Search

Abstract—The capabilities of a combination of passive seismic methods to study the geological structure of the upper part of the Earth's crust compared to active methods are analyzed using case examples. The passive methods include microseismic sounding, Nakamura's horizontal-to-vertical spectral ratio method (HVSR), seismic interferometry, and, for anthropogenic sites, ambient vibration testing using industrial signals. Three examples are considered: a zone of a platform tectonic earthquake, a kimberlite pipe, and a hydroelectric dam with foundation site. The results of the passive and active seismic methods agree well. Passive methods give more diffuse horizontal boundaries but clearly identify near-vertical heterogeneities. Combining passive methods is effective for reconnaissance studies and in the remote regions that are difficult to access by active observation techniques. Combination of passive methods enables simultaneous processing of seismic records obtained through different passive methods, with a minimum of two sensors required. [ABSTRACT FROM AUTHOR]

Abstract—Peculiarities of microseismic ambient noise are studied based on the data from the stations of regional seismic network located in the central part of the Baikal rift. The probabilistic approach is used to thoroughly investigate the pattern of diurnal variations in microseisms and to analyze the amplitude level and frequency content of the spatial anomalies and the temporal changes (seasonal and annual). Based on the 2020–2021 data, a regional probabilistic model of the microseismic noise is built in a wide range of periods. The study of microseisms in the frequency band of about 1 Hz revealed a seasonal anomaly against the level of the global minimum in the microseismic noise power spectrum. The anomaly is observed from May to December at seismic stations surrounding Lake Baikal except for its northern part. The back azimuth direction in the frequency range of about 1 Hz indicates the arrivals from the location of the lake, suggesting that these signals can be identified as lake-generated microseisms. The high values of the coherence function testify to a linear relation between the wind velocity and the occurrence of lake microseisms. The detailed analysis of the spectral and polarization parameters of the seismic ambient noise revealed two types of lake-generated microseisms with frequencies of 0.4–0.7 and 0.7–1.5 Hz. The first frequency interval is likely to correspond to the single-frequency lake-generated microseisms, while the second interval covers the frequency ranges of the dual-frequency microseisms. [ABSTRACT FROM AUTHOR]

Abstract—The study estimates the impact level of storm microseisms on long-term gravimetric measurements. The gravimetric measurements were conducted at the Zapolskoe, Obninsk, and Murmansk sites using CG-5 Autograv relative gravity meters. seismic measurements were conducted simultaneously with the gravimetric measurements at Zapolskoe. The analysis of these measurements has demonstrated the feasibility of using seismic data as control information to estimate the high-frequency noise component in gravimetric data. Additional seismic information was taken from the services of the Incorporated Research Institutions for Seismology. The analysis of these data established that the attenuation of the noise component in gravimetric measurements is consistent with the data from the modeled sensitive element of the gravimeter, which utilizes seismic series as an input. The first characterization of the storm-induced background noise in gravimetric measurements at the Murmansk site is obtained. Also, the possibility of predicting measurement errors based on meteorological forecasts is ascertained, which can aid in the planning of gravimetric work. [ABSTRACT FROM AUTHOR]

Abstract—The vertical movements measured by broadband seismic stations located around the epicenter of the December 26, 2004 Sumatra 2004 mega-earthquake of magnitude 9.1 are analyzed. It is shown that during the five years before the earthquake (1996 to 2000), the COCO station closest to the epicenter, located 1700 km away, recorded quiet daily variations. Step-like distortions in the level of the recorded seismic noise appeared at this station in 2001 and continued up to the time of the earthquake. The station has also recorded pulses lasting a few minutes, with amplitudes gradually increasing to several times the amplitude of diurnal variations, followed by a decrease. The pulses occurred under quiet meteorological and geomagnetic conditions. No such pulses were found in the records of stations more than 2000 km from the epicenter. It is hypothesized that the sharp changes in the low-frequency seismic noise reflect tectonic slips on the faults in the lithosphere of the Indian Ocean. [ABSTRACT FROM AUTHOR]

