Izvestiya, Physics of the Solid Earth

The article analyzes the aftershock activation observed in the south of Avacha Bay (Kamchatka) as a result of the Vilyuchinsk earthquake on April 03, 2023 MW = 6.6 (j = 52.58°N, l = 158.78°E, h = 105 km). The total duration of this activation is estimated at 450 days. The linear dimensions of the studied area are limited horizontally by an ellipse with axes a = 34 km (azimuth a = 117°), b = 22 km, in the depth range h = 95-105 km. The orientation of the ellipse corresponds to one of the two nodal planes of the main event mechanism. Two stages in the attenuation of the aftershock flow were distinguished: 1) in accordance with the Omori law and with a duration of 8.4 days; 2) with exponential (t = 140 days) attenuation until reaching the background seismicity level. In terms of the number of earthquakes, the two stages are comparable. The phase of regime change corresponds in time to the interval between the strongest aftershocks — April 11, 2023 ML5.5 and April 21, 2023 ML4.9. It is shown that these two earthquakes are a doublet based on the high correlation of waveforms and the proximity of their epicenters. A sharp postseismic decrease in the slope of the Gutenberg-Richter graph bby 1.5 times after the strongest aftershock on April 11, 2023 ML5.5 is shown, with subsequent recovery throughout the exponential aftershock regime. No variations were detected in the bparameter in the Omori regime.

A number of specialized software packages have been now developed to calculate seismic hazard curves. Their algorithmic basis is based on the use of numerical methods. The key feature of such calculations is the discretization of the selected areas of seismic source zone (SSZ) — each zone is divided into finite elements, and each element has seismic activity proportional to its area. Besides, the values of earthquake magnitudes in the calculations also take a discrete set of values. Numerical methods certainly open upthe possibility of solving problems that cannot be described analytically, but they also introduce certain errors. Too large a division may not take into account local features of the seismic regime. The accuracy of the final results depends significantly on two key parameters: the selected magnitude discretization step and the principles of dividing the SSZ zones. In this study, weconsider a localized seismic source whose event flow strictly corresponds to the GutenbergRichter law. To determine the intensity of seismic vibrations, the Shebalin macroseismic field equation is used.

The properties of the Earth’scrust of the western Orenburg region are discussed. The data analysis of the broadband seismic station ORR (Orenburg), which is part of the “Neftegaz-Seismika” network, was carried out using the receiver function method. The Zhu-Kanamori method was used to estimate the average parameters of the crust — the thickness and the ratio of the longitudinal and transverse velocities of seismic waves — within the framework of a single-layer model. A one-dimensional section of seismic wave velocities under the station was obtained by inversion of the waveforms of the receiver function. It is shown that the upper part of the constructed section is characterized by low values of transverse wave velocities. This is consistent with the presence of terrigenous and oil- and gas-containing rocks under the seismic station. The obtained section, in general, does not contradict the interpretation results of two DSS profiles performed in this region, except that the crustal thickness calculated by the receiver function method and approximately equal to 35–37 kmturns out to be 7–10 kmless compared to the DSS results. In order to make a final conclusion about the crustal thickness in the region, additional research on observations of a larger number of broadband seismic stations is required.

The article is devoted to the creation of maps of the Earth’sgravitational field parameters using the measurement data of a gravimetric complex, including an astrometer of the plumb line deviation and a high-precision relative gravimeter. Conducting joint area measurements using the complex allows determining the following Earth’sgravitational field parameters: acceleration of gravity and gravity anomalies, components and the total value of the plumb line deviation, components of acceleration of gravity and gravitational gradients (second derivatives of the anomalous potential). The total measurement time at a single point using the complex does not exceed 1 hour, the standard deviation of the measurement series of the plumb line deviation components is from 0.1″ to 0.3″, the acceleration of gravity is 10 μGal. The paper presents examples of maps of the above-mentioned Earth’sgravitational field parameters, created based on the measurement results using the gravimetric complex in the Moscow region, including the measurement profile across the gravitational-anomalous Moscow attraction.

This paper is devoted to the study of the features of thermoremanent magnetization (TRM) acquisition in brick fragments from a Roman kiln, uncovered by archaeological excavations in the vicinity of the village of Dragovishtitsa (Western Bulgaria), in experiments using the Thellier method as modified by Coe. The aim was to determine the magnitude of the geomagnetic field in ~300 AD. Initially, petromagnetic and archeomagnetic studies were carried out in the Paleomagnetic Laboratory of the National Institute of Geophysics, Geodesy and Geography of the Bulgarian Academy of Sciences. Then, experiments using the Thellier-Coe procedure at two cooling rates of samples taking into account the TRM anisotropy were carried out in the Laboratory of the Main Geomagnetic Field and Petromagnetism of the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences. Based on 18 archaeointensity determinations, the average weighted value of the magnetic field B_av= 56.5±0.8 μTwas obtained, corresponding to the archaeological age of 260±20 CE. The dating of 281-342 CEobtained by the archaeomagnetic method is consistent with the archaeological estimate of the kiln operation time. The calculated value of archaeointensity is in satisfactory agreement with the reference values of the magnetic field for Bulgaria and confirms its decrease in the time interval of ~200-300 CE.

In order to prove the synchronicity of magnetization to the Formation of rocks of the Neoproterozoic Katav Formation (Southern Urals), a paleomagnetic conglomerate test was performed. Samples of breccias from two sections of the carbonate Katav formation located in the vicinity of Tolparovo and Katav-Ivanovsk were studied. Petrographic and electron microscopic studies of thin sections and polished sections have been carried out, confirming the similarity of the mineral composition in the fragments and matrix. The qualitative assessment of the chaotic directions of magnetization of the fragments according to Graham was verified by more stringent Rayleigh and Hodges-Ajne criteria. The results allowus to state that the conglomerate test is positive, and the high-temperature magnetization component in the rocks of the Katav Formation is primary.

On the example of transient electromagnetic sounding (TEM) of moraine deposits the efficiency of the TEM-FAST technology is demonstrated. This technology uses a compact 25 m × 12.5 mcombined transmitterreceiver antenna for measuring transient responses in the microsecond range. The research depth with specific resistance of sandy-clayey deposits of 30–300 Ohm·mis notless than 50 mwith a “dead” zone thickness of not more than 6 m. For mobile pedestrian sounding, an antenna fixed on 4 flexible poles at a height of 2.5 mabove the earth′ssurface was used. The measuring system, with a total mass of 2.5 kg, was transported along the profiles by a team of 4 people at a speed of ~1 m/s. Every 20 seconds after processing 13,000 pulses, the system in the “autopilot” mode recorded the measurement results and coordinates. The repetition rate of current pulses with an amplitude of I = 3.3Аwasf = 3.2 kHz, the measured range of transient response timest = 4 — 64 μs. Vibration noises arising during movement due to deformations of the antenna contour in the Earth′smagnetic field reduced the sounding depth to 50–60 m. Based on the area sounding data, 3D geoelectric model of the territory was constructed, which was used to assess the capabilities of the TEM-FAST system in flight mode. Transient responses were synthesized, corresponding to different antenna heights above the surface of the studied environment. Despite the decrease in response amplitudes when raising the antennas to a 30 mheight, the sounding depth remained within 50 m. Based on mass-produced light UAVs (unmanned aerial vehicles) equipped with real-time dynamic positioning systems, it is proposed to tow the TEM system with a convoy of devices flying or hovering above the earth′ssurface in autopilot mode. When positioning antenna angles with decimeter accuracy, the noise threshold of transient response measurements will not exceed the levels obtained during mobile pedestrian sounding, and the research depth will not fall below 50 m.