JPH11183631A - Method for estimating amplification characteristic of ground at earthquake - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate estimation of an amplification characteristic of the ground in a wide area. SOLUTION: A fine topographical segment diagram of an area to be estimated is obtained (a2). A point for constant micro-motion observation is selected (a3). The result of the constant micro-motion observation is Fourier transformed (a5). The ratio H/V of the spectrum is integrated thereby calculating a Vi value (a6). A correlation formula between the Vi value and the natural period Tg of the ground is set for each fine topographical segment (a8). For a point where the constant micro-motion is not observed, the Vi value is calculated on the basis of the correlation formula (f) of the natural period Tg of the ground and Vi value obtained for each fine topographical segment (a9). An amplification characteristic of the ground in a wide area can be estimated through comparison of the Vi values (a10).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for broadly estimating a ground amplification characteristic that greatly affects the magnitude of a ground motion on a ground surface.

[0002]

2. Description of the Related Art Conventionally, when considering the seismic resistance of a structure constructed on the ground or buried underground, it is necessary to consider the magnitude of a seismic motion that can occur. It is known that the magnitude of seismic motion is strongly influenced by surface ground characteristics. Surface soils vary according to the composition and structure of the soil between the ground surface and the underlying subsurface solid basement. Large ground motions propagate from the epicenter using the surface basement as a medium, pass through the surface ground from the surface basement, and reach the ground surface. As the ground motion passes over the surface ground, the amplitude, velocity and acceleration increase and are amplified. Therefore, in the surface layer properties,
The ground amplification characteristic, which indicates the degree to which the wave of the seismic motion traveling on the surface ground is amplified before reaching the ground surface, is important.

Methods for evaluating the ground amplification characteristics of earthquake motion include a method based on boring data, a method based on the results of elastic wave exploration, and a method based on observation results of microtremors. The method based on the boring data of
Drilling is performed until it reaches the surface foundation, and the ground amplification characteristics are evaluated based on the thickness of the surface ground and the properties of the soil. The method based on the results of the elastic wave exploration described above transmits an elastic wave from the ground surface into the surface ground, observes the reflected wave from the surface base, estimates the structure of the soil, and evaluates the ground amplification characteristics. The method based on the observation results of microtremors of
H / VR, which is the ratio between the horizontal and vertical components of the observed waveform
And evaluate the maximum value as the ground amplification factor.

[0004]

The above-mentioned acquisition of boring data and the above-mentioned elastic wave exploration require costly surveys at each point. Therefore, evaluation is performed at a plurality of points to obtain a wide-area ground amplification characteristic. It is very difficult to evaluate.
Although the method based on the results of microtremor observation is relatively inexpensive in terms of observation cost for each point, it is necessary to always observe microtremors, and cannot be applied to points where microtremors are not always observed. Microtremor observation must be performed, for example, at midnight, which is not easily affected by disturbance. However, when performing observations at many points over a wide area, it takes a long time to move between the points, and it takes a long time to perform the entire observation. In addition, there may be points where it is difficult to observe microtremors at all times due to structures and the like.

It is an object of the present invention to provide a method for estimating a ground amplification characteristic during an earthquake that enables a wide area estimation of a ground amplification characteristic based on a result of observation of microtremors including a point where microtremors are not always observed. .

[0006]

SUMMARY OF THE INVENTION The present invention relates to a method for estimating a ground amplification characteristic in which a ground motion on a surface basement is amplified in a surface ground until the ground motion reaches the ground surface. Within, a microtremor is always observed at a point belonging to each of the plurality of micro topographical divisions, and for the observed microtremor, the ratio between the horizontal component and the vertical component of the spectrum is integrated within a predetermined range,
The result is obtained as a Vi value, the natural period Tg of the ground is calculated for each observation point, the correlation formula between the Vi value and the natural period Tg of the ground is set for each micro-topographic section, and a point where the microtremor is not always observed. , The Vi value is calculated from the natural period Tg of the ground at the point using the correlation equation for the micro-topographic section to which the point belongs, and the relative comparison of the Vi value at each point indicates a wide area amplification. This is a method for estimating ground amplification characteristics during an earthquake, characterized by estimating characteristics.