Abstract—A laboratory setup was constructed in IDG RAS to investigate the process of shearing the contact of rock blocks of one-meter scale. It was used to investigate deformation processes in a fault with a heterogeneous structure of the sliding interface, which contained strong contact patches—analogs of the asperity in the well-known model of Hiroo Kanamori (Kanamori and Stewart, 1978). It is shown that when a large slip occurs, the rupture, which starts in the zone of maximal deficit of interblock displacement, cuts the segments of the fault with lower effective strength, the latter being decreased in previous deformation events. Those previous events may be "slow" slips with low seismic efficiency. In nature the events that "prepare" the fault interface for a large slip may be smaller earthquakes—foreshocks, or they can be either low frequency earthquakes or slow slip events, both can hardly be detected in seismic records. Thereupon a promising diagnostic indication is the shift of the spectrum of ambient seismic noise to lower frequencies caused by the decrease of fault stiffness. [ABSTRACT FROM AUTHOR]

Abstract—In response to the kind invitation of the Editorial Board of the journal "Izvestiya, Physics of the Solid Earth" to take part in the discussion concerning the methodology of statistical study of correlations between geophysical processes, we have done a small research and written this paper. Being predominantly geomagnetologists, we focused on the topical question of a correlation between the earthquakes and geomagnetic storms and briefly touched upon the connection of the earthquakes with seismic noise. The main result of our analysis is as follows. The correlation between the earthquakes and magnetic storms exists objectively. The problem deserves a further study using statistical hypothesis testing methods, and a special attention should be paid to clearly distinguishing between causal and acausal correlations. [ABSTRACT FROM AUTHOR]

Abstract—Increasing the quality of seismic data is the basis for subsequent seismic data processing and interpretation. In the acquisition of seismic records, the introduction of noise is the main factor that affects the quality of seismic data. Especially in the desert areas, the complex desert noise has the characteristics of high energy, nonstationary, nonlinear, low frequency, non-Gaussian, which brings great difficulties to the conventional denoising methods. Due to the powerful learning ability, we have been devoted to use convolutional neural networks (CNN) to suppress the desert noise of seismic data. However, as the depth increases, network might be faced with the problem that the influence of shallow layers on deep layers is reduced, which is not conducive to the suppression of complex desert noise. This paper raises a new network named a feature enhancement denoising network (FEDnet). First, the proposed model increase the width of the network and fuse the feature information of several different layers to fully utilize the influence of the whole network. It is beneficial to capture more desert noise features concealed in the complicated background. Second, we integrate the hybrid dilated convolution design into our model to improve the receptive field, which plays an important role for getting more context information in the denoising task. Finally, we also adopt residual learning to facilitate the network training. The experimental results demonstrate that the proposed network can obtain superior performance for the denoising task of seismic data compared with existing denoisers. [ABSTRACT FROM AUTHOR]

The possibility of jointly inverting the receiver function waveforms and the seismic noise horizontal-to-vertical spectral ratio to study the Earth's structure is substantiated. Both data types are widely used for constructing a velocity model beneath a single seismic station. The main difference between the methods is associated with the different frequency content of input data which is 0.02–0.2 Hz in receiver functions and 0.5–20 Hz in seismic noise. It is shown that notwithstanding these differences, the joint inversion approach more effectively reconstructs the model of the medium in case when a station is underlain by a complexly structured sedimentary cover. In the practical implementation, the parameters of both seismic methods are described in a flat-layer representation of the medium. Besides, both methods are most sensitive to the depth distribution of S-wave velocities. In this work, we use records from the Monakovo seismic station, Nizhny Novgorod region, Russia, to construct a model of the medium consistent with both data types. It is shown that the allowance for the H/V spectral curve in the receiver function interpretation provides additional constraints on the small-scale structure of the upper part of the velocity section thus stabilizing the reconstruction procedure. [ABSTRACT FROM AUTHOR]

