Evaluation of seismic shaking intensity gains by high frequency microseism registration method (as exemplified by a developable site in Uzhgorod)

Objective. To refine seismic hazard parameters by registering high-frequency microseisms within the site under reconstruction in connection with the land plot enlargement of a plant intended for electronic components manufacturing. To quantify the estimated intensity of seismic shakings (in MSK-64 scale scores) accounting for the effects associated with local engineering and geological conditions at the study site. Methods. Seismic microzonation practical works at construction sites implies the application of short-period microseism registration method, which is considered to be one of the most efficient and unbiased instrumental SMZ methods when the field seismological studies are to be performed in a short period of time. The method relies on comparing parameters of soil micro-vibrations generated by natural and anthropogenic sources at the studied and the reference sites. At that, the soil is regarded as a filter capable of modifying the amplitude and phase oscillation spectra of seismic waves hitting the sedimentary cover basement. The seismic intensity gains were determined by comparing the amplitudes of soil oscillations at registration points over several sections of the site and at a reference point. Microseisms were recorded by using two identical three-channel digital seismic stations DAS-05 being the newest ones out of the model series of automatic seismic stations developed at S. I. Subbotin Institute of Geophysics of the NAS of Ukraine. VEGIK seismometers were used as seismometers. Results. Microseismic oscillation recording analysis has revealed that the main contribution to the formation of a wave field is due to the urban background disturbances falling within the frequency range of f = 8.0 - 18.0 Hz, as well as low-frequency natural oceanic effects amounting to f = 0.4 - 8.0 Hz while high-frequency vibrations are caused by anthropogenic factors amounting to f = 18.0 - 27.0 Hz (Fig. 3). Data of synchronous 24-hour microseism registering have indicated a sufficiently high stability of the amplitude level and frequency composition of microseismic oscillations, which suggests that the microseismic processes approximate stationary ones, provided that non-stationary events are removed from records. Plots of seismic intensity gain values at different frequencies caused by soil conditions at the studied site, determined according to the relation of averaged microseismic amplitude spectra both at the studied and reference site, are shown in Fig. 4. The average estimates of seismic intensity gains in the frequency range of 0.1 - 20.0 Hz for the construction site soil conditions, calculated with respect to microseismic spectral densities per all three vibration components, are presented in Table 1. The seismic intensity gain in relation to the initial (background) one for the engineering and geological conditions of the site equals to ΔIr = -0.21. Scientific novelty. Given the amplitude ratio and amplitude spectra of microseisms recorded at different sites and at the reference point, refined parameters of seismic hazards for the developable site have been obtained with consideration of the local soil conditions effects. Evaluation ratings of seismic shaking calculated intensity (in MSK-64 scale scores) based on effects associated with the local engineering and geological conditions of the study site have been provided. Practical significance. Construction site SMZ yields updated values of seismic forces relative to the general seismic zonation of the country, which allows taking into account possible gain in seismic severity at the design stage of earthquake-proof construction. Consideration of SMZ results at construction of engineering structures prevents human casualties and reduces economic losses in case of seismic manifestations.