Seismic Instruments最新文献

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.

A new method for estimating the self-noise of the measuring channel of a seismometer is proposed, taking into account the linear relationship between the signal and noise. The method extracts noise using records of two identical measuring channels on the example of the SM-3KV short-period seismometer with an operating frequency range of 0.5–40 Hz. The method was tested on model signals for channel noise with a normal distribution, as well as microseismic noise records recorded on a pedestal by seismometers with locked and free inertial masses. Work with the model signals demonstrated that the accuracy of the numerical result when assessing the level of isolated noise depends on the value of the cross-correlation of the initial seismograms. Consideration of this dependence when calculating the self-noise of real measuring channels yields a noise level similar to the standard method based on separation of the incoherent component of the initial signals. The noise values in the 0.5–40 Hz range with a locked mass of the seismometer are 2.1 ± 0.3 nm/s according to the standard method and 2.2 ± 0.4 nm/s according to the new methods. The obtained values do not contradict the manufacturer’s data of the SM-3KV seismometer, which state that the self-noise level does not exceed 2 nm/s in the operating frequency range.

The article discusses the results of processing instrumental and macroseismic observations of the strongest earthquake in Central Baikal in the last 60 years, which occurred on December 9, 2020 (M_W = 5.5), as well as its aftershocks. The event was named the Kudara earthquake, after the locality where the maximum intensity of shocks was recorded. The epicenter of the main shock is confined to Proval Bay (Lake Baikal), which formed as a result of the catastrophic Tsagan earthquake of 1862. The sufficiently high density of seismic stations in the region allowed us to obtain reliable estimates of the position of epicenters and source depths for the main shock and aftershocks. In total, more than 70 earthquakes were recorded, located within an area extending in the sublatitudinal direction. The strongest aftershock (K_R = 12.6) occurred to the west of the main source on the first day after the main shock. To estimate the seismic moment and focal spectrum of the earthquake, the coda envelope inversion method was tested. The moment magnitudes for the main shock and three aftershocks have been obtained.

An automatic vertical alignment method is presented and analyzed for an absolute laser ballistic gravimeter. The method uses a high-speed video camera to record the displacement trajectory of glare from the measuring beam reflected from a corner reflector when the test body is in free fall. The magnitude and direction of the measuring beam displacement angle from the vertical is determined by frame-by-frame processing of the obtained video, and the vertical of the working gravimeter beam is corrected by actuators installed on the movable supports of the gravimeter base. Experimental verification showed that the vertical alignment error does not exceed 3 × 10^–5 rad.

This article, continuing the study of the powerful event of February 18, 1772, on the northern Kola Peninsula according to written sources about it (Nikonov, 2020a), provides materials and analyzes several groups of natural processes in the epicentral and adjacent areas in terms of natural manifestations of seismic disturbances with arguments in favor of their occurrence as result of the earthquake of February 18, 1772. The groups include materials of a geophysical profile-section along the seafloor north of the Cape Pogan-Navolok, changes in the nature of new accumulations of debris at the bottom along the Murman Coast west of Cape Pogan-Navolok, in the area on the eastern coast of Kola Bay. Signatures of seismic disturbances in all groups are quite consistent with the event of 1772 and, thus, allow an increase in the shaking intensity score by VI–VII points independent of written data (in the 2020 article there were only two points: the settlement of Kola and cape Pogan-Navolok at the NW outlet of Kola Bay. On this basis, the source parameters of the event are determined anew. In addition, some features of the new tsunami at the outlet of Ura Bay to the Barents Sea are considered. The earthquake of February 18, 1772, according to the set of revealed signatures, is recognized as the most powerful of the currently known historical earthquakes in the Murmansk seismogenic zone, which is today acknowledged as a higher-order seismically active zone.

The article presents the results from a study of dynamic characteristics of the response of a high-rise building under the seismic impact of a series of southern Baikal earthquakes in 2007–2008. One earthquake used in the study, the Kultuk, occurred on August 27, 2008, at 10:35 LT (UTC +8). The epicenter of tremors was at the bottom of Lake Baikal, 30 km from Baikalsk, 75 km south of Irkutsk. The data were obtained by an engineering seismometric station installed on a large-panel nine-story building of the 135 series. The station consists of a 16-channel 24-bit digital-to-analog converter, 5 OSP-2M two-component accelerometers installed in the basement and on the third, fifth, seventh, and ninth floors of the building, and two SK-1P seismic sensors installed in the basement and on the ninth floor. The paper compares the peak ground acceleration values at the levels of the basement and ninth floor. The relative amplification of the acceleration amplitudes of the ninth floor with respect to the base are obtained, while it is noted that the time references of peak ground accelerations do not correlate with each other. Further study of the vibration acceleration waveforms using wavelet analysis showed that the energy of forced vibrations of the building under seismic action is redistributed from higher frequencies towards low-frequency range, close to the eigenfrequencies of the building vibrations. Based on the obtained vibration spectrograms, the frequency–amplitude gains of the building’s response to seismic events were calculated for various altitude levels. The maximum gain for the ninth floor is 17 units at the eigenfrequency of the building.

The source parameters of earthquakes in the Arctic during the entire instrumental period were calculated using a small number of stations, which in addition were remote from each other. Furthermore, during the 20th century, the source parameters of Arctic earthquakes were most often calculated from bulletin data from only part of the seismic stations operating at that time, using outdated velocity models and localization algorithms. The present article describes an approach that has already been successfully used by the authors to refine the source parameters of early instrumental earthquakes in the Arctic. The approach uses all currently available archives of bulletins and seismograms from the seismic stations that operated in the early 20th century; it also employs the modern ak135 velocity model and an improved localization algorithm implemented in the NAS program. We have relocated the epicenters of earthquakes recorded within the Arctic in the early 20th century and compiled an updated catalog of relocated seismic events. The relocation procedure was applied to 18 out of 25 earthquakes in the Arctic. The new coordinates of some earthquakes appeared to significantly differ from the previously determined ones. As a result, this may significantly affect the ultimate seismic hazard assessment of such areas as Severnaya Zemlya and Franz Josef Land, which are characterized by weak seismicity. Most of the relocated earthquake epicenters are confined to the main seismically active zones of the Arctic, namely, mid-ocean ridges, the Svalbard archipelago, and the Laptev Sea shelf.