No 4 (2019)

ARTICLES
Spatial and temporal structure of global low-frequency seismic noise
Sobolev G.A., Zakrzhevskaya N.A.

Abstract

The paper aims to study in detail the structure of seismic noise before and after the Sumatra mega-earthquake with M = 9.1 of December 26, 2004. The records by the IRIS seismic stations in different regions of the world, equipped with STS-1 seismometers providing ground motion velocity recording in a broad range of periods from 0.2 to 360 s with the use of standard equipment, form the empirical base of the research. In the records of each station, intervals free of earthquakes, interference from manmade impacts, and noise enhancement due to cyclone propagation were selected. The amplitude of steady seismic noise in the range from 40 to 360 s, not complicated by the recordings of earthquakes and effects of meteorological origin, is in the order of 20 to 40 nm, which characterizes the actual resolution of a broadband seismic station. The noise bursts in the ranges 40–80, 80–160, and 160–320 s differ by the shape and time of occurrence suggesting different sources of their generation. The absence of the correlation between the noise recordings at the neighboring seismic stations spaced 102–103 km apart indicates the influence of local processes. The noise reflects turbulent processes in the Earth’s atmosphere, whereas the exponential growth of the noise with the increase of the oscillation period is consistent with A.N. Kolmogorov’s theory of locally isotropic turbulence in the atmosphere. The noise amplitude after the Sumatra mega-earthquake with M = 9.1 of December 26, 2004 increased by a factor of 1.5–2 in January 2005 compared to January 2004, irrespective of the location of the seismic station.

Физика Земли. 2019;(4):3-14
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Forecasting aftershock activity: 4. Estimating the maximum magnitude of future aftershocks
Baranov S.V., Pavlenko V.A., Shebalin P.N.

Abstract

In this paper, we consider the problem of forecasting the magnitude of the future, starting from a certain instant of time, strongest aftershock. This problem is topical since the later strong aftershocks occur against the background of the less frequently repeating shocks, are less expected and thus pose an independent hazard. At the same time, the magnitudes of the strongest aftershocks decrease with time after the main shock. The purpose of accurate forecasting is to minimize the underestimation or overestimation of the magnitude of future risks. In this study, the aftershock process is represented by the superposition of the Gutenberg–Richter and Omori–Utsu laws whose parameters are estimated by the Bayess method using the data on the aftershocks that have already occurred to a given time point and the a priori information about the probable values of the parameters. This significantly improves the forecast compared to the estimates that are based on the magnitude of the main shock alone. The quality of forecasting is estimated relative to the Båth’s dynamic law with the use of two independent criteria. The first criterion is based on the similarity estimates, and the second, on the error diagram.

Физика Земли. 2019;(4):15-32
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Strong-earthquake-prone areas recognition based on an algorithm with a single pure training class: I. Altai–Sayan–Baikal region, M ≥ 6.0
Dzeboev B.A., Gvishiani A.D., Belov I.O., Agayan S.M., Tatarinov V.N., Barykina Y.V.

Abstract

A new version of the Barrier algorithm is proposed for recognition of strong-earthquake prone regions based on training over a single reliable training class. The modification of the algorithm consists in creating blocks that reveal the geological–geophysical features (attributes) characteristic of the recognized highly seismic objects and provide their quantitative estimates. The recognition of the areas prone to earthquakes with М ≥ 6.0 is carried out for the Altai–Sayan–Baikal region. The results of the recognition are used for assessing the effect of the remote earthquakes that occurred in the Altai–Sayan orogenic region on the stability of structural-tectonic crustal blocks in the contact zone of the West Siberian platform and the Siberian plate.

Физика Земли. 2019;(4):33-47
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Near islands Aleutian earthquake with Mw = 7.8 on july 17, 2017: i. Extended rupture along the commander block of the Aleutian island arc from observations in Kamchatka
Chebrov D.V., Kugaenko Y.A., Lander A.V., Abubakirov I.R., Gusev A.A., Droznina S.Y., Mityushkina S.V., Ototyuk D.A., Pavlov V.M., Titkov N.N.

