Seismic Instruments最新文献

The Kolba permanent seismic station was installed in 2020 at the Kolba geophysical station (affiliated with the Northern Territorial Administration for Hydrometeorology and Environmental Monitoring), near the settlement of Dikson, Krasnoyarsk krai, in order to increase the sensitivity of the Arkhangelsk Seismic Network (affiliated with the N. Laverov Federal Center for Integrated Arctic Research, Ural Branch, Russian Academy of Sciences), which carries out seismic monitoring in the Barents–Kara region and adjacent areas. For regional earthquakes in the European sector of the Arctic, the representative magnitude was determined at M_Lrep = 3.4. The Kolba seismic station records local events of different nature with magnitudes from 0.8 to 1.7. The dependence of the number of revealed earthquakes on the level of microseismic noise is revealed.

Earlier, the authors carried out a unique experiment on long-term continuous precision monitoring of crustal resistivity variations in a highly seismic region. The result of this experiment can be considered a special type of VES profile, in which, instead of a linear coordinate, the sounding date changes from picket to picket. When processing precision monitoring data, it is necessary to solve the inverse VES problem with the highest possible accuracy. Standard programs for inversion of VES curves do not allow this, and even with very small fitting residuals, the actual error in reconstructing the resistivity can be huge due to equivalence effects. The authors have previously developed a special method for regularizing the residual functional, which multiply increases the accuracy in solving the inverse problem for the considered type of resistivity section, and a method for obtaining realistic, rather than underestimated estimates of the solution error. To do this, a package of synthetic resistivity profiles is formed that imitates a real section, the VES direct problem is solved, and time series of apparent resistivity are constructed, on which noise similar to real noise is superimposed. After that, the VES inverse problem is solved and the errors in reconstructing the model resistivity curves are analyzed. Such calculations were carried out both for the total signals and their components, obtained as a result of decomposition of the apparent resistivity series into physically determined components. The developed approach makes it possible to solve the inverse VES problem with heretofore unattainable accuracy. We emphasize that a reliable estimate of the solution errors is provided not by the convergence criteria of the inversion algorithm (they are almost always overly optimistic), but by direct calculations of the direct and inverse problems for synthetic profiles similar to real signals. In the present article, the profile of the experimental VES curves obtained in the course of this experiment is inverted. Series of resistivity variations are calculated in four layers of a geoelectric section with a duration of more than 12 years. It has been established that the upper layer of the section is characterized by trend and seasonal changes in resistivity with a large amplitude. Significant anomalous seasonal effects were found in the second layer of the section. For the third layer, the presence of small-amplitude seasonal effects was established, while there are no significant resistivity trends. Variations in the resistivity of the fourth layer are less reliably estimated; to detect the effects of external factors on electrical resistivity, it is necessary to use signal stacking methods.

The paper proposes a method for searching for the M2 tidal harmonic in seismicity, based on the construction of time series of conditional accumulated seismic energy (CASE) with a given discretization and their adaptive spectral analysis. The preliminary results of testing this method on seismic observations throughout the territory of California for the period 1983–2012 are presented. For the first time, the fundamental harmonic of the lunar tide M2 was reliably identified in weak seismicity variations at a significance level above 5σ in the range of energy classes 5–9. Confinement of areas with significant M2 amplitudes to the San Andreas Fault and its branches was established.

At the stages of the FENICS experiment conducted in 2014 and 2019, unique data on deep electromagnetic sounding with grounded sections of industrial power lines were obtained at distances from 180 to 940 km from the center of the supply line to the measuring installation. The electromagnetic field components were recorded by VMTU-10 measuring equipment (VEGA, St. Petersburg). The fluctuations of the current in the supply line were recorded as a time series with a sampling rate of 1 kHz. Based on synchronous time series of the field components at the observation points and the current strength in the supply vibrator, estimates of the power spectra of the autocorrelation and cross-correlation functions of the recorded values were calculated using fast Fourier transform (FFT). The resulting spectral characteristics were used to determine the amplitudes of the field components of the source and phase shifts between them, as well as to estimate the impedance tensor components. To do this, correction factors were calculated for the ratio of the apparent resistivity values for the horizontal magnetic field component to those for the impedance and electric field. A 2D interpretation is made and estimates of the resistivity allocation at depth are obtained for two sublongitudinal and sublatitudinal profiles of the Karelia–Kola region.

To solve many geological and geophysical problems, in particular, prediction of strong earthquakes, seismic zoning, and earthquake-resistant construction, it is necessary to have reliable data on the distribution of earthquake hypocenters in the study area. In turn, the completeness of data on the distribution of earthquake hypocenters and accuracy of their determination largely depend on the efficiency of the observation system. We evaluate the efficiency of the actual network of seismological observations in Karelia, which consists of seven seismic stations. The efficiency of the seismological observation network is estimated by the minimum magnitudes of seismic events recorded in the considered region and the minimum errors in determining the main parameters (origin time and hypocentral coordinates) of the recorded events. Calculation of the minimum earthquake magnitudes for the Karelian seismological network shows that a station amplification of 30 000, the network is able to record earthquakes with a magnitude slightly less than 2, with a tendency towards a decrease in this minimum magnitude, provided that the event is recorded by at least three stations. The errors in determining earthquake epicentral coordinates in latitude δφ (km) and longitude δλ (km) do not exceed 1–2 km in the center of the network, and 6 and 11 km on the periphery, respectively. The system’s errors in determining the depths of earthquake sources H (km) do not exceed 5 km in the vicinity of the stations and 25 km throughout the territory of Karelia.

