No 3 (2019)

ARTICLES
Ideas and methods for local recovery of tectonic stresses from fault-slip data: a critical review
Mukhamediev S.A.
Abstract

In the research aimed at determining tectonic stresses from fault-slip data (the seismological data on the focal mechanisms of earthquakes, geological data on slickensides, etc.), in the past few decades, it has become a predominant practice to use the approach that we refer to as the method of the local kinematic reconstruction (MLKR) of stresses and paleostresses. In the MLKR, ignoring the equilibrium conditions, the authors assign a studied block (macrovolume x) a certain symmetric tensor T which they call without explanation a stress tensor and which is, in their opinion, the only cause of the observed slips. In the MLKR, the principal axes and the ratio of the differences of the principal values of tensor T (the so-called reduced tensor TR) are reconstructed locally, without taking into account the interaction of x with the contacting blocks, i.e., in such a manner as if macrovolume x were isolated. Tensor TR is determined based on the analysis of N events (N ≥ 4) that occurred in x over the time span Δt using only the data on the slip directions and on the orientation of the slickensided planes. This approach ignores the rate of change of the stresses, previous deformation history, and mechanical properties of the block, as well as the ratio of Δt to the stress relaxation time in the block.

In this review, the key ideas of MLKR are discussed and it is shown that the underlying concept of this method is fundamentally fallacious and can lead to results that are arbitrarily inconsistent with reality since under a change in the ignored factors, tensor TR can become almost arbitrary with the same set of slips. According to the mechanics of deformable solids (MDS), uniform stresses in a quasi-statically deformed macrovolume x are genetically related to the self-equilibrated surface forces acting on x and are completely independent of deformations. In contrast, the “stresses” in MLKR are genetically caused by strains and not related to surface forces. As a result, MLKR misses the possibility to balance x, i.e., to satisfy the inviolable conservation laws of momentum and angular momentum. Besides, the TR object that is reconstructed in MLKR is not objective: frame indifferent. In the attempts to achieve the desired objective, followers of the MLKR have to implicitly return to the representations that have been rejected as early as in Cauchy’s works: they do not separate the universal laws of dynamics from the mechanical properties of a particular medium. Specifically, they postulate some a priori subjective interrelations between the elements of the sought tensor TR and the slip directions, thus formulating the constitutive relations of the medium which differ from author-to-author but are attributed a meaning of the universal laws. The information about TR in the MLKR is derived from these relations rather than from the laws of mechanics. Due to this, the notions of stresses and constitutive laws in the MLKR fundamentally differ from the respective notions in the MDS. The followers of the MLKR constantly neglect the fact that the observed slip pattern not only reflects the sought stresses but also the other factors – at least, the mechanical properties of a particular medium, which should also be reconstructed from the observations rather than postulated speculatively. In the Appendix to the review, by the example of a perfectly plastic medium, we recapitulate our previously suggested scheme in which the problem of reconstructing the field of equilibrium stresses and the problem of reconstructing the constitutive relations (in this case, it is the form of the plastic potential of the medium) are separated and solved sequentially. In media that are not perfectly dissipative, separating these problems is problematic. Together, these problems constitute an absolutely new problem that has no analogs in the MDS and waits for its solution from ambitious and competent researchers.

Физика Земли. 2019;(3):3-40
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Estimation of free core resonance parameters based on long-term strain observations in the diurnal frequency band
Milyukov V.K., Amoruso A., Crescentini L., Mironov A.P., Myasnikov A.V., Lagutkina A.V.
Abstract

The free core nutation (FCN) is one of the Earth’s rotational eigenmodes, which is caused by the retrograde motion of the liquid core relative to the mantle. The FCN period and Q-factor are determined by the elastic properties of the core/mantle boundary (CMB) and their electromagnetic interaction. In the celestial coordinate system, the FCN period is about 430 days; in the Earth-fixed reference frame this effect manifests itself in the form of the free core resonance (FCR) whose frequency falls in the diurnal tidal band. FCR observation requires highly accurate measurement of the amplitudes and phases of the near-diurnal tidal waves. In particular, the parameter estimates for minor waves K1, P1, Ψ1, and Φ1 are critically important for evaluating the FCR effect, i.e., the period and decay of this resonant mode. The progress in the experimental study of FCR is mainly due to the accumulation of the data from superconducting gravimeters and VLBI; at the same time, also the data of the precision laser strainmeters were used. In this work, the FCR effect is studied based on the long-term strain precision records by two European stations: Baksan, Russia (laser interferometer–strainmeter with a measuring armlength of 75 m [Milyukov et al., 2005; 2007] and Gran Sasso, Italy (two perpendicular laser interferometer–strainmeters, BA and BC, each with a measuring armlength of 90 m [Amoruso and Crescentini, 2009]).