According to the present invention, tremor is constantly observed at a point belonging to each of a plurality of micro topographical divisions in an area where the ground amplification characteristic is to be estimated. The ratio between the horizontal component and the vertical component of the spectrum of the observed microtremor is integrated in a predetermined range, and the Vi value that is the integration result is set as a correlation equation of the natural period Tg of the ground at each observation point for each micro-topography section. . For a point where microtremor is not always observed in the area where the ground amplification characteristic is to be estimated, the Vi value is calculated from the natural period Tg of the ground at that point using the correlation formula for the micro-topographic section to which the point belongs. . If the Vi value at each point in the area is compared relatively, the Vi value corresponds to the amplification characteristic of the ground, so that a wide-area amplification characteristic can be estimated.

In the present invention, the natural period Tg of the ground at the point where the microtremor is observed is obtained from the peak of the spectrum.

According to the present invention, the natural period Tg of the ground at the point where microtremors are always observed is determined from the spectrum value of the result of microtremors observation. Of the periodic components of the microtremor, the component close to the natural period of the ground is considered to resonate and form a peak.Therefore, there is a large possibility that the spectrum corresponds to the natural period, and the natural period of the ground can be easily determined. You can ask.

In the present invention, the natural period Tg of the ground at the point where the microtremor is not always observed is determined from the thickness of the surface ground from the ground surface to the surface base.

According to the present invention, the natural period Tg of the ground at a point where the microtremor is not always observed cannot be obtained from the peak of the spectrum of the microtremor. Therefore, the determination is made based on the thickness of the surface ground from the ground surface to the surface base. The thickness of the surface ground can also be estimated from the change in gravity, and the data can be used to calculate the natural period Tg of the ground.

Further, in the present invention, the micro-terrain division is interpreted based on a micro-terrain division map prepared in advance.

According to the present invention, a micro-topography classification map, such as a back-top wetland, an alluvial fan, an alluvial terrace, a low terrace, a middle terrace, a high terrace, a mountain ridge, or an old river road, is created. Since the information is interpreted based on the information, it can be easily estimated without newly observing.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a procedure for estimating ground amplification characteristics over a wide area according to an embodiment of the present invention. The procedure starts from step a0, and in step a1, an estimation target area is determined. In step a2, a micro-topographic map of the determined estimation target area is obtained. In step a3, a plurality of microtremor observation points are selected for each micro-topography section. There are several types of micro-topography, for example, back marsh, alluvial fan, alluvial terrace, low terrace, middle terrace, high terrace, foothills or old river channel. During the soil formation process and the cause of the formation.

In step a4, microtremor is always observed at the selected point. The direction of the microtremor to be observed is the horizontal direction and the vertical direction. In the horizontal direction, observations are made in two orthogonal directions, and the observation results are combined. In step a5, the observation result of the microtremor is changed from data on the time axis to data in the frequency domain by Fourier transform. In step a6, H / V, which is the ratio between the horizontal component and the vertical component of the spectrum obtained by converting the observation data into data in the frequency domain,
The ratio is obtained, and the value is integrated in a predetermined cycle range to calculate the Vi value. In step a7, the cycle of the peak of the spectrum is set as the natural cycle Tg of the ground. This is because, considering a stratified structure without irregularities, both the Vi value and Tg are correlated with the basement depth. In step a8, a correlation equation f is set from the correspondence between the Vi values obtained at a plurality of microtremor observation points belonging to the same micro-terrain category and the natural period Tg of the ground. In step a9, the Vi value at another point where the microtremor is not always observed is calculated as a function of the natural period of the ground and the micro terrain classification. In step a10, V including the points where microtremors are always observed and the points where
Wide-area ground amplification characteristics are estimated by comparing the i values.
In step a11, the procedure ends.

