Seismic Instruments最新文献

In 2012–2018, at the geodynamic survey area of the ultraviscous oil field of PJSC Tatneft, results of measurements of formations of the Earth’s surface were obtained using discrete geometric leveling and continuous satellite receivers (GNSS). Analysis of the results made it possible to compare the vertical displacement component obtained by two completely different measurement methods. The analytical model of the deformable formation made it possible to simulate subsidence of the Earth’s surface over the entire deposit, while showing consistency with the exceedance results obtained by the leveling method. The conducted spectral analysis according to GNSS data revealed a vast number of different spectra and also allowed us to calculate the trend velocity of the Earth’s surface.

The article presents a brief overview of the currently existing ideas about the seismotectonic situation in the Earth’s crust of Iran, which is experiencing intense compression in the northeastern direction as a result of collision of the Arabian and Eurasian lithospheric plates. The survey also involved geodetic data in the form of modern GPS measurements of horizontal surface displacements. The stress-strain state of the Earth’s crust of Iran (construction of the average focal mechanism) was assessed based on data on the total set of 945 focal mechanisms of earthquakes of average strength (4.4 ≤ M_W  ≤ 6.5) according to the ISC catalog, which occurred from 1975 to 2020, within 12 spatial samplings. The focal mechanisms of the strongest earthquakes in the last 50 years (M_W = 6.0–7.4) for one event in each of these samplings are also considered. The calculated parameters of the average mechanisms and focal mechanisms of strong earthquakes are compared both with each other and with the surrounding tectonic situation and the distribution of deformation velocity vectors according to GPS observations. A satisfactory correspondence has been established between all the comparable values. The differences in the type of formation of the seismogenic layer of the Earth’s crust of Iran in different regions are demonstrated. These differences are manifested in different ratios of shear and thrust components in the reconstructed mean mechanism in different spatial samplings. A similar difference is noted in individual focal mechanisms of the strongest earthquakes. However, it is possible to describe the observed nature of deformation of the crust of Iran within a single concept of collisional tectonics, caused by collision of the Arabian and Eurasian plates in the last 5 Ma.

The paper presents the results of long-term (55 years) ground-based geodetic observations with increased spatial and temporal detail (the distance between reference points is 0.3–0.5 km, and the intervals between measurements are 1–2 months) in the Ashgabat fault zone (Northern Kopet Dag); local (with a width of 0.5 km) anomalous deformations of the Earth’s surface with average annual deformations rates of 1–3 × 10^–5/year are detected. The results of comparing the velocities of vertical and horizontal displacements of the Earth’s surface in the Ashgabat fault zone with data on the modern kinematics of interaction of the Turan and Iranian plates, obtained from GNSS measurements according to the Iranian Geodetic Network, are discussed; it is shown that the rate of reduction of the Kopet Dag based on ground geodetic measurements is almost 100 times less than in GPS observations. This proves that local vertical displacement anomalies occur under conditions of a quasi-static regional stress field. Spectral analysis of long-term series of leveling observations was carried out, and the prevailing periods of anomalies of vertical displacements of the Earth’s surface were revealed. It is demonstrated that the annual harmonic dominates in the block parts of the leveling profiles. The maximum periods in the fault zone do not correspond to the annual component and have a shorter duration. It is established that the annual harmonic is associated with seasonal thermoelastic strain and local changes in the fault zone are caused by periodic changes in the pore pressure associated with the precipitation regime. A mechanism of parametric excitation of local vertical displacements of the Earth’s surface is proposed, when changes in the internal parameters of the medium (bulk elastic modulus) occur under stationary regional loading.