Физика Земли. 2019;(3):41-50
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Rapid growth of shear strain in weakened zones of the lithosphere
Birger B.I.
Abstract

A weakened zone in the lithosphere plunging into the mantle can lead to an earthquake after the application of a shear stress only in the case if the effective viscosity of this zone is very low. At low viscosity, in the short time that elapses after the application of stress, significant displacements of the walls of the zone emerge causing high-amplitude seismic waves. The Andrade law describing the transient creep under constant stress applied at the initial time instant leads to very low effective viscosity a few first seconds after the initial time instant. The effective viscosity also decreases due to the temperature rise in the weakened zone caused by the dissipative release of heat. However, this process is not rapid enough to noticeably change the temperature and effective viscosity in a short time.

Физика Земли. 2019;(3):51-57
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Surface wave tomography of the arctic from seismic Rayleigh and Love wave group velocity dispersion data
Seredkina L.V.
Abstract

The results of studying the deep structure of the Earth’s crust and upper mantle of the Arctic from surface wave data are presented. For this purpose, based on the frequency-time analysis procedure, a representative dataset of group velocity dispersion curves of seismic Rayleigh and Love waves (1555 and 1265 paths, respectively) in the period range from 10 to 250 s is obtained. With the use of a two-dimensional tomography technique for a spherical surface, group velocity distributions are calculated at separate periods. Overall, 18 maps for each type of surface waves are constructed and the horizontal resolution of the mapping is estimated. For four tectonically different regions of the Arctic, the dispersion curves calculated from the tomography results are inverted for the velocity sections of the SV- and SH-waves. Based on the obtained distributions, the main large-scale features are analyzed in the deep structure of the Earth’s crust and upper mantle of the Arctic, and the revealed velocity irregularities are correlated to various geological structures. The results of the study are of considerable interest for further constructing the three-dimensional model of the shear wave velocity distributions and for studying the anisotropic properties of the upper mantle of the Arctic, as well as for building the geodynamical models of the region.

Физика Земли. 2019;(3):58-70
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Localization zones of ancient and historical earthquakes in Gornyi Altai
Deev E.V.
Abstract

The conducted paleoseismological and archaeoseismological studies reveal three zones of concentration of the ancient and historical earthquakes in Gorny Altai which are related to the Kurai Fault zone, Katun, and South Terekta faults. The surface ruptures are detected within the Kurai Fault zone, which were formed in the epicentral zones of the paleoearthquakes that occurred 6500, 5800, 3200, and 1300 years ago and had magnitudes Mw = 6.7–7.6. The recurrence period of the paleoearthquakes is 700 to 2600 years. The detected secondary seismogenic deformations indicate that an epicentral zone of the paleoearthquake with an age of less than 12.5 ka (Mw = 7.2–7.6, intensity I = 10–11), the traces of earthquakes and their clusters with M ≥ 5–5.5 and I ≥ 6–7, which occurred about 150 and 90 ka ago, in the intervals of 38–19 ka ago (with a recurrence period of about 2 ka), and 19–12.5 ka ago are related to the southern part of the Katun Fault. The earthquake of I ≥ 5–6 which damaged the constructions of the Chultukov Log 1 burial mound in the period from IV century B.C. to the beginning of I century A.D. is associated with the northern part of the Katun Fault. In the zone of the South Terekhta Fault, the seismogenic displacements that occurred in VII–VIII centuries A.D. (Mw = 7.4–7.7, I = 9–11) and about 16 ka ago (M ≥ 7, I = 9–10) are revealed. The latter triggered the formation of a landslide-dammed lake which was destroyed by the earthquake about 6 ka ago (M ≥ 7, I = 9–10). Secondary paleoseismic deformations of the ancient earthquakes (M ≥ 5–5.5, I ≥ 6–7) are recorded in the sediments of the Uimon Basin with an age of 100–90 ka and about 77 ka. These results should be taken into account in designing a gas pipeline in the People’s Republic of China, building infrastructure for tourism, and elaborating the seismic zoning maps for the territory of the Russian Federation.