FIG. 2 shows a schematic configuration of a measuring device for constantly observing microtremors. The fine movement sensors 1, 2, 3 are installed at a stable position on the ground surface, for example, on an asphalt road. The two fine movement sensors 1 and 2 detect the fine movement in the horizontal direction orthogonal to the vertical movement, and the fine movement in the vertical direction, that is, the vertical direction is detected by the third fine movement sensor 3. Outputs from the fine movement sensors 1, 2, 3 are amplified by the amplifier 4. The signal conditioner 5 performs, for example, a low-pass filter process on the signal amplified by the amplifier 4. As the cutoff frequency of the low-pass filter, for example, about 30 Hz is used. In the signal conditioner 5, the signal is sampled at a period of, for example, 100 Hz, and is sampled every 0.01 second. The position of the measurement point is detected as longitude and latitude data using, for example, GPS. The outputs of the fine movement sensors 1, 2, 3 are stored in the memory 8 and recorded by the recorder 6 on the surface of recording paper or the like. The data obtained from the signal conditioner 5 is data that changes over time,
That is, the FFT analyzer 7 for performing the fast Fourier transform (hereinafter, abbreviated as “FFT”) on the data belonging to the time domain and converting it into the frequency domain data, that is, the spectrum data about the microtremor, Generate. The generated spectrum data is stored in the memory 8. Such an observation device operates with power from a power supply 9 such as a battery, and can be carried to an observation point. The spectral data stored in the memory 8 is subjected to data processing using a personal computer 10 or the like, and is used as an index for evaluating the ground amplification characteristics.
A value or the like is calculated.

FIG. 3 shows a procedure for analyzing observation data on microtremors performed using the observation apparatus shown in FIG. The analysis is started from step b0. In step b1, data of 10 sets of each of two components in the horizontal direction and one component in the vertical direction are obtained. At each measurement point, 10 or more sets of signals from the movement sensors 1, 2, and 3 are measured with a basic unit of 20.48 seconds. The data is selected for each group and analyzed for each micro-topographical section. Next, in step b2, each component is converted to a Fourier spectrum using the FFT analyzer 7. In step b3, two components in the horizontal direction are combined, and the combined direction is set as the horizontal direction again. In step b4, a smoothing process is performed using a 0.3 Hz Parzem window. Next, at step b5, the geometric mean of the horizontal component and the vertical component are obtained, and at step b6, the horizontal two-dimensional spectrum and the upper and lower spectrum are obtained. In step b7, the horizontal top and bottom spectrum ratio (H
/ VR), and the procedure ends at step b8.

FIG. 4 shows the frequency distribution of the horizontal and vertical spectral ratios (H / VR) calculated in step b7 of FIG. In step a6 of FIG. 1, H / VR is integrated in the section from T1 to T2, and the Vi value is calculated according to the following first equation. The Vi value calculated in this way corresponds to the area indicated by hatching in FIG.

[0019]

(Equation 1)

The integral range from the periods T1 to T2 is set so that the Vi value calculated according to the first equation corresponds to the amplification factor obtained from the strong motion records of the past earthquake motion. This range has been found to be preferably, for example, in the range of 0.1 seconds to 5 seconds. Further, from such spectrum data, a natural period Tg of the ground can be obtained corresponding to the peak value.

FIG. 5 shows the correlation between the natural period Tg of the ground and the Vi value for a certain micro-terrain section, for example, a fan. As the correlation equation between Tg and Vi, it is determined that an equation using a logarithmic function as shown in the following second equation is appropriate. Vi = a × lnTg + b (2) In some cases, the coefficient a of the logarithmic part of the second equation is 0, and the Vi value may be treated as a constant b.

If a correlation equation for obtaining a Vi value as shown in the following equation (2) is obtained for each micro-topography section, for other points, the micro-topography and the natural period of the ground are determined only from existing data, and the Vi value is obtained. Is estimated and the V obtained at the microtremor observation point
By relatively comparing with the i value, it is possible to estimate a ground amplification characteristic over a wide area.

As shown in FIG. 6, at points belonging to the same micro-topographical section, the ground amplification factor and the Vi value have a fairly good linear relationship as long as it is judged from past resonance data and the like. doing. Therefore, even when the Vi value itself is different from the ground amplification factor, the ground amplification characteristics can be evaluated for each point by relative comparison. In this embodiment,
As long as the micro-topography classification and the natural period Tg of the ground are obtained from the existing data, it is possible to easily estimate the ground amplification characteristics over a wide area including the points where no microtremors are observed, based on the results of microtremors observation. . Note that the natural period T of the ground
g can also be obtained by estimating the depth of the surface layer from observation records of gravity anomalies.

