No 6 (2024)

Using the example of two tsunamigenic earthquakes, a method for determining the magnitude of the displacement of the seabed, which is the cause of the tsunami, is being worked out. It is shown that with an average coefficient of divergence of deformation anomalies recorded by a laser strainmeter for the entire planet, it is possible to approximately estimate the amount of displacement of the seabed at the site of tsunami formation. For each region where the tsunami originated, there are more accurate divergence coefficients that can be estimated experimentally and from which the values of seabed displacements can be more accurately determined.

The article considers the problem of determining the parameters of the seismic regime for the territory of the Irkutsk region. To solve this problem, a complete catalog of earthquakes within the studied region with a unified magnitude scale was created for the time period from 1962 to 2021. Determination of seismic regime parameters is an important step for subsequent seismic hazard assessments. The solution of this problem is extremely important for insurance and reinsurance companies, as it makes it possible to use the probabilistic approach in the tasks of earthquake risk assessment, which in turn allows to make the most correct management decisions and ensure the stability of the company’s financial system.

We consider two earthquakes which occurred on October 18, 2017 and October 25, 2017 in Western Transbaikalia. In spite of both the events are of moderate energy level, they can be regarded as significant for the study region, as such events are rarely registered there in comparison with the adjacent active areas of the Baikal rift zone. Focal mechanisms of both the earthquakes, based on surface wave amplitude spectra, demonstrate that their sources were formed under influence of dominating subhorizontal latitudinal compression and inclined or subvertical NW-SE extension that is common for Western Transbaikalia. Scalar seismic moments (M₀), moment magnitudes (M_w) and source depths (h) have also been determined: M₀ = 5.0·10¹⁵ N·m, M_w = 4.4 and h = 7 km for the October 18, 2017 earthquake and M₀ = 3.5·10¹⁵ N·m, M_w = 4.3 and h = 29 km for the October 25, 2017. The considered events caused noticeable macroseismic effects in the near field zones; maximum observed intensity is IV–V and V (MSK-64) for the October 18 and 25, 2017 earthquakes, respectively. Based on the data obtained, the current activity of faults in the study area is considered. The obtained results can be used in seismic hazard assessment for the territory of Western Transbaikalia.

Geochemical and isotopic–geochemical (Sr–Nd–Pb) data on the quaternary intraplate hauyne basanites and ordanshites in the Lesser Caucasus provide an insight into the most probable nature (characteristics of the composition and the depth of occurrence) of the EM II type enriched mantle source, from which, along with the Caucasus OIB-type plume–asthenospheric source, the parental melts of the rocks were derived. The source of the type was demonstrated to have been modified by a subduction-related component. Data are presented on the likely timing and mechanisms of contamination of the magma-generation regions with slab material. Our data suggest that the residue in the mantle source contained garnet, amphibole, and rutile. Our results led us to conclude that the source enriched in incompatible elements was most probably subduction-modified (in the course of Mesozoic and, perhaps, also Paleogene subduction events) subcontinental lithospheric mantle of the EM II type, which likely corresponded to rutile-bearing amphibole–garnet peridotite.