Historical earthquakes of Minusinsk okrug of Yenisei Province are studied. Information in mass media and professional literature of the second half of the 1800s–early 1900s make it possible significantly to enrich the very limited knowledge on regional seismicity. Materials on six earthquakes missing in former basic macroseismic catalogs are presented, as well as original information from unknown sources on four earthquakes. As a result of the study, one can classify the territory as an area of moderate seismicity. The maximum observed intensity does not exceed 7 on the MSK-64 scale.

An improved statistical method for determining the resonance frequencies of soils from spectral H/V ratios is presented. The combination of developed methodological and computational techniques makes it possible to identify with high reliability the resonance frequencies of the soil sequence and fundamental frequencies of the natural vibrations of structures and their elements based on engineering seismological observations. The authors demonstrate the possibility of refining the seismic soil model of a site and compiling spatial models of the change in depths of seismic boundaries of layers of the soil sequence from the results of frequency analysis of records of microseisms and seismic events. The use of the described engineering and seismological observations and methodological techniques makes it possible to instrumentally estimate the increase in amplitudes of vibrations of structures and their individual elements at resonance frequencies and obtain a sufficiently detailed seismometric floor-by-floor description of the vibrational characteristics of buildings and structures.

The study compares seismic hazard assessments of the territory of Uzbekistan, obtained with the same input parameters, but using different methodological approaches: the Riznichenko approach based on the theory of macroseismic and spectral-time shaking and the classical Cornell probabilistic approach based on the full probability theorem. As seismic source models, linearly extended sources (seismogenic zones) and area sources (quasi-uniform seismological provinces) were considered. The authors used a number of their own damping dependences, established from analysis of isoseismic earthquake patterns in Central Asia, when assessing the seismic hazard of the study area in terms of macroseismic intensity, along with the Shebalin dependence, obtained from global data (I = 1.5M – 3.5 log R + 3). To estimate seismic hazard in engineering seismic indicators, the dependences built into the R-CRISIS software package, developed over the past 10–12 years for shallow active crust and stable regions, were used as the ground motion equation. For a 50-year seismic impact nonexceedance probability P = 0.90, the maximum differences in seismic hazard assessments using the two considered approaches for the entire seismically active part of the study area are ∆I = 0.39; for P = 0.95, ∆I = 0.54; for P = 0.98, ∆I = 0.61; and for P = 0.99, ∆I = 0.76. A similar comparison of seismic hazard assessments in the values of maximum ground motion accelerations leads to the following figures: for P = 0.90, ∆a_max = 75 cm/s²; for P = 0.95, ∆a_max = 111 cm/s²; for P = 0.98, ∆a_max = 167 cm/s²; for P = 0.99, ∆a_max = 273 cm/s².

The Yuzhno-Sakhalinsk Seismological Regional Information Processing Center (RIPC) recently used the Richter magnitude M_wa to estimate the energy level of Far Eastern earthquakes. To date, this is the most common energy characteristic in the operational catalog of the RIPC. The basis is digital waveforms of regional seismic stations emulated to the characteristics of the Wood–Anderson seismograph. In the present paper, the results of determining the magnitude M_wa in Russian Far Eastern conditions are analyzed. The magnitude determination accuracy is estimated, and station corrections are obtained. The regression relation M_wa and Japanese Meteorological Agency magnitude M_j showed a high similarity between these scales. This made it possible to use M_j as the reference when studying the features of M_wa. The regional seismic network makes it possible to correctly estimate the magnitude M_wa at a regional distance (up to about 1000 km), including for deep-focus earthquakes (up to 600 km). Transfer relations between M_wa and other mass-determined regional energy characteristics of earthquakes are obtained. The M_wa scale can be used to estimate the energy level of any earthquakes in the Kuril–Okhotsk and Sakhalin regions in the magnitude range of weak and moderately strong events and is recommended for inclusion in regional earthquake catalogs for the Russian Far East.

The paper discusses the applicability of the horizontal-to-vertical spectral ratio for ground motion (H/V method) to estimate the seismic intensity increment and trace the bedrock surface covered by soil layers. The first issue is the classic seismic microzoning problem; the second is related to structural mapping of the upper part of the geological profile. The authors assess the possibility of using a single station to estimate the intensity increment without synchronization of data at a given point at a site with records based on reference rocks. It is shown that such an approach is invalid, because the maximum variations in the H/V ratio during a day may reach 2.7 times; a degree of intensity of 0.8 will correspond to such variation. At the same time, it is shown that the resonance frequencies of soil layers are stable. If the mean velocities of seismic waves are known at some points of profile surface, the position of the underlying bedrock top can be traced with confidence.