[0024]

As described above, according to the present invention, in order to estimate the ground amplification characteristics during an earthquake, a microtremor is always observed at a point belonging to each of a plurality of micro topographical divisions, and a spectrum obtained from the observation result is obtained. The value of Vi obtained by integrating the ratio of the horizontal component and the vertical component in a predetermined range, and the natural period T
g is set, and the Vi value is calculated from the correlation equation set for the micro-topographic section to which the point belongs even at a point where the microtremor is not always observed. The ground amplification characteristics during an earthquake can be estimated. Since it is not necessary to calculate the Vi value based on the observation result of the microtremor at all the points, it is possible to easily compare the amplification characteristics over a wide area.

According to the present invention, the natural period Tg of the ground at the point where microtremors are always observed is obtained from the peak of the observed spectrum, and the natural period Tg and the Vi value of the ground are obtained using the observation results. Can easily be set.

Further, according to the present invention, the natural period Tg of the ground at a point where microtremors are not always observed is determined from the thickness of the surface ground from the ground surface to the surface base, so that the thickness of the surface ground is determined. If this is the case, the natural period Tg of the ground can be calculated without newly performing measurement, and the Vi value can be easily estimated using a correlation equation set at a point where microtremors are constantly observed. Can be.

Further, according to the present invention, the micro-topographic section at the point where the ground amplification characteristic is estimated is read based on the micro-topographic section map prepared in advance, so that the micro-topographic section is checked again when estimating the ground amplification characteristic. It is not necessary to perform the above, and as long as the micro-topographic map is created, it is possible to easily estimate the ground amplification characteristics over a wide area that seems to have a common ground structure to a slightly deeper place.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating a procedure for estimating a ground amplification characteristic according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a schematic electrical configuration of a device for observing microtremors at all times;

FIG. 3 is a flowchart showing a processing procedure for constantly performing microtremor observation using the apparatus of FIG. 2;

4 is a graph showing a Fourier transform result of the H / V ratio of the microtremor spectrum obtained by the processing procedure of FIG.

FIG. 5 is a graph showing a correlation between a natural period Tg of the ground and a Vi value obtained for a fan which is one of the micro-topographic classifications.

FIG. 6 is a graph showing a correspondence relationship between a ground amplification factor obtained from a record at the time of a past strong earthquake and a Vi value.

[Explanation of symbols]

 1, 2, 3 Fine movement sensor 4 Amplifier 5 Signal conditioner 6 Recorder 7 FFT analyzer 8 Memory 10 Personal computer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Joji Ejiri 4-640 Shimoseito, Kiyose-shi, Tokyo Obayashi-gumi Technical Research Institute, Inc. (72) Inventor Daigo Maeshima 4-640 Shimoseito, Kiyose-shi, Tokyo, Japan (72) Inventor Yozo Goto 4-640 Shimoseito, Kiyose-shi, Tokyo Inside Obayashi Corporation Technical Research Institute

Claims (4)

[Claims] 1. A method for estimating a ground amplification characteristic in which a ground motion on a surface basement is amplified in a surface ground until the ground motion reaches a ground surface, comprising: Microtremors are constantly observed at points belonging to each of the categories, and the ratio of the horizontal component to the vertical component of the spectrum is integrated over a predetermined range for the observed microtremors, and the result is obtained as a Vi value. Calculate the natural period Tg of the ground, set the correlation equation between the Vi value and the natural period Tg of the ground for each micro-topographic section, and for the point where microtremors are not constantly observed, the correlation equation for the micro-topographic section to which the point belongs Is used to calculate the Vi value from the natural period Tg of the ground at that point, and to estimate the wide-area amplification characteristics from the relative comparison of the Vi values at each point. Method of estimating characteristics. 2. The method according to claim 1, wherein the natural period Tg of the ground at the point where the microtremor is observed is obtained from a spectrum peak. 3. The natural period Tg of the ground at a point where no microtremor is observed is determined from the thickness of the surface ground from the ground surface to the surface base.
Estimation method of ground amplification characteristics at the time of earthquake described. 4. The method for estimating a ground amplification characteristic during an earthquake according to claim 1, wherein the micro-topographic section is read based on a micro-topographic section map created in advance.