Физика Земли. 2019;(3):71-96
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Systematics of paleomagnetic directions from early–middle Devonian rocks of Minusa troughs: new data and old problems
Shatsillo A.V., Pavlov V.E.
Abstract

Paleomagnetic results from numerous Early–Middle Devonian volcanic sequences of the Minusa trough, southern Siberia, are presented. The analysis of these data definitely indicates that the geomagnetic field in the Devonian had a specific character, different from both the present field and the field of more ancient geological epochs, and was extremely variable (hyperactive). The anomalies in the paleomagnetic record of the Early–Middle Devonian are not local, peculiar to a particular region, but have a global occurrence. The synthesis of the obtained results with the paleomagnetic data from the coeval volcanics from Scotland shows that during a relatively short time (10–20 Ma), the geomagnetic pole repeatedly changed its location, significantly deviating from the Earth’s rotation axis up to the (paleo)equator and assuming some quasi-stable positions. The arguments suggesting that the specific features of the Devonian paleomagnetic record could probably be induced by the significant contribution of the equatorial dipole to the main geomagnetic field are presented.

Физика Земли. 2019;(3):97-116
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On 100 000-year rhythmicity in geodynamics and paleoclimate
Bezverkhnii V.A.
Abstract

The 100 000-year rhythmicity in paleoclimate variations in the Quaternary can be associated with the respective oscillations in insolation and seafloor volcanism forced by Solar-Moon gravitation (SMG). This conclusion is based on the wavelet analysis of long time series of the variations in the eccentricity of the Earth’s orbit, different paleoclimatic characteristics, and the existing spectral estimates of variations in SMG and seafloor volcanism.

Физика Земли. 2019;(3):117-125
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Modeling the spectral features of acoustic noise produced by gas flow in rock samples based on the theory of ensemble of interconnected pore resonators
Ivanova E.A., Mikhailov D.N.
Abstract

A new model describing the pore space as an ensemble of interconnected Helmholtz resonators is proposed. This model makes it possible to improve the description of spectral peculiarities of the experimentally recorded acoustic noise during gas flow through a porous medium. The results of the resonance frequency calculation are presented by the example of the pore space model of the Indiana Limestone. Microvortexes in the pores are considered as the main mechanism of acoustic noise generation by gas flow. The presented numerical simulations on COMSOL Multiphysics show that the generation of microvortexes begins when the Reynolds number in the pores reaches 1 to 10.

Физика Земли. 2019;(3):126-134
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Experimental methods for studying the wetting properties of oil reservoirs: a review
Ivanova A.A., Mitiurev N.A., Shilobreeva S.N., Cheremisin A.N.
Abstract

The theoretical principles of the laboratory methods for studying the wettability of unconventional oil formation rocks are discussed and examples of their practical implementation are presented. The comparative analysis of the advantages and disadvantages of each method is presented. It is shown that despite the recent progress in the development of methods for determining the wettability of rocks, they still need to be improved. Examples of their possible improvements are discussed.

Физика Земли. 2019;(3):135-149
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Studying the dynamics of cosmic dust flux on the earth’s surface from peat deposits
Tselmovich V.A., Kurazhkovskii A.Y., Kazansky A.Y., Shchetnikov A.A., Blyakharchuk T.A., Philippov D.A.
Abstract

Peat cores sampled from different climatic zones are studied. The  petromagnetic and microprobe methods are used to find peat layers enriched with cosmic dust. It is established that the behavior of saturation isothermal remanent magnetization (SIRM) of peat deposits from the zones where the aeolian transfer of terrigenous particles is negligible can be used for studying the dynamics of the fall of cosmic matter on the Earth’s surface. The cosmic dust flux can be conditionally divided into the background and burst components. Here, the background flux of cosmic dust varies cyclically. The characteristic times of these cycles are about 100 years. The cyclicity in the background flux of cosmic material most clearly manifested itself in the interval of 1200 to 500 years ago. The most significant burst in the influx of cosmic material (by an order of magnitude above the background) is revealed in the layer that was formed about 5000 years ago. The microprobe studies established that the mineralogical content of cosmic dust differs between the background and burst fluxes.

Физика Земли. 2019;(3):150-160
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