卷 55, 编号 8 (2019)

The characteristics of the short-period S-wave attenuation field in the source zone of the strongest Tohoku earthquake of March 11, 2011 in the northeast of Japan are considered (M_w = 9.0). The records of shallow local earthquakes obtained by the MAJO station at distances of 250 to 700 km are processed. A method based on the analysis of the ratio of the maximum amplitudes of the S_n and P_n waves (the S_n/P_n parameter) is used. The source zone is divided into four regions bounded by coordinates 36°–37°, 37°–38°, 38°–39°, and 39°–40°18′ N; and 140°30ʹ–145° E. It is established that all the regions contain segments of a rapid decrease in the S_n/P_n values, which are followed by segments of their sharp growth at small epicentral distances in all the areas. Another segment of rapid decrease in the S_n/P_n parameter is identified in all regions at relatively large distances. It is assumed that the first segments of rapid decrease in the S_n/P_n values are related to the gradual sinking of the S-wave rays in the mantle wedge. In this case, the minimum values of the parameter correspond to rays partly moving along the foot of this wedge. Such an effect is explained by the fact that the biggest content of the deep-seated fluids ascending due to the dehydration of the oceanic crust rocks correspond to the bottom of the mantle wedge. The segments of a sharp increase in the S_n/P_n values are likely to correspond to the propagation of rays within the upper part of the plate characterized by very weak attenuation. The second segments of the rapid decrease in the S_n/P_n parameter are related to the ray penetration into the waveguide formed in the bottom part of the plate formed as a result of the dehydration of the mantle rocks. The mean values S_n/P_n(Δ) in the four areas are much lower than in the source zone of the strongest Maule earthquake of February 27, 2010 (Chile, M_w = 8.8). This effect agrees with the earlier assumption about the larger fluid content in the subduction zones in the west of the Pacific Ocean compared to the east. Furthermore, this allows us to explain the features of the aftershock processes in these two enormous regions of the Pacific Ring.

This paper analyzes the article published in Sankt-Peterburgskie Vedomosti (SPV) and a number of foreign studies addressing the natural cataclysm that occurred in Savoy (Western Alps) in 1751 and was recognized by witnesses as a volcanic eruption. This event was observed by V. Donati, a famous natural sc-ientist of the 18th century, and described by H.B. de Saussure, a prominent researcher of the Alps. Other evidence of similar phenomena in the Alps is presented from the historical data. It is suggested that a special type of explosive eruption, not accompanied by lava pouring out but still very dangerous may occur in the Alps.

The history of lacustrine sedimentation in the Sevan depression is traced. The potential of using lacustrine sediments for the analysis of paleoclimate and neotectonic development of the region is shown. The long-term preservation of the lacustrine regime in the Sevan depression is mainly due to large lava flows that repeatedly create dams in the northwestern part of the depression. A coupled analysis of three dated sections of sediments near the coast of Lake Sevan (Dzknaget, Norashen, and Argichi) shows that the main factor determining the lake level fluctuations in the Holocene is the climate: the alternation of eras with varying moisture amounts. Transgression periods are characterized by the accumulation of shallow–lake and beach facies and regression periods are characterized by alluvial and subaerial technogenic (cultural layers) facies. Holocene sediments of Lake Sevan contain synchronous transgressions of the Subarctic, Atlantic and Sub-Atlantic periods and regressions of the Boreal and Subboreal periods. The transgression periods are associated with climate moistening and the development of tree–shrub vegetation in its watershed area, and the regressions correspond to epochs of a decrease in humidity and a reduction of this type of vegetation. The minimum lacustrine sedimentation rates are typical for beach facies that are remote from river mouths (0.1–0.3 mm/year), and the maximum values are typical for delta facies (0.7–1.5 mm/year). The results can be used to analyze and forecast long-term trends in the development of the natural environment in the watershed area of Lake Sevan and changes in its level.

In this work, an improved approach of the pixel-based analysis of the Terra/MODIS imagery is proposed. The approach allows us to improve the accuracy in estimating characteristics of the combustion zone when detecting thermal anomalies. The investigation is carried out based on the imagery of active vegetation fires in Siberian forests by the MODIS radiometer in the spectral ranges of 3930 to 3990 and 10 780 to 11 280 μm (bands 21 and 31, respectively). It is proposed to describe the approximation of the temperature profile of the fire front using an exponential function. Using the nonuniform approximation of the temperature distribution on the surface in the vicinity of the active combustion zone allows us to determine the portion of the active pixel of the Terra/MODIS image with the given temperature excess over the background temperature in it. This improves the accuracy in extracting active combustion zones and classifying the heat release rate at the subpixel level. This approach is applicable to monitoring fire development phases in the near real time mode.

The features of the geomagnetic noise distribution over frequencies in different bandwidths and in the signal accumulation mode are investigated. It is shown that the amplitude of geomagnetic noise, measured by a magnetomodulation magnetometer in the passband of 0.1 to 230 Hz at the latitude of the Middle Urals in a quiet geomagnetic field can vary from 0.2 to ~2 nT from peak-to-peak, depending on the time and place of the measurement. The conditions for measuring a weak magnetic field against a background of geomagnetic noise and the limitations associated with the random nature of the changes in the amplitude of geomagnetic noise and the lack of correlation of its amplitude at various measurement points are considered. The obtained data can be used to estimate the amplitude of a weak external magnetic field, which can be recorded in an undisturbed geomagnetic field in unshielded space, which is of interest in magnetobiology and biomedicine.