Seismic Instruments最新文献

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.

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.

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.

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.

The article presents a description and the first results of an experiment on deformation-seismic monitoring of the technical condition of a residential building located in a subsidence zone above mine workings. Deformation processes are monitored by hydrostatic leveling. Dynamic monitoring is implemented by controlling the change in the resonance (eigen) frequency using seismological equipment. A description of the hardware and software for the deformation-seismic monitoring subsystems is given. The first results for an 8-month continuous observation period are presented. Conclusions are made about the prospects of this type of monitoring in the event of a threat to the mechanical safety of a protected building. The material is addressed to specialists engaged in solving problems in the inspection and monitoring of the technical condition of buildings and structures.

In order to clarify the genesis of liquefaction folds (convolutions) developed at lithological boundaries in lacustrine sediments, such structures in three regions were compared. Folds in each region differ in morphology, composition of sediments, and the vertical gradient of their density and viscosity upon deformation. It is proposed to use the ratio of the widths of syn- and anticlinal folds in the convolution horizon (K_s) to analyze the latter, in which K_s > 1 corresponds to the normal viscosity gradient, and K_s < 1, to its inversion. Convolutions of the Baltic Shield and Yakutia from K_s ≥ 1 are noted in the most liquefied sediments with unstable density stratification (sands-on-silts), which indicates the possibility of their spontaneous formation during lithogenesis. Folds with K_s ≈ 1 are widespread in Yakutia, which indicates their cryogenic genesis. Convolutions in the Tien Shan were formed with stable sediment stratification in terms of density (silts-on-sands); low-fluidized coarse-grained sediments with viscosity inversion were also involved in deformation. These features indicate the seismic initiation of liquefaction processes. The results substantiate the lithogenic genesis of convolutions in lacustrine complexes of the Baltic Shield, cryogenic and lithogenic in Yakutia, and seismogenic in the Tien Shan. It is proposed to determine the scatter of the K_s value in the diagrams for diagnosing the genesis of convolutions. For lithogenic structures, this parameter is shifted to the region with K_s > 1, and for seismites, it is relatively symmetric with respect to K_s = 1.

The characteristics of the attenuation field in the lithosphere of South Asia are considered. Records of local earthquakes, obtained at the NIL station, and ratios of maximum amplitudes of S_n and P_n waves within the distance range of ~300–1900 km are analyzed. About 200 earthquake seismograms were processed in aggregate. It is established that generally lower attenuation is observed in the uppermost mantle beneath the Indian Plate (for the meridional profile directed toward the source zone of large Bhuj earthquake of January 26, 2001, with M_W = 7.7). Considerably higher attenuation corresponds to the regions of Himalayas and, especially, Southern Tibet. It is shown that increased attenuation is observed in the source zone of the recent large Nepal earthquake of April 25, 2015 (M_W = 7.8). Additionally, lower and intermediate attenuation is reported within the source zones of large and great interplate events (M_W = 7.0–8.1), occurred in the Himalayan region in 1897–1930. Conversely, substantially decreased attenuation corresponds to the source zone of the Bhuj intraplate earthquake. These new results are consistent with earlier data, which indicate concentration of mantle fluids below source zones prior to large interplate earthquakes, as well as ascent of fluids into the crust after these events. High attenuation zones are distinguished in the regions of West Himalayas and central Pakistan, where large earthquakes have not occurred for a long time. It is suggested that processes related to the preparation of large seismic events can occur there.