Fizika zemli

In contrast to reversals, excursions of the geomagnetic field can occur at lower convection intensities in the Earth’s core. Since in such geodynamo regimes the behavior of the magnetic field is still quasi regular, a reduction of the dipole field during an excursion may indicate a global failure of the dynamo process. As a consequence, it is possible that during the excursion, not only the dipole component, but also the higher harmonics of the field decrease. This hypothesis is tested in a three-dimensional (3D) dynamo model.

The paleodirection and paleointensity of the geomagnetic field are determined in seven andesibasalt–basalt samples from three lava flows (L1–L3) of the Avacha volcano. The reliability of the Thellier–Coe method for geomagnetic field paleointensity determination is demonstrated on the sample from the modern lava flow TTI-50 (the Tolbachik Fissure Eruption): the difference between the calculated Han and IGRF12 values is within 3% with a quality factor q > 13. The age of lava flows L1–L3 is refined using paleomagnetic data. Flows L1, L2 of the Young Cone were formed in 1827 and 300–600 years ago, respectively, and the age of flow L3 on the crest of the somma is determined at 30–32 ka, which agrees with the known age estimate (14C dating) of the avalanche associated with the catastrophic destruction of the Avacha volcano 29 900 ± 900 years ago.

The manifestation of coast effect is studied based on the magnetotelluric (MT) soundings made on three profiles in the southern part of the Sikhote-Alin folded system (SAFS) surrounded by the Sea of Japan in the south and east. It is found that the coast effect is poorly expressed in the MT sounding amplitude curves while well pronounced in the behavior of the magnetovariation response functions. The analysis of the complex Wiese tipper and its real induction arrows in the north and east directions identified the intensity of the coast effect within the study area and the influence of the main deep faults and conductive zones in the Earth’s crust and upper mantle on the effect. Based on the three-dimensional (3D) numerical modeling, the main factors affecting the behavior of the amplitude MT sounding curves in the presence of coast effect are determined for the conditions of the southern part of SAFS and different geoelectric models.

It is shown that most of the epicenters of marsquakes are located in the zones of extension and fairly large shear stresses associated with the deviation of Mars from hydrostatic equilibrium. Non-hydrostatic stresses in the interior of Venus are calculated for two types of models: an elastic model and a model with a lithosphere of varying thickness (150–500 km) overlying a weakened layer that has partially lost its elastic properties. Numerical modeling of the system of elastic equilibrium equations for a gravitating planet is carried out with a step of 1°×1° in latitude and longitude up to a depth of 480 km – the first phase transition zone in the mantle. The topography and the gravitational field of the planet are the boundary conditions of the problem. Overall, the level of nonhydrostatic stress on Venus is not very high. On the surface and in the crust, the highest shear stresses are observed in the region of the Maxwell Monte on Ishtar Terra. Beneath the Maxwell Monte, shear stresses in the crust reach 80 MPa and compressive stresses, 125–150 MPa, depending on the model. Tensile stresses around this region are about 20 MPa. The highest tensile stresses occur in the regions beneath structures such as Lavinia Planitia, Sedna Planitia, and Aino Planitia.

For the eastern zone of the North Caucasus, including the Tersko-Caspian trough, the Dagestan wedge and adjacent structures, with the maximum coverage of the data from the Federal network of seismological observations of the Geophysical Survey of the Russian Academy of Sciences (GS RAS), the regional frequency-dependent relations for estimating the seismic Q-factor of the Earth’s crust and upper mantle are determined by the coda-wave envelope method. In total, waveforms of 394 crustal earthquakes with source depths from 1 to 42 km and magnitudes from 2.2 to 5.5 are analyzed using the Coda Q module of the SEISAN software package (Havskov et al., 2020). The seismic-Q estimation technique using the single-scattering model is described in detail. Recommendations are given how to practically use the obtained estimates characterizing the wave properties of different-scale volumes of the fault-block geological environment to make attenuation corrections when calculating the source spectral parameters in the eastern zone of the North Caucasus. The results of the study will help to improve the information content of earthquake catalogs, in particular, to fill them with data on the energy characteristics of seismic events (universal values of moment magnitudes, Mw) in the most seismically active regions of the Russian Federation.

There are regions of moderate seismicity that have long lacked a dense network of seismic stations due to their sparse population, inaccessibility, and low economic levels. Characterization of earthquakes in such regions is typically limited to the small amount of macroseismic information available and data from only one or two seismic stations. Most often, the location of such earthquakes was based on the use of macroseismic information alone. We propose a method for probabilistic estimation of the location of such earthquakes based on a joint analysis of their macroseismic and instrumental data. The technique is implemented in the ProLoM program (Probabilistic Locator by Macroseismics). In this study, we test the performance of the method on a test earthquake of May 20, 1967 and present the results of the analysis of the earthquakes that occurred in the north of the East European Platform on June 30, 1911 and January 13, 1939.

This study estimates the impact level of storm microseisms on long-term gravimetric measurements. Gravimetric measurements were conducted at the Zapolskoe, Obninsk, and Murmansk sites using CG-5 Autograv relative gravity meters. Seismic measurements were carried out concurrently with gravimetric measurements at Zapolskoe. The analysis of these measurements demonstrates the feasibility of utilizing seismic data as control information to estimate the high-frequency noise component of gravimetric data. Based on the additional seismic information from the services of the Incorporated Research Institutions for Seismology, the correspondence between the attenuation of the noise component in gravimetric measurements and the data from the modeled sensitive element of the gravimeter, which utilizes seismic series as an input, is identified. The first characterization of the storm-induced background noise of gravimetric measurements at the Murmansk site is obtained. Furthermore, the possibility of predicting measurement errors based on meteorological forecasts is refined, which can aid in the planning of gravimetric work.