Vol 17, No 6 (2023)

Using the results of instrumental observations made at the INTERMAGNET network and at the ionospheric sounding station “Wakkanai”, it is shown that a strong eruption of the volcano Shiveluch of an effusive-explosive nature, which occurred on April 10, 2023, was accompanied by variations in the Earth’s magnetic field, as well as changes in the critical frequency of the ionosphere F2-layer.

The 2003 Chuya earthquake had a noticeable impact on the seismicity of the whole Altay region. The article presents data on changes in the seismic regime of Altay from 2003 to 2021. It is shown that in the first six months after a major earthquake in 2003, earthquakes occur in the epicentral zone, and other structures of Altay are aseismic. After 2009, changes are formed both in the near zone, covering geological structures adjacent to the epicenter, such as the Aigulak, Kuray, South Chuya, North Chuya ridges, and in the far zone at distances of 250‒300 km from the epicenter and in different directions from it. After the Aigulak earthquake of 2019, an aftershock process was formed, which ensured the displacement of the center of Altay seismicity into the ridge of the same name. The explanation of the seismicity development in space and with time delays for years can be given on the basis of the influence of a large earthquake on the multi-layered lithosphere in accordance with models that assume the occurrence of disturbances during a large earthquake not only in the fragile part of the earth’s crust, but also in layers with high plasticity.

The article describes the behavior of the series of seismic precursors before the activation of 2016‒2019 in an area with a tectonically complex geodynamic environment at the boundaries of the Pacific, North American, and Eurasian lithospheric plates. In order to clarify the location of possible focal areas of future strong earthquakes, a comprehensive approach to the application of allocated precursors was considered: quiescence by parameters RTL and Z-function, variations of the slope of the recurrence graph γ and areas of seismic ruptures dS. For the first time, in relation to Kamchatka, for anomalies to parameter dS, interpreted as seismic activations, the presence of a predictive component before the onset of foreshock activation is shown by the example of an increase in its values before the events that are the strongest for the areas where their focal areas are located: the Near islands Aleutian earthquake with M_w = 7.8 on July 17, 2017 and the Uglovoye Podnyatiye earthquake on December 20, 2018, М_w = 7.3, is demonstrated. The total length of the anomalous region, currently existing and including successive stages of quiescences and foreshock activations is ~900 km, which allows for the possibility of a new strong earthquake with a magnitude exceeding all previously recorded in this area in the junction zone of three lithospheric plates.

The article presents new petrogeochemical data of rocks from the area of the Outer Islands of the Gulf of Finland. Previously, quartz porphyries of Gogland Island were considered as a standard of volcanic comagmates of Rapakivi granite of the Vyborg massif. There are two effusive tolshas on Sommers Island. The “Southern” is composed of quartz porphyries, with an isotopic age of 1.66 Ga old, so they can be considered as the earliest manifestations of Riphean magmatic activity in this area. In addition to quartz porphyries, trachybasalts, andesibasalts, and dacites are present in the “Northern” strata, and its isotopic age is 1.59‒1.57 Ga old, which does not correspond to the values of the nearby Vyborg massif 1.65‒1.63 Ga old. Therefore, despite the pro-ximity of the compositions of quartz porphyry and rapakivi granites, volcanites are not products of differentiation of the magma initial for granites, but were formed independently of them from the same type of source, but in a slightly different structural environment, allowing their free flow to the Earth’s surface.