the hard rock with a very thin superficial layer, and no predominant period in the long-period range was observed in the microtremors. This must be the same reason why there were no long-period amplitudes in the acceleration records in Miyako.Combining these results with that in Hachinohe we can conclude that the correspondency of the predominancy and the predominant period between the microtremors and the strong motion records is very remarkable and thus the observation of the long-period microtremors is a good alternative to estimate the long-period input motions to large-scale structures by future earthquakes.Finally a short recommendation on the observation schema of long-period microtremors was added. The problem left untouched is the wave-theoretical considerations of the microtremors, which will be scheduled before long.

Microtremor observations at the Osaka-Tsuruyama Plateau

In order to investigate the ground conditions at the Osaka-Tsuruyama Plateau, measurements of microtremors at different depths have been carried out. The purpose of this investigation was to obtain clear information of the underground structure at this site. The period distribution curves obtained by surface stations are characterized by a sharp peak around 0.20 sec which indicates rather good ground conditions. Upon examining period distribution curves obtained at underground conditions the predominant periods were almost the same but a secondary maximum appears at periods longer than 0.5 sec. These results were confirmed by applying Fourier transformation of the respective sample function of the microtremors.

Investigations of Microtremors in the Akita plain in Japan

Microtremor observations at 59 stations in the Akita plain in Japan were carried out during November 23 through November 28, 1969 using magnetic tape recorder. The period distribution curves from these measurements showed that only very few locations (7 out of 58) indicated good ground conditions in the low land along the coast of the Japan sea from Akita to Oga. Most of the locations fell under the category of C type distribution curve (predominant periods in the range 0.35–0.40) suggesting underground alluvium of very soft nature over wide area.