Journal of Volcanology and Seismology最新文献

OBS observations in the Persian Gulf during a short time span have revealed the occurrence of low magnitude (M_L = –0.2–2.9) earthquakes with hypocenters in the mantle beneath the Gulf and beneath the Zagros Mountain Massif. A cross-section across the shoreline of the Persian Gulf shows the projections of these hypocenters beneath the Zagros Mountains to make inclined layers that dip northeast at a high angle into the mantle down to depths of 120‒180 km. The 3D distribution of large and moderate magnitude earthquakes based on an improved earthquake catalog as reported by the US Geological Survey and by the International Seismological Centre (ISC) is not at variance with the distribution of microearthquakes and low magnitude earthquakes but seems rather to supplement it, forming a separate seismic dipping layer. According to the data acquired by OBS observations, seismic activity occurs throughout the entire crust and upper mantle of the region rather than in the upper crustal layers only as was asserted in previous publications. It is possible that collision and accompanying phenomena (mantle seismicity and destruction of the granitic layer in the crust) are related to the hypothetical rotation of the Earth around the center of rotation placed at Cyprus Island.

This paper reports results from an analysis of swarm seismic events that were recorded in 2018 in the area of the Elbrus Volcanic Center (EVC) by the North Caucasus Seismological Network of the Federal Research Center, Geophysical Survey, Russian Academy of Sciences (FRC GS RAS). The standard location procedure was used to find the hypocenters of recorded events in the EVC area. In the LOS program package, a complex combined location algorithm, methods for minimizing the time residuals at the source, and search over a grid with several trial depth values, were used to relocate the hypocenters of seismic events in swarm sequences. This approach enhances location reliability due to incorporation of the uncertainties related to erroneous onset identification and insignificant deviations of travel times that depend on the earth velocity model used here. A study of earthquake mechanisms for the larger earthquakes has determined the type of slip in agreement with the kinematics in the Elbrus–Mineralnye Vody normal-oblique fault zone. Correlation analysis of waveforms on the vertical component that have been filtered in the frequency range 1‒10 Hz, which includes P and S waves, at the nearest station shows a high agreement among the data under analysis (the correlation coefficient is R ≥ 0.85), thus indicating a high probability of all events having similar sources and propagation paths of seismic waves. Comparison of power spectral densities for individual earthquakes in the swarm sequences also demonstrates a high similarity.

This paper describes the behavior of several seismic precursors prior to the 2016–2019 activation in an area of a complex geodynamic setting at the boundary of the Pacific, North American, and Eurasian plates. We used an integrated approach to combine identified precursors with a view to getting more accurate locations of possible rupture zones of future large earthquakes. The precursors include quiescence based on the RTL parameter and the “Z-function”, variations in the slope of the earthquake recurrence graph (γ-value), and the areas of seismic ruptures dS. For the first time in Kamchatka, we analyze anomalies in the parameter dS interpreted as seismic activations to show the presence of a precursory component before the onset of the foreshock activation. Examples include increases in the values of that parameter before events that were the largest for the areas where they occurred: the Near-Islands Aleutian earthquake of July 17, 2017, M_w = 7.8 and the Uglovoe Podnyatie earthquake of December 20, 2018, M_w = 7.3. The overall extent of the anomalous area, which still exists and includes consecutive quiescence and foreshock phases, is ~900 km, which creates conditions for possible occurrence of a new great earthquake in the junction zone of the three plates with a magnitude that can exceed whatever has been recorded in the region so far.

We know that tectonomagmatic activity periodically increased during the Earth’s history without any visible external factors to cause these occurrences. This is obviously related to the evolution of petrological processes at depth that produce events in the outer shells of the modern Earth (the tectonosphere). However, the essence of these processes and the mechanisms that translate them to the tectonosphere remain little known. We have examined this problem for the particular case of the Late Cenozoic (Neogene to Quaternary) global activation. We know that the modern Earth is a cooling body with a solidifying liquid iron core. The process must be accompanied by a number of thermodynamic, physical, and physicochemical effects, and it is these which might cause the inner activation of our planet. We have tried to shed some light on these problems using available modern geological, petrological, geochemical, and geophysical data on the activation that is just now occurring before our eyes. We have shown that the main active element on the modern Earth must be a thin crystallization zone that is constantly rising; that zone is between the wholly solidified part of the core (the solid inner core) and its completely liquid part (the outer liquid core). It is this zone which harbors various phase transitions in a cooling melt as the melt is passing bifurcation points. The phase transitions are both of the type like a change in released solid phases that accrete to the inner core and as retrograde boiling producing drops of core fluids. It is shown that the drops are rising in a high-Fe host melt and are accumulated at the base of the mantle. Once there, they participate in the generation of mantle plumes which are the chief translators of deep impulses to the outer geospheres, and leave the core for good simultaneously with impulses. It is supposed that at one such point, fluid solubility experienced a sharp drop in the cooling high-iron liquid of the outer core. This must have led to a simultaneous intensification of retrograde boiling of this melt throughout the entire surface of the core crystallization zone, that is to say, on a global scale. It is this phenomenon which must have supplied the excess of core fluids necessary for mass generation of mantle plumes and have served as a trigger for processes involved in the Late Cenozoic global tectonomagmatic activation of the Earth.