Abstract—The connection between the properties of seismic noise continuously recorded by the network of 21 broadband seismic stations in Kamchatka during nine years of observations (2011–2019) and the nonuniform rotation of the Earth is studied. The daily time series of median values calculated ​​over all network stations are analyzed for three noise parameters: multifractal singularity spectrum support width, generalized Hurst exponent, and minimum entropy of the distribution of the squared orthogonal wavelet coefficients. For identifying the common components in the variations of these parameters, their adaptive first principal component was calculated in a half-year moving window. The quadratic coherence spectrum between the first principal component of seismic noise characteristics and the time series of the length of the day was calculated in a 182-day moving time window. The time–frequency diagram of the coherence spectrum characterized by a sequence of coherence bursts concentrated in a narrow frequency band with periods from 11 to 14 days is analyzed. The time delays between the coherence bursts and the release of seismic energy in Kamchatka are estimated in a 5-year moving time window. [ABSTRACT FROM AUTHOR]

Abstract—The Slichter mode (1S1) is the longest-period mode of the free oscillations of the Earth. The period of the Slichter mode directly depends on density jump between the outer liquid and the inner solid core which makes the detection of this oscillation very important for gaining a more detailed insight into the structure of the Earth's interior. Reliable empirical data on the detection of Slichter mode are absent, which is associated with the rather low amplitude of this mode on the surface. In this work, for the first time, an attempt is made to detect the Slichter mode using the strain data from the largest 2010 Chilean earthquake recorded by the Baksan laser interferometer–strainmeter (Sternberg Astronomical Institute of the Moscow State University (SAI MSU)) with a measuring arm length of 75 m in the Elbrus region, North Caucasus. An asymptotically optimal data analysis algorithm that allows for the properties of seismic noise and peculiarities of the mode is developed. Simultaneously with mode detection, the algorithm provides evaluation of mode parameters (frequency and splitting magnitude). Statistical reliability of the detection is estimated; the parameters of the Slichter mode and the corresponding density jump between the inner and outer core of the Earth are determined. [ABSTRACT FROM AUTHOR]

Abstract—The response of seismic noise in the minute range of periods to the strong magnetic storms is studied. The noise is analyzed from the records by IRIS broadband seismic stations located in the different regions of the world. The minute variations in the X-, Y-, Z-components of the magnetic field at different observatories were obtained through the INTERMAGNET world data system. Seismic pulses that emerged at rapid variations in the rate of change of the magnetic field components dX, dY, dZ are detected. The pulses have the amplitudes of ~2 μm and duration of a few min. The high variability in the ratio of the amplitude of seismic pulses to the magnitude of changes in dHx, dHy, dHz indicates a nonlinear process. The amplitudes of seismic pulses are approximately identical at the stations located in the seismically active or quiet regions. The properties of the pulses also do not depend of weather conditions. The pulses are revealed in the records by all seismic stations located in the continents. In the records from the identical stations located on the volcanic islands in the deep part of the Pacific, such pulses are not detected. It is hypothesized that abrupt changes in the electromagnetic field during a storm act as a trigger for the release of energy accumulated in the Earth. [ABSTRACT FROM AUTHOR]

Abstract—In this paper, we consider the problem of recording and processing the microseismic data with their subsequent interpretation and making the decisions concerning the probable threats associated with the violation of integrity of terrestrial environment due to mining activity. These decisions are made based on the detection of microseismic sources and recognition of their types by processing the multichannel seismograms from small-aperture seismic arrays. The procedures of seismogram processing include the algorithms for detecting the signals of microseismic sources in the noisy seismograms from the array sensors and the procedures for estimating various parameters of these sources. In this paper, we propose a new detection algorithm which is based on evaluating multiple coherence of the array seismograms and is capable of identifying the waves radiated from microseismic sources with complex focal mechanisms against the diffuse seismic noise. In contrast to the widely used STA/LTA array detector which is aimed at separating plane seismic waves, the proposed detector is intended for the recognition of waves from local seismic events in the complexly structured subsurface environment. Based on the numerical experiments with model and real seismic array data containing signals from the sources with complex focal mechanisms, it is shown that the types of microseismic sources can be recognized with the use of various algorithms of seismic array data analysis: with new algorithms for estimating the coordinates of microseismic sources and with traditional algorithms of space-time spectral analysis. [ABSTRACT FROM AUTHOR]

