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编号 2 (2023)
Articles
How to Choose a Magnitude Interval to Evaluate the Slope of the Magnitude-frequency Graph
摘要
In modern seismological practice, to describe the distribution of magnitudes, the Gutenberg-Richter law is widely used, one of the parameters of which is the b-value (the slope of the magnitude-frequency graph on a log scale). Authors propose new approaches to the problem of adequate and efficient statistical estimation of this parameter. The problem of the correct choice of the magnitude interval is discussed, on which the straightness of the Gutenberg-Richter law is observed with an acceptable degree of accuracy and which should be used to estimate the b-value. An efficient method of accounting for discreteness and aggregation of magnitudes in earthquake catalogs (the maximum likelihood method for discrete distributions) is proposed. The problem of changes in time of the lower limit of representative earthquakes registration is considered and a statistical approach is proposed for their description.
3-11
Microseisms as a Tool for Geophysical Research. A Review
摘要
Considering seismic ambient noise as a tool for geophysical research, the spatial and temporal characteristics of the noise itself are of a great interest. Characterization of energy and frequency distributions of the microseisms is an important stage of the research. The review considers the main mechanisms of generation of microseismic oscillations in a wide frequency range, including primary and secondary microseisms (0.05–0.3 Hz), low-frequency oscillations (0.2–50 mHz), high-frequency oscillations (2–60 Hz), lake microseisms (0.5–2 Hz). The paper also describes the most popular techniques used for processing and analyzing a continuous data of seismic ambient noise; a wide range of geophysical problems was demonstrated, which use the results of seismic monitoring.
12-32
Peculiarities of the Magnetic Field of Geothermal Systems of the Pauzhetsky Area (South Kamchatka)
摘要
Based on many years of research, maps of magnetic field anomalies ΔTa for large geothermal systems of the Pauzhetsky region of South Kamchatka were constructed. Magnetic fields have both general characteristics and individual features for each research object. In the area of Nizhne-Koshelevsky steam-dominated geothermal field identified a system of linear negative magnetic field anomalies, confined to thermal controlling discontinuous tectonic disturbances. Pauzhetsky geothermal field is characterized by a heterogeneous structure of the anomalous magnetic field ΔTa: NW region is marked by a quiet weakly negative magnetic field, indicating the predominance in this part of the field lateral spreading of hydrotherms from the upper aquifer; SE – a large number of alternating magnetic anomalies of high intensity, confined to subvolcanic bodies of acidic to moderate composition. The South Kambalnaya group of thermal fields is characterized by a lower magnetic induction modulus T as compared to the Pauzhetsky and Lower Koshelevsky geothermal fields, indicating a more intense alteration of the Kambalnaya Ridge rocks by hydrothermal-metasomatic processes, apparently as a result of long-term exposure to convective heat flow.
33-51
Volcanism of the Initial Stage of Subduction of the Northern Part of the Pacific Plate (Kamchatka Peninsula, Kumroch Ridge)
摘要
We studied whole rocks compositions of the Baydara and Semkorok Mountains, which are located at the north-western part of Kumroch Ridge. Rocks are represented by Amf-Px basaltic andesites and andesites, and have microelements distribution typical for island-arc type of rocks. Some mineralogical and geochemical characteristics of the studied lavas of the Baydara Mt. (low concentrations of K2O, all REE, LILE, Th and U) and the Semkorok Mt. (low LREE concentrations) make them principally different from the rocks of the located nearby Late Pleistocene-Holocene Shiveluch volcanic massif. Isotopic K-Ar age of lavas (0.7 Ma for Baydara and 1.3 Ma for Semkorok) allow us to propose that their eruptions might be caused by the initial phase of the northern segment of the Pacific plate subduction. The Early Paleocene age (~62 Ma) of the Khapitsa series rocks, which compose north-western part of Kumroch Ridge, is confirmed for the first time by the isotopic-geological methods.
52-68
Results of Long-Term Observations of the Electric Field of the Atmosphere During the Eruption of Volcano Ebeko in the Period 2018‒2020
摘要
In order to study the processes of formation of Volume charges in the eruptive clouds of the eruptions of the Ebeko volcano in the period 2018‒2020, observations of the atmospheric electric potential gradient in the city of Severo-Kurilsk were carried out. 179 cases were recorded when the propagation of an eruptive cloud occurred in cloudless or low-cloud conditions and was accompanied by a response in variations of the atmospheric electric potential gradient. Four types of responses in variations of the potential gradient of the electric field of the atmosphere are revealed, it is shown that the type of recorded response is determined by the conditions of propagation of the eruptive cloud relative to the point of registration, and also determined by the relative location of the lower and upper regions of the eruptive cloud at the time of registration of the response. At the same time, the negative volume charge prevails in the eruptive cloud, which is localized in the upper region of the eruptive cloud, the positive volume charge is localized in the lower region. The data of field observations are consistent with the results of numerical modeling.
69-82
Deep Velocity Structure and Seismicity of the Trans-Baikal Region (Along the Reference Geological and Geophysical Profile 1-SB)
摘要
The paper presents analysis of the seismicity and deep structure of the Trans-Baikal region in the section of the reference geophysical profile 1-SB. It was determined that the Earth’s crust and upper mantle has a complex heterogeneous structure. The thickness of the Earth’s crust varies from 40 km in the South-Eastern part of the profile and in the areas of intermountain depressions in the North-Western part, and up to 48 km in the areas of mountain ranges. The values of the boundary velocities along the M boundary also vary greatly, from higher values of 8.4‒8.5 km/s for P-waves and 4.9‒4.95 km/s for S-waves (especially in the South-Eastern part of the profile) to reduced values of 7.8‒8.0 km/s for P-waves and 4.6‒4.7 km/s for S-waves in the section of the Baikal rift zone in the North-Western part of the profile. A strong inhomogeneous structure of the medium in terms of elastic wave velocities, Vp/Vs velocity ratios, and the Poisson’s ratio is determined for the upper and the middle crust at depths of 8‒20 km. The authors determined that zones of increased seismicity are referred to blocks of the Earth’s crust with inhomogeneous velocity structure according to data of differently polarized P- and S-waves. The area of the Baikal rift zone, in the immediate vicinity of the largest Muya earthquake of 1957 with M = 7.6, is characterized by elevated inhomogeneity in the upper part of the Earth’s crust according to the elastic wave velocities and secondary parameters of the medium (Vp/Vs ratio, K* = = Vp/(γ – 1), where γ = Vp/Vs, Poisson’s ratio (σ)). A number of other inhomogeneous deep zones have also been identified in the profile based on anomalies of P- and S-waves velocities and secondary parameters of the medium, which correlate to varying degrees with seismically active sites according to long-term instrumental observations. The established unambiguous connection of large inhomogeneous zones of the upper crust of the Trans-Baikal region with the accumulation of stresses and their discharge in the form of strong earthquakes allows us to make a reasonable medium-term forecast of catastrophic events.
83-96