Journal of Volcanology and Seismology最新文献

This is the first petrophysical study of extrusive rocks (dacites to andesites) discharged by Bezymianny Volcano. We provide a comparative description of properties for extrusive rocks in accordance with identified age groups. We show the dynamics in the variation of extrusive rock properties in relation to their ages, with the result that the older a rock the higher are its density, strength, and elastic parameters. Rocks petrophysical features are compared between extrusive domes and lava flows. We argue for petrophysical properties to be applicable for deriving more accurate results for the genesis of rocks having similar petrophysical properties, in particular, rocks of extrusive and effusive origin.

This paper reports results of mathematical modeling applied to the stress and strain in epicentral zones before and after the large earthquakes that occurred on June 22, 2002 in the Qazvin Province, northwestern Iran (M_w = 6.4) and the Gujarat, India earthquake of January 26, 2001 (M = 6.9). The modeling relied on a method for calculating stress and strain in a blocky elastic isotropic heterogeneous medium disturbed by a set of faults that are due to an external tectonic stress field. The boundary conditions were specified based on geological and seismological data. It has been shown that the epicenters of large crustal earthquakes occur in zones of high stress concentration at the ends of tectonic faults. Rupture occurs when the relationship between acting tectonic stresses satisfies the requirement σ_yy/σ_xx >3, thus connecting zones of high stress. The evolution of the aftershock process is controlled by the stress drop caused by a new rupture, while the resulting aftershock clusters are in spatial correlation with the stress drop zone. The new rupture propagates in the direction of dominant tectonic fault orientation in the region. We show a relationship to exist between rupture length and the possible retrospectively predicted location and magnitude of the earthquake depending on the elastic energy buildup and its possible release under specified structural tectonic conditions.

Modern seismological practice makes a wide use of the Gutenberg–Richter relation to describe the magnitude distribution. One of its parameters is the b-value (the slope of the earthquake recurrence curve in a log scale). This study proposes some new approaches to the problem of adequate and efficient statistical estimation of the parameter. We discuss the correct choice of the magnitude range over which the Gutenberg–Richter relation is a straight line to an acceptable degree of accuracy, and which should be used to estimate the b-value. We propose an efficient technique for incorporating discreteness and the grouping of magnitudes in earthquake catalogs (the method of maximum likelihood for discrete distributions). We discuss the problem of time-dependent variation for the lower threshold of complete earthquake reporting and propose a statistical approach to this variation.

The research in seismic ambient noise as a tool for geophysical studies must primarily rely on the space-time characteristics of the noise itself. What is important in this research is to characterize the distribution of noise sources both over frequency and in energy content. The present review considers the main mechanisms for the generation of microseisms, including primary and secondary microseisms (0.05–0.3 Hz), low frequency oscillations (0.2–50 mHz), high frequency oscillations (2–60 Hz), and lake-generated microseisms (0.5–2 Hz). We also describe the most popular procedures in use for the processing and analysis of continuous seismic ambient noise arrivals; we demonstrate a wide range of geophysical problems based on recordings of microseismic ground motion.

The gradient of potential in the atmospheric electric field was recorded in the town of Severo-Kurilsk in order to study the formation of 3D electric charges in eruption clouds due to explosions during the 2018–2020 eruptions of Ebeko Volcano. A total of 179 cases were recorded in which the eruption cloud was propagating under cloudless conditions (or nearly so) accompanied by responses in the variations of electric potential in the atmosphere. Four characteristic types of response have been identified in the variations of atmospheric electric field potential. It is shown that the type of recorded response was controlled by the conditions of propagation for the eruption cloud relative to the recording site, as well as being influenced by mutual positions of the charges in the lower and upper regions of the eruption cloud at the time the response was recorded. The eruption cloud was dominated by a negative 3D electric charge localized in the upper region of the eruption cloud, while the positive 3D electric charge was localized in the lower region. The field observations are in agreement with the results of numerical simulation.

The present paper reports a study of rocks in Mts. Baidara and Semkorok, which are situated in the northwestern flank of the Kumroch Range. The rocks are Amf-Px basaltic andesites and andesites, and are characterized by an arc type of trace element distribution. Some mineralogical and geochemical features in the composition of lavas sampled at Mt. Baidara (low concentrations of K₂O, as well as of the entire range of REEs, of large-ion elements, thorium, and uranium) and at Mt. Semkorok (low concentrations of light REEs) are substantially different from the rocks of the adjacent Shiveluch Late Pleistocene–Holocene volcanic massif. The K-Ar isotopic lava ages are ~0.7 Ma (Baidara) and ~1.3 Ma (Semkorok), suggesting the inference that the eruptions might be related to the initial phase in the formation of the northern subduction segment in the Pacific plate.