Abstract—In this work, we study the parameters of microseismic noise in the vicinity of the Chilean subduction zone in order to detect oscillations of a block–fault system, similar to those identified by processing the results of the previous laboratory experiment. The analysis is based on the data recorded by the broadband seismic stations of the international seismic network IRIS. We calculated the spectral parameters of microseismic noise before and after several earthquakes. It is established that before the earthquakes with Мw ≥ 8, the calculated value of spectral centroid decreases by 0.12–0.26 Hz. The decrease in the value of spectral centroid in the range from 0.008 to 0.45 Hz can be as long as up to two days. The proposed approach can be a new instrument useful for real-time monitoring of active faults of various scales. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The group velocity dispersion curves are constructed for Rayleigh and Love waves along the paths intersecting the European continent in the interval of periods from 10 to 100 s from the records of earthquakes and seismic noise. The radial anisotropy of the European upper mantle is estimated based on these data. At first, the average velocity sections of the SV- and SH-waves are calculated along each trace from the Rayleigh and Love wave data, respectively. Based on these sections, the average anisotropy coefficient is determined for each path in four depth intervals (the crust + the three 30-km upper mantle layers). These results are used for identifying the variations in the lateral anisotropy in the studied region with the use of tomographic inversion. This approach makes it possible to exclude the different degrees of smoothness of the lateral variations of the SV- and SH-waves if these variations are determined separately from the Rayleigh and Love waves: in this case, large errors in the anisotropy coefficient are possible due to different sets of the paths. The resolution of the data used for tomography was estimated by a "checkerboard test," which demonstrated the possibility to resolve structural features with a linear size of 1200 to 1300 km in the central part of the studied area, i.e., approximately 15°–50° in longitude and 40°–65° in latitude. The tomographic inversion of the lateral variations in the anisotropy coefficient shows that in the continental part of the studied region, the anisotropy coefficient at all depths in the upper mantle is zero within the error limits, whereas in the region of the Black and Baltic seas, it is positive and equal to 4–4.5% in the subcrustal mantle at the depths of 34 to 64 km. In the underlying layer in the Baltic Sea region, this parameter is close to zero, whereas beneath the Black Sea Basin it remains positive although it decreases to 2–3%. In the lowermost layer, anisotropy is not observed anywhere in the entire region; however, this can be due to the lack of the data for large periods. Positive anisotropy () is typical of the oceanic areas which can testify in favor of the oceanic hypothesis of the origin of the Black Sea basin. [ABSTRACT FROM AUTHOR]

10.1007/s10950-014-9468-6 13 Lyubushin A.A. Coherence between the fields of low-frequency seismic noise in Japan and California, Izv. 10.1134/S1069351314030069 11 Lyubushin A.A. Dynamic estimate of seismic danger based on multifractal properties of low-frequency seismic noise. It is the analysis of multifractal properties of low-frequency seismic noise that made it possible to predict the mega-earthquake in Japan of March 11, 2011 well in advance (Lyubushin, 2009; 2010a; 2010b). This was initially established in (Sobolev et al., 2005; 2008; 2010; Lyubushin and Sobolev, 2006; 2007; Sobolev and Lyubushin, 2006; 2007; Lyubushin, 2008). [Extracted from the article]