On September 21, 2004, the series of perceptible earthquakes occurred near the city of Kaliningrad. This series of seismic events affected not only the Sambia Peninsula, but also the entire Baltic region up to southern Sweden and eastern Denmark. The following events were recorded instrumentally: the foreshock with a magnitude of 5.0, the main shock with a magnitude of 5.3, and several aftershocks. The above magnitude estimates correspond to the highest values provided by various agencies. The magnitude estimates are conservative, but in this case it is even more difficult to explain the abnormally high intensities in some locations. Abnormally high seismic effects can be explained in only two ways: either the influence of the geological medium structure, or the occurrence of new earthquakes, which were triggered by the fore- and mainshock. The first assumption is not supported by the results of seismic microzoning, because the maximum amplification of seismic intensity was only 0.6 at two sites. To explain the observed intensity, such amplification is too weak. We attempted to find factual material confirming the possibility of earthquakes triggered by the fore- and mainshock. The article analyzes the distribution of seismic effects from the September 21, 2004 Kaliningrad earthquakes within Kaliningrad oblast. The observed intensities are compared with the calculated seismic effects in terms of the seismic intensity scale. As a result, it was found that the observed level of seismic effects significantly exceeds the calculated one and cannot be explained by the effects from the fore- and mainshock because of ground conditions. The macroseismic surveys in the territory of Kaliningrad oblast revealed excessively large cracks in the ground with significant horizontal movements along them and an anomalous lowering of the water level in a pond near the village of Veselovka, despite the torrential rains the day before. According to the macroseismic data, the average value of the attenuation coefficient for the region under consideration was b ≈ 2.7, which is significantly lower than the world average. The acquired data gave us grounds to hypothesize about certain earthquakes that accounted for the anomalously high seismic effects in Kaliningrad oblast: these seismic events were triggered by the fore- and mainshock of the 2004 Kaliningrad earthquakes, but were not recorded by seismic stations. A conclusion is drawn about the stress state of the crust beneath the Sambia Peninsula. Therefore, trigger earthquakes are quite possible here.

The article presents data on the main stages of creating a network of multidisciplinary borehole measurements at the Petropavlovsk-Kamchatsky geodynamic testing area, its current configuration, the composition of the measurements, and technical support. Matters related to the choice of measuring boreholes are discussed, as well as organizational and technical solutions that ensured the creation and successful operation of the network for more than 20 years. Currently, the network includes five radiotelemetric points created on the basis of boreholes, where geoacoustic measurements, electromagnetic measurements with underground electric antennas, and other types of measurements are carried out. The network makes it possible to conduct promising fundamental scientific research in the study of endogenic processes associated with preparation of strong earthquakes. In the course of long-term measurements, it was found that the developed methods for monitoring changes in the stress-strain state of the geomedium, which are based on data from borehole geoacoustic measurements and measurements with underground electric antennas, can be successfully used in regional systems for medium- and short-term earthquake forecasting. Most of the technical tools used in the borehole measurement network are the authors’ own developments. In fact, the network is an experimental base for studying the processes of preparation of strong earthquakes in one of the most seismically active regions of the world, as well as the information base of a system for medium- and short-term forecasting of strong Kamchatka earthquakes, operating in the area of Petropavlovsk-Kamchatsky.

The characteristics of the attenuation of seismic coda waves in the Fergana Basin are studied. The study analyzes short-period coda waves of local earthquakes recorded by frequency-selective seismic stations in 1960–1984 and digital seismograms of more than 100 earthquakes that occurred in the Fergana Basin from 2005 to 2021 and were recorded by the stations of the KNET network. Detailed analysis of variations in the attenuation field within the Fergana Basin showed that it is characterized by a mosaic structure that changes with depth. This attenuation field structure is consistent with the structure of the velocity field obtained by seismic tomography, as well as with the conclusions drawn from analysis of the density model. The features of the seismicity distribution relative to the position of blocks and weakened zones at different depths are considered. The correlation between contrasting objects of the attenuation field in the Fergana Basin and objects in the pre-Mesozoic strata of contrasting petrophysical characteristics has been revealed. Oil and gas fields in the Fergana Basin are associated with zones of strong attenuation, which are probably conduits for migration of fluids, including hydrocarbons. A possible explanation is also considered for the increased sensitivity of the atmospheric baric field near the Namangan hydrometeorological station to processes associated with preparation and occurrence of significant seismic events here.