This study is concerned with an analysis of seismicity and deep structure in the Transbaikalia along the 1-SB reference geophysical traverse. We have found a complex inhomogeneous structure of the crust and upper mantle. The crustal thickness varies between 40 km in the southeastern part of the traverse and in intermontane troughs in its northwestern part on the one hand and 48 km in mountain ranges. Strong variation also affects boundary velocities at the Moho, ranging from 8.4‒8.5 km/s for compressional waves and 4.9‒4.95 km/s for shear waves (especially in the southeastern part of the traverse) to lower values of 7.8‒8.0 km/s for compressional waves and 4.6‒4.7 km/s for shear waves in the area of the Baikal Rift Zone in the northwestern part of the traverse. A strongly inhomogeneous earth structure based on elastic wave velocities, Vp/Vs velocity ratios, and Poisson’s ratio was found for upper and middle crust at depths of 8‒20 km. It was also found that zones of higher seismicity tend to coincide with crustal blocks with inhomogeneous velocity structure based on differently polarized compressional and shear waves. Higher inhomogeneity in upper crust as inferred from elastic wave velocities and secondary earth parameters (Vp/Vs velocity ratios, the parameter K* = Vp/(γ ‒ 1), where γ = Vp/Vs, and Poisson’s ratio (σ)) characterize the area of the Baikal Rift Zone in an immediate vicinity of the great Muya earthquake of 1957 with М = 7.6. As well, several other deep zones of inhomogeneity have been identified along the traverse line based on anomalies of Р and S velocities and secondary earth parameters that correlate to varying degrees with seismically active areas based on multiyear instrumental observations. We have identified an unambiguous relationship of large inhomogeneous zones in the Transbaikalia crust with stress buildup and stress release in the shape of large earthquakes, thus substantiating the intermediate-term prediction of catastrophic events.

Our multiyear studies have enabled us to make maps showing magnetic anomalies ΔT_a for major geothermal systems in the Pauzhetka area of southern Kamchatka. Magnetic fields possess both general characteristics and individual features for each object of study. The Nizhne-Koshelev vapor-dominated geothermal field is identified on the basis of a set of linear negative magnetic anomalies confined to thermal-controlling tectonic faults. The Pauzhetka geothermal field is characterized by an inhomogeneous structure of the anomalous magnetic field ΔT_a: the northwestern area has a quiet, slightly negative magnetic field, showing that this part of the field is dominated by lateral spreading of hydrothermal fluids from the upper aquifer; the southeastern part has numerous sign-varying magnetic anomalies of high intensity confined to acid–intermediate subvolcanic bodies. The South Kambalnyi group of thermal fields is characterized by a lower absolute value of magnetic induction Т compared with the Pauzhetka and the Nizhne-Koshelev geothermal fields, showing that the rocks of the Kambalnyi volcanic range are more intensely altered by hydrothermal and metasomatic processes, probably as a result of a long-continued action of convective heat flow.

This paper reports the results of seismic, electromagnetic, and gravity surveys, as well as seismicity data in the lithosphere at depth for the active volcanoes in the Avacha–Koryaksky group which is part of the East Kamchatka volcanic belt. We have developed an integrated geophysical model for the crust and lithospheric mantle in the area of study. The resulting distribution of crustal geophysical inhomogeneities, in particular, beneath Avacha Volcano, was used to study the main features of intracrustal fluid saturation and of pathways as channels for deeper fluids to rise to the upper crust. The integrated model assumes that the stresses that arise at boundaries of zones with different conditions of defluidization constitute one of the factors to produce seismicity beneath the active volcanoes. We also incorporate data from regional seismic tomography to deal with a general scheme for deep processes in the lithosphere and the system that supplies magma to the volcanoes. It is assumed that active volcanoes, in particular, Avacha, are connected to the asthenospheric layer of the lithosphere mantle at a depth of about 70–120 km, whence fluids/melts are coming into the magma chamber in the lower crust with subsequent formation of a peripheral chamber in the upper crust beneath the volcanic cone due to the heat supplied by the lower crustal source.

We have identified signs showing mixing of moderately potassic magnesian and high alumina magmas on Klyuchevskoi Volcano based on the analysis of mineral compositions and the geochemistry of trace elements during crystallization. The mineralogy and distribution of Mg, Fe, Cr, Ni, Co, and Al in olivines and clinopyroxenes which are contained in the magnesian basalts and high alumina basaltic andesites discharged by the flank and summit eruptions of Klyuchevskoi in 1938, 1966, 1945, and 1994, all provide evidence of an injection of magnesian basaltic melts into high alumina magma.