Abstract—In my comment on the paper by A.A. Lyubushin, G.N. Kopylova, and Yu.K. Serafimova, I focus on the problem of legitimacy of seismological data pertaining to the unconventionally low frequency range from the standpoint of metrology and question the validity of the obtained results. The latter aspect is associated with the fact that in the discussed paper of Lyubushin et al. (2021), the estimates of confidence intervals in the calculated statistics are completely absent, which is at odds with the generally accepted approaches to the presentation of the results of measurement data processing. Without conducting such estimates, the authors made an unjustified conclusion about the correlation of the studied processes and, as a consequence, about a triggering effect of the irregularity of the Earth's rotation on the seismic process. [ABSTRACT FROM AUTHOR]

With the use of the method of low-frequency microseismic sounding, the configuration of the magmatic feeding system of the Tolbachinsky Dol—a regional zone of areal basaltic volcanism in the southern part of the Klyuchevskoy volcano group in Kamchatka—is studied. The initial data are obtained by a stepby-step recording of the background microseismic noise in 2010-2015 within a thoroughly marked-out survey area covering the zones of fissure eruptions in 1975-1976 and 2012-2013 and, partly, the edifice of the Ploskii (flat) Tolbachik volcano. The depth sections reflecting the distributions of the relative velocities of seismic waves in the Earth’s crust are constructed. For a more reliable interpretation of the revealed deep anomalies, the results of independent geological and geophysical studies are used. The ascertained low-velocity structures are closely correlated to the manifestations of present-day volcanism. It is shown that the feeding structure of the Tolbachinsky Dol is spatially heterogeneous, incorporating subvertical and lateral pipeshaped magma conduits, closely spaced magma feeding channels, and shallow magma reservoirs. A longlived local transcrustal magma conducting zone is revealed, and regularities in the deep structure of the feeding systems of fissure eruptions are identified. The configuration of the established subvertical magma conduits permits basalts moving to rise to the surface by different paths, which, inter alia, explains the contrasting magma compositions observed during a single eruption. Thus, based on the instrumental data, it is shown that the magmatic feeding structure of the Tolbachinsky Dol has a number of specific peculiarities and is significantly more complicated than has been previously thought about the areal volcanic fields. [ABSTRACT FROM AUTHOR]

The results of the long (2011-2016) investigation of background seismic noise (BSN) in Kamchatka by the method suggested by Doct. Sci. (Phys.-Math.) A.A. Lyubushin with the use of the data from the network of broadband seismic stations of the Geophysical Survey of the Russian Academy of Sciences are presented. For characterizing the BSN field and its variability, continuous time series of the statistical parameters of the multifractal singularity spectra and wavelet expansion calculated from the records at each station are used. These parameters include the generalized Hurst exponent α*, singularity spectrum support width Δα, wavelet spectral exponent β, minimal normalized entropy of wavelet coefficients En, and spectral measure of their coherent behavior. The peculiarities in the spatiotemporal distribution of the BSN parameters as a probable response to the earthquakes with Мw = 6.8-8.3 that occurred in Kamchatka in 2013 and 2016 are considered. It is established that these seismic events were preceded by regular variations in the BSN parameters, which lasted for a few months and consisted in the reduction of the median and mean α*, Δα, and β values estimated over all the stations and in the increase of the En values. Based on the increase in the spectral measure of the coherent behavior of the four-variate time series of the median and mean values of the considered statistics, the effect of the enhancement of the synchronism in the joint (collective) behavior of these parameters during a certain period prior to the mantle earthquake in the Sea of Okhotsk (May 24, 2013, Mw = 8.3) is diagnosed. The procedures for revealing the precursory effects in the variations of the BSN parameters are described and the examples of these effects are presented. [ABSTRACT FROM AUTHOR]

