Seismic Instruments最新文献

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.

To determine the coordinates of earthquakes, the Republic of Armenia (RA) Seismic Survey uses algorithms based on the Geiger method. All these algorithms use only one travel time curve (one velocity curve) for the entire study area and all stations of the RA seismological network. Despite the fact that a number of studies have been devoted to the structure of the crust and upper mantle of the Caucasus and, in particular, the RA, however, so far most of the existing travel time curves cannot be used to determine earthquake hypocentral coordinates and, in particular, the depth of the source with sufficient accuracy. Quite obviously, to determine the coordinates of earthquakes, it is desirable for the crust to have travel-time curves (or velocity columns) constructed not according to local earthquake data, but other seismological data, in particular, deep seismic sounding (DSS) and earthquake converted wave method (ECWM), and for the upper mantle, according to data on distant earthquakes. Currently, information has been collected on longitudinal wave velocities in the crust within the territory of Armenia, which makes it possible to present an averaged velocity model of the crust in Armenia. For this, the data of all available velocity sections were used, compiled from materials obtained along the network of regional profiles by ECWM and DSS data. The velocity curve was constructed from DSS and ECWM data obtained in different years in the territory of Armenia. The use of one velocity curve by the seismological networks in Armenia will make it possible to obtain homogeneous data from these networks.

Catalogs of weak seismic events (M ≤ 4) in Turkey and Iran were analyzed to identify signs of their contamination with blasts. A well-pronounced predominance of the number of seismic events and a decrease in their average magnitude in the period from 10:00 to 17:00 LT were found. The epicenters of seismic events that occurred in this time interval at depths of less than 3 km are located much more compactly than other seismic events. Such indications can be considered as an evidence of contamination of the earthquake catalogs with road, mine, and quarry blasts. The presence of such an anthropogenic effect in the earthquake catalogs is also confirmed by the presence of a well-defined weekly periodicity of seismic events. It manifests in the form of a significant weakening of the discussed daytime extremum on Saturday and Sunday in Turkey and on Friday in Iran. Using the example of Turkey, where the observation service attempted to separate blasts and earthquakes in the seismic catalog, it is shown that in an earthquake catalog, which was purified this way, the prevalence of seismic events in the daytime over their number in the nighttime is still observed. The opposite effect was also revealed, namely, the presence of earthquakes in the blast catalog; i.e., there is a mutual contamination of the catalogs of earthquakes and blasts. However, it cannot be excluded that the relative increase in the number of earthquakes in the daytime is only partially caused by the presence of undetected explosions due to imperfect algorithms for discriminating between blasts and earthquakes. The daytime extremum in the number of earthquakes can be partially generated by the occurrence of additional weak earthquakes triggered by relatively strong blasts (i.e., by the triggering effect of explosions on the natural process of seismogenesis). The obtained results show that even in case of earthquake catalogs that are claimed to be cleared of blasts, it is necessary to carry out preliminary study these catalogs in order to assess their contamination with blasts.

The article presents the results of studying the source of the 2015 Kenekesir earthquake and its aftershock sequence. The earthquake occurred in the Archman–Nokhur tectonic node zone, where the northwest orientation of the Central Kopet Dag faults changes to the northeast orientation of the Western Kopet Dag faults. The actual rupture plane at the Kenekesir earthquake was determined from the dataset of the focal mechanism, three-dimensional orientation of the aftershock cluster, orientation of the nearest faults, and first isoseismals of previous earthquakes. The rupture plane strikes southwest and dips to the northwest. The displacement type is oblique slip with equal normal-fault and left-lateral strike-slip components. The aftershock series lasted 186 days and consisted of 1249 aftershocks of the representative level (K_R ≥ 5.6). At its initial stage, the 11-day period of regular development of the aftershock process is identified, when the Omori law is fulfilled with the highest correlation coefficient and aftershock attenuation parameter p = 1.35. Then, the aftershock process assumes a pulsating character, passing to the stage of stress relaxation in the medium. Accelerograms and velocigrams of the Kenekesir earthquake and its aftershocks are of undoubted interest for assessing the seismic hazard in this area. It was found that the instrumental intensities determined from the velocity (I_PGV) and seismic wave power (I_{PGA ⋅ PGV}) agree the best with the regional macroseismic field equation.

A new energy classification method is proposed for acoustic events recorded in laboratory experiments on rock destruction. The method analyzes the coda waves of acoustic emission (AE) events. Coda waves are considered as reverberation of the acoustic field in the test sample. The new classification was tested on two experiments carried out on different rocks: granites of the Voronezh massif and Berea sandstone, on an INOVA-1000 controlled hydraulic press at the Borok Geophysical Observatory (GO), Schmidt Institute of Physics of the Earth, Russian Academy of Sciences (IPE RAS). Comparison of the new classification with the one used at Borok GO showed that both methods give well consistent results in the middle range of energies of AE events. For strong events with the saturated initial parts of the signals due to the limitations of the recording equipment, the new technique demonstrated better results, leading to energy estimates of such events from the undistorted coda of the signal.