Abstract

The largest instrumentally recorded regional back-arc earthquake with MW = 7.8, MC = 8.2 occurred on July 17, 2017 on the Bering transform fault (the boundary between the Beringia minor lithospheric plate and the Komandorskii (Commander) block of the Aleutian island arc). This seismic event, called the Near Islands earthquake or Near Islands Aleutian earthquake, caused the ground to shake with intensity I = 5–6 on Bering Island and a small tsunami wave on the Near Islands. The specific aspects of the peculiarities of the pattern of seismicity of the northwestern segment of the Aleutian arc and the tectonic position of the Near Island Aleutian earthquake, the details of its prompt processing, macroseismic manifestations, analysis results of ground motion peak amplitudes, focal mechanisms, and earthquake source models are discussed. The coseismic displacements according to the GNSS data are presented. It is concluded that the source of the Near Islands Aleutian earthquake did not fill the seismic gap near the Commander Islands, and the possibility of the strongest earthquake hitting the northwestern part of the Aleutian arc remains.

Физика Земли. 2019;(4):48-71
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The Mw = 7.8 earthquake of july 17, 2017 near the Commander islands: tectonic position and geodynamic setting
Rogozhin E.A., Lutikov A.I., Dontsova G.Y., Zhikovets V.N.

Abstract

The tectonic position and overall geodynamic setting, as well as the seismological characteristics and peculiarities of the aftershock process, are considered for the source of the strongest Near Island Aleutian earthquake of July 17, 2017, МW = 7.8, on the Commander Islands. It is shown that in contrast to the eastern segments of the Aleutian island arc, the subduction of the Pacific lithospheric plate beneath the Commander block is not observed. The analysis has shown that, according to the distribution of the aftershock epicenters in the form of a linear elongated narrow zone with a length of about 400 km, the seismic source occupied almost entire northern slope of the Commander Island rise and spread in the Bering fault zone. It spanned the whole of this seismogenic zone up to the transverse structure west of the Near Islands (Attu islands). The focal mechanism solutions and the pattern of displacements in the sources of the main shock, as well as the strongest foreshocks and aftershocks, suggest that the slip in the source was an almost pure right-lateral shear. The aftershock process of the earthquake on July 17 developed quite feebly for an earthquake of this magnitude. Besides, it has two specific features distinguishing it from the aftershock processes of most of the Kuril-Kamchatka earthquakes: (1) a low release of the cumulative scalar seismic moment (M0cum aft), which, according to different estimates, made up from 0.75 to 1.0% of the main-event seismic moment (M0me); and (2) a very slow growth of the deficit in the release of seismic moment (M0). At the same time, the duration of the quasi-stationary phase of the M0cum release in the aftershocks, which is estimated at approximately half-a-year and which took a considerable span of the total length of the aftershock process from this earthquake, appears to be untypically long. These features of the aftershock process of the Near Island Aleutian earthquake of July 17, 2017 distinguish it from the aftershock processes peculiar to most of the strong Kuril–Kamchatka earthquakes. Overall, its source can be considered as a transform one between the two Benioff zones, the Aleutian and Kuril–Kamchatka ones, rather than as a subduction source characteristic of the last two zones.

Физика Земли. 2019;(4):72-88
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Regularities in the spatiotemporal variations of deformation processes in the region of Japan subduction zone
Steblov G.M., Sidel`nikova I.A.

Abstract

The approach is proposed for modeling deformations in the subduction zones from the surface displacement data recorded by the methods of satellite geodesy. The technique is developed for revealing spatial and temporal variations in the deformations in the subduction zones. Based on the suggested approach, the variations in the deformation processes are studied in the interplate contact zone in the region of the Japanese islands at different stages of the seismic cycle. The possibility of identifying the preparation process of the strongest interplate earthquakes based on the satellite geodetic data is explored and the probable indications of this process are suggested.

Физика Земли. 2019;(4):89-98
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Seismicity, tectonics, and GPS geodynamics of the Caucasus
Lukk A.A., Shevchenko V.I.

Abstract

The style of the seismotectonic deformation of the Earth’s crust in the Caucasus and its immediate surroundings corresponds to the thrust setting with a subhorizontal principal compression axis oriented north-northeast across the strike of the Caucasian structures and a subvertical principal extension axis, as established by the reconstruction of a representative set of focal mechanisms of earthquakes. Overall, this deformation style quite closely agrees with the notions developed in plate tectonics according to which the Caucasian segment of the Alpine–Indonesian mobile belt is experiencing strong transversal (across the strike) narrowing as a result of the convergence of the Arabian and Eurasian lithospheric plates. At the same time, the detailed geodetic measurements conducted in the territory of the Greater Caucasus established the GPS site displacements, testifying to the increase of its width. This widening cannot be attributed to the extension across the strike of this mountainous edifice because the focal mechanism solutions of earthquakes in this territory clearly indicate a setting of compressive stresses across the strike of the geological structures. We suggested interpreting this combination of the geodetic and seismological data by the active growth of the volume (and area) of layered rocks of the Greater Caucasus and the emergence of the setting of outward pressure probably as a result of the inflow of the additional mineral material carried by the ascending deep fluids.