The question of the existence of foci of deep earthquakes in the Caucasus is extremely important both from the viewpoint of geodynamics and seismic hazard and seismic zoning of the region. Earlier, it was believed that earthquakes with depths not exceeding 150 km could occur in the Caucasus. The existence of deeper earthquakes in the Caucasus has not previously been discussed in the scientific literature. In recent years, discussion of this issue has found reflection in works by various authors. However, consideration of the existence of deep foci based only on seismological data gives no idea of the geodynamic reasons that could cause the accumulation and discharge of energy in the form of earthquakes at great depths in the Caucasus. The article discusses the geodynamics of the Caucasus with the involvement of modern studies on seismic tomography and deformation conditions for recording movements of the crust at reference GPS stations. Analysis of the focal mechanisms of earthquakes in the North and Central Caucasus obtained for different depths, data on the distribution of hypocenters, seismic tomography sections, and geodynamic conditions have made it possible to choose the optimal versions of geodynamic models and draw appropriate conclusions about the causes of earthquakes both at great depths and in the crust of the region under study.

According to records of a 75-m laser interferometer over a 15-year observation period, the deformation component of the lunar–solar tide is distinguished as a result of the reaction of the Earth’s crust to this tide. The tidal response depends on the mechanical properties of the geophysical medium, or, in other words, it is determined by the elastic coefficients at the observation point. If the medium experiences variable tectonic or other mechanical loads, then at extreme values, within the framework of the considered concept, the elastic parameters of the medium should depend on the magnitude of this load or degree of the stress state and thus change the crustal response to the tide. The article demonstrates that, for a quantitative analysis of the stress level, it is necessary to select only the main lunar wave M₂ from the total tide. The main advantage of this wave, as the article shows, is that it is less affected by variations in meteorological factors. Moreover, a complex parameter is required, namely, the amplitude factor and phase value of the observed tidal wave M₂ with respect to the theoretical value of these parameters of this wave. Only the complete set of these parameters makes it possible to correctly assess the level of stress in the geophysical medium and, as a consequence, the ability to predict the formation of an active seismic source in a local zone.

Systematic damage to building structures of the Armenian Church of the Archangels Gabriel and Michael Church in Kaffa–Feodosia (southeast Crimea) are investigated. The deformations include tilts, rotations, and drag displacements of building elements, sagging of the hinge parts of arched structures above windows and entrances, and a wide range of fissures, including continuous joints and joint assemblies in the form of “flower” structures. Sets of systematic damage were formed by four earthquakes. The epicentral area of the earliest earthquake (event A) with a local intensity I_L = VIII–IX (MSK-64) was located along the submeridional axis in the South Crimean seismogenic zone. After this event, the church was repaired: a chapel with counterforce function was built, and many windows were filled with stones for stability. The earthquake occurred in 1423 with a high probability. This dating is supported by seismogenic deformation data on 1423 earthquake in the walls of the Funa fortress. The next earthquake (event B) occurred with a high probability at the turn of the 17th–18th centuries along one of the segments of the South Azov seismogenic zone. The local seismic vibration intensity was I_L = VII–VIII (MSK-64). Earthquake C occurred in the second half of the 18th century after large restoration works, which repaired the damage of event B. Its consequences have been especially well preserved in the arches and platbands of the western facade of the church. Maximum seismic oscillations that led to these damages acted along the ≈130°–160° axis. The local seismic intensity was I_L = VII–VIII (MSK-64). The last earthquake (event D) occurred in 1875. Our data generally coincide with the available parameterization of this event. Its epicentral area was in the South Crimean seismogenic zone; intensity was I_L = VII–VIII (MSK-64).

The efficiency of the seismic station network on the territory of the Uda Volcanic Complex (UVC), which consists of 29 irregularly distributed instruments, was assessed. At different points of the UVC, earthquakes of different minimum energy classes were recorded with different accuracies. Calculation of the minimum energy classes for the UVC seismological network of 29 stations shows that, when the number of stations is magnified to ~30 000, such a network throughout the considered territory reliably detects earthquakes of 7.0 minimum energy classes, which corresponds to magnitudes of ~2.5. The errors in determining the earthquake epicentral coordinates in latitude (δφ) and longitude (δλ) within the network do not exceed 0.4 km. The errors in determining the depths of earthquake sources (H) in the center of such a network do not exceed 0.2 km, and within the entire territory of the UVC, 5.0 km. For these calculations, the errors in determining the travel time and propagation velocities of seismic waves were set equal to 0.1 s and 0.1 km/s, respectively, and could be both random and systematic.