RU2771156C1 - Method for seismic microdistricting using the vulnerability coefficient - Google Patents

Abstract

FIELD: seismic research.SUBSTANCE: invention relates to the field of seismic research and can be used in engineering seismology to assess the intensity of seismic vibrations, taking into account the properties of soils composing the territories of cities and construction sites. To increase the accuracy of determining the increments of seismic intensity caused by seismic events of natural or artificial origin, micro-vibrations are recorded at the research site using three-component seismic receivers installed on soils with different engineering and geological conditions, the amplitude-frequency spectra of horizontal (X, Y) and vertical (Z) components are calculated from the obtained seismograms, after which the resulting spectrum of transverse vibrations H is calculated and the transfer functionsfor the studied soils are calculated andfor reference soil. From the obtained spectral ratios, the maximum values of the amplitude A and the corresponding frequency f are distinguished, the use of which makes it possible to calculate the vulnerability coefficient:which, in turn, makes it possible to calculate the increment of seismic intensity ΔI at each observation point according to the formula:whereis the maximum value of the vulnerability coefficient at the measurement points on the studied soils;is the maximum value of the vulnerability coefficient at the measurement points on the reference soils; A is the maximum value of the amplitude of the transfer function H/V; f is the frequency corresponding to the maximum H/V.EFFECT: increase in the stability and accuracy of determining the increments of seismic intensity.1 cl, 2 dwg

Description

Application area

The present invention relates to the field of seismic research and can be used in engineering seismology to assess the intensity of seismic vibrations, taking into account the properties of soils that make up the territories of cities and construction sites.

The technical result of the proposed invention is to increase the stability and accuracy of determining seismic intensity increments.

It is known that seismic microzoning (SMR) is carried out in order to identify areas with different seismic intensity (shaking intensity) within the study area, which may differ from the intensity determined by the general seismic zoning map of the territory of the Russian Federation (OSR-2015). The differences are due to a number of reasons. Firstly, the difference in the physical and mechanical properties of soils and the features of their structure; secondly, the location of study areas in relation to seismotectonic zones; finally, the features of the characteristics of the foci of influence.

There is a method of performing seismic microzoning by engineering-geological method using a model of seismic-soil conditions and local classification of soils according to seismic properties [1].

However, in this method there is no instrumental assessment of seismic intensity increments under conditions of real strong earthquakes, which reduces the accuracy of determining the magnitude increments.

There is also known a method of seismic microzoning, including the excitation of seismic vibrations by a low-power pulsed source, the registration of these vibrations by seismic receivers located in areas with different engineering and geological conditions, the determination of the value of the velocities of longitudinal or transverse waves, the densities of the corresponding soils and the assessment based on these characteristics of the increment of the score [2 ].

The disadvantage of this method is the low productivity, reliability and stability of the results.

The closest in technical essence and the achieved result to the proposed one is the method of seismic microzoning using microseismic oscillations [3], in which the following formula is used to determine the change in the intensity of a strong earthquake by the maximum amplitude of microoscillations:

where (A _max ) _i , (A _max ) _o are the maximum amplitudes of microseismic vibrations for the test and reference soils, respectively.

The disadvantages of this method are the low stability and accuracy of taking readings A _max directly from the seismogram, which leads to a decrease in productivity and the correctness of taking into account the nonlinear elastic properties of soils when calculating the increment in the intensity of seismic vibrations.

The technical result of the proposed invention - increasing the productivity and accuracy of determining the score (intensity) - is achieved through the use of the so-called vulnerability coefficient when implementing the method. Let us dwell on this concept in more detail. The concept of coefficient (index) of vulnerability (K _y ) was introduced by the famous Japanese researcher I. Nakamura in 1997 [6]. In 1989, he proposed a technique based on the notion that the influence of a “thin layer” located directly under the seismic sensor on the object under study contributes to the amplification of the transverse wave (S) to a greater extent and practically does not change the longitudinal wave ( P) [5]. Based on this position, the ratio of the spectral characteristics of the horizontal components X and Y to the spectrum of the vertical component Z will characterize the so-called transfer function, which depends on the "thin layer" of the object under study. The horizontal component H in this case is determined by any of the following relationships:

average

geometric mean

vector sum

root mean square

since in [4] these relations were subjected to statistical analysis, as a result of which it turned out that the calculation of the resulting value of the horizontal component of the spectrum is practically independent of the choice of the calculation option.