Физика Земли. 2019;(4):99-123
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Seismotectonic model of the northwest Caucasus: geological-geophysical aspect
Stogny G.A., Stogny V.V.

Abstract

In connection with the seismic hazard assessment of the Black Sea coast of Russia, the problem of the formation of seismicity in the Northwest Caucasus in the context of recent kinematics of the East Black Sea microplate is relevant, and the boundary of the microplate with the Scythian plate, in our opinion, should be considered among the most debatable issues. This problem is studied based on the tectonic scheme of the consolidated crustal layer of the Northwest Caucasus relying on the analysis of the geological and geophysical data. It is shown that the interplate boundary is represented by the Crimean and West Caucasian faults and that the crust of the Scythian plate is tectonically stratified in the region of the plate interaction which led to the overthrusting of its layers with the subsequent isostatic downwarping of the Moho to a depth of 55 to 60 km beneath the Greater Caucasus Orogen. The tectonic structure of the northeastern flank of the East Black Sea microplate is determined by the Anapa and Sochi blocks having significant excess density. The formation of seismicity of the Northwest Caucasus is mainly associated with the reverse stresses created by the seismogenic Anapa and Sochi blocks and with the tectonic stresses in the zone of the West Caucasian interplate fault. The area of the Anapa and Sochi blocks is at most 4000 square kilometers, which constrains the limiting source size and the maximum (6.0) magnitude of the earthquakes.

Физика Земли. 2019;(4):124-132
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The traces and chronology of the strong historical earthquakes on cape Zyuk, east Crimea
Ovsyuchenko A.N., Korzhenkov A.M., Maslennikov A.A., Vakarchuk R.N., Lar`kov A.S., Sysolin A.I.

Abstract

The results of studying the geological and archaeological traces of the strong earthquakes on Cape Zyuk where the ancient settlement existed from the turn of the 6th to the 5th centuries B.C. to the first-third–middle of the 6th century A.D. are presented. The detailed knowledge gained on this monument allowed us to compile the timeline of the strong earthquakes for the past 2500 years. The detection of the seismotectonic deformation of the Late Holocene sediments indicates that the rupture of the source of a strong earthquake at Cape Zyuk extended to the surface. The dating of this event is constrained by the upper age of the second half of the 4th century B.C. to the beginning of the third century B.C. The previous event is dated to before the turn of the 6th–5th centuries B.C. Besides these events, there were presumably another four earthquakes in 63 B.C., before the second half of the 4th century B.C., in the first third to the mid-6th century A.D., and in the 18th century. Presumably Cape Zyuk was struck by four or five strong earthquakes during the past 2500 years. The lack of information about the earthquakes between the 6th and 18th centuries, i.e., during more than a 1000-year period, can probably be related to both the long period of quiescence between seismic activations and to the incompleteness of the collected data.

Физика Земли. 2019;(4):133-152
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Probabilistic estimates of hypocenters from the data of Kamchatka seismic network stations
Droznin D.V., Droznina S.Y., Senyukov S.L., Chebrov D.V., Shapiro N.M., Shebalin P.N.

Abstract

A new approach is proposed for determining the earthquake hypocenters aimed at more comprehensive characterization of its uncertainty and ambiguity. Application of the new approach to studying the seismic focal subduction zones and volcanic seismicity is discussed by the example of the data of the Kamchatka Branch of the Geophysical Survey of Russian Academy of Sciences.

Физика Земли. 2019;(4):153-165
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Nonhydrostatic stress state in the Martian interior for different rheological models
Batov A.V., Gudkova T.V., Zharkov V.N.

Abstract

We perform the analysis of nonhydrostatic stress field in the Martian interior for two types of heterogeneous elastic models: the ones with a lithosphere and the ones with a lithosphere and probable melting zones within it. Numerical modeling of the system of elastic equilibrium equations for a gravitating planet is carried out on a 1x1 arc degree spherical grid down to a depth of 1000 km. The boundary conditions are specified by the topography and gravity field data determined relative to a hydrostatic equilibrium spheroid taken as a reference surface. High maximum shear stresses in the zones of high tensile stresses are assumed as the criterion for selecting the probable marsquake sources. Irrespective of the type of a rheological model, the zones of the high shear and tensile stresses in the crust and mantle of Mars arc revealed beneath Hellas Planitia, Argyre Planitia, Mare Acidalia, Arcadia Planitia plain, and Valles Marineris.

Физика Земли. 2019;(4):166-180
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