Returning to the concept of the vulnerability coefficient, we note that this parameter successfully describes the dynamic characteristics of the upper part of the studied soils, while marking the weakest areas that are most susceptible to elastic vibrations. Using Nakamura's technique, spectral plots of the H/V transfer function can be obtained, i.e. shaking amplification graphs at dominant frequencies, which allows calculating the values of the vulnerability coefficients for each measurement point using the formula:

where A is the maximum value of the gain in accordance with the spectral characteristic H/V, f is the frequency corresponding to this value.

As an example, consider the processing of one seismogram with the recording of microseismic vibrations according to the described method. In FIG. 1 shows a fragment of a seismogram with a duration of 8 seconds. For the X, Y, and Z components, the spectra were calculated (Fig. 2a) and then the transfer function H/V was calculated (Fig. 2b). From the drawing in FIG. 2b shows that despite the fact that the spectra of all components have a maximum in the high-frequency region (19.2 Hz), the H/V gain reaches a maximum value of 8 a.u. at a frequency of 1.8 Hz. Thus, at this observation point, the coefficient of vulnerability of the studied soils will be equal to 35.6 c.u. Using this technique, K _y is calculated for all observation points on the studied and reference soils, and the resulting data bank allows you to calculate the seismic intensity increment at each observation point using the formula:

- максимальное значение коэффициента уязвимости в точках измерения на исследуемых грунтах;where

- the maximum value of the vulnerability coefficient at the measurement points on the studied soils;

- максимальное значение коэффициента уязвимости в точках измерения на эталонных грунтах;

- the maximum value of the vulnerability coefficient at the measurement points on the reference soils;

A - the maximum value of the amplitude of the transfer function H/V;

f is the frequency corresponding to the H/V maximum.

Brief description of the drawings

Fig. 1 - A fragment of a seismogram recording micro-oscillations, the duration of the recording is 8 seconds.

Fig. 2a - Amplitude-frequency spectra of the seismogram in the X, Y, Z components.

Fig. 2b - Spectral ratio H/V (transfer function) for a fragment of the seismogram shown in FIG. one.

Information sources

1. Nikitin S.N., Pogrebchenko V.V., Nikitina I.A. The method of seismic microzoning by engineering-geological method. Engineering survey. 2017; (6-7): pp. 118-132.

2. Recommendations on seismic microzoning. M: Nauka, 1985. p. 72.

3. Seismic microzoning. // Edited by d.t.s. S.V. Medvedev // M: Nauka, 1977. S. 67-74. - PROTOTYPE.

4. Lunedei E, Malishewsky R., 2015. A Review and some new issues on the theory of the H/V technique for ambient vibrations. In: (Eds Ansal A.) Perspectives on European Earthquake Engineering and Seismology, Geotechnical and Earthquake Engineering, vol. 39. Springer, Cham. P. 371-394.

5. Nakamura Y. A method for dynamic characteristic estimation of subsurface using microtremor on the ground surface // Quarterly report of Railway Technical Research Institute. 1989. V. 30. No. 1. P. 23-33.

6. Nakamura Y. Seismic Vulnerability indices for ground and structures using microtremor. In World Congress on Railway Research. Florence, 1997. P. p. 1-7.

Claims (10)

A method for seismic microzoning using a vulnerability factor, including recording microseismic ground vibrations using seismic recorders and seismic receivers installed in areas with different engineering and geological conditions, highlighting the maximum amplitudes on seismograms and judging the nonlinear elastic properties of soils based on the ratio of the maximum amplitudes of seismic signals on the studied and reference soils, characterized in that the registration of soil microoscillations is carried out by three-component seismic receivers, the amplitude-frequency spectra of the horizontal (X, Y) and vertical (Z) components are calculated from the received seismogram, after which the resulting spectrum of transverse vibrations H is calculated by the formula:

while the vertical spectrum V is calculated from the vertical component Z, the transfer functions are calculated

for the studied soils and

for the reference soil, from the obtained spectral ratios, the maximum values of the amplitude A and the corresponding frequency ƒ are selected, the use of which makes it possible to calculate the vulnerability coefficient:

which allows you to calculate the seismic intensity increment ΔI at each observation point using the formula:

where

- the maximum value of the vulnerability coefficient at the measurement points on the studied soils;

- the maximum value of the vulnerability coefficient at the measurement points on the reference soils; A - the maximum value of the amplitude of the transfer function H/V; ƒ- frequency corresponding to the maximum H/V.

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