Based on the theory of surface waves in a media with weak lateral heterogeneities, an explanation is given for the peculiarities that are considered as prospecting indicators in the microseismic sounding method (MSM). According to this theory, during the propagation of surface waves in such media, their local characteristics are the same as in the case of propagation in a horizontally homogeneous medium characterized by the same vertical velocity section as underlying a given point. Since the wave energy flux through a semiinfinite vertical strip is conserved, the amplitude of the wave is redistributed along the vertical. Based on this principle and simple physical considerations, it is shown that above a low-velocity domain, the amplitudes of microseisms should increase at wavelengths approximately exceeding the depth of the anomaly by a factor of three and decrease at small wavelengths. The model calculations show that variation of spectral amplitudes only allows a depth to the anomaly to be estimated rather than its average extent. Practically identical spectra characterize the anomalies in which the product of the vertical extent of the anomaly and the velocity contrast is approximately similar. [ABSTRACT FROM AUTHOR]

The data from the seismic networks of the Kamchatka Branch of the Geophysical Survey of the Russian Academy of Sciences are used for calculating the cross correlations of seismic noise for the stationary digital stations over 2013 and for radio telemetric stations (RTS) in the region of the Klyuchevskoy volcano over the period from January 1, 2009 to May 31, 2013. Four hundred and two correlations overall are calculated. The fundamental-mode group velocities of the Rayleigh waves are calculated in the periods ranging from 5 to 50 s. The calculations for the region of the Klyuchevskaya group of volcanoes are based on the RTS data and cover the periods from 2 to 8 s. The two-dimensional (2D) maps of group velocity distributions in different periods are constructed with the use of the algorithm of surface wave tomography (Barmin, 2001). The velocity sections for the selected Kamchatka regions are reconstructed by the dispersion curve inversion technique (Mordret, 2014). For each region, the structure of the Earth's crust and upper mantle down to a depth of 50 km was obtained. [ABSTRACT FROM AUTHOR]

The earthquake forecasting technique based on the effect of the modulation of high frequency seismic noise (HFSN) by the Earth's tides is discussed and the experience of its practical application in Kamchatka is described based on the extensive measurement data for 1996-2013. The empirical development of the lower magnitude threshold on the epicentral distance is refined. The reliability and validity of the precursor are assessed. The efficiency of the precursor is estimated in two ways. It is shown that the results of applying the prognostic procedure statistically significantly differ from a random guess. The presented estimates are based on the catalog containing 68 earthquakes of 1996-2013 in the predicted magnitude interval M ≥ 5.0 at the epicentral distances that are linked to the magnitude by functional dependence. [ABSTRACT FROM AUTHOR]

The implications of the Poisson ratio of a heterogeneity for the microseismic image of the latter reconstructed by the microseismic sounding method are studied in the numerical experiments. In particular, the cases are considered with anomalous effective Poisson ratios, which may probably occur if the microseismic waves propagate through fractured zones where the crack opening is commensurate with or larger than the characteristic amplitude of the vibrations. Based on the numerical simulations, the nonlinear distortions in the microseismic sounding method, which arise due to the perturbation of the microseismic (probing) signal by introducing the heterogeneities, are estimated. [ABSTRACT FROM AUTHOR]

The model of the magmatic system beneath the Uzon-Geizernaya volcano-tectonic depression and adjacent Kikhpinych volcano in Kamchatka is constructed to a depth of 30 km based on the microseismic sounding data. For doing this, measurements of the natural microseismic field by the Guralp CMG-6TD portable broadband seismometer were carried out at 60 points along three profiles with a total length of about 28 km. The revealed structural heterogeneities were interpreted in the common context with the previous geological, geological-morphological, and petrological results. The area of a shallow crystallized magmatic reservoir is identified and spatially localized below the depression. The zones of the presumed concentration of the basaltic melts probably responsible for the local geodynamic activation of the region during the past 15 years are revealed as the peripheral magmatic chamber of the Kikhpinych volcano at a depth of 5-12 km and a deeper (15-20 km) magma storage. The geometry of the identified deep structures is consistent with the local microseismicity and the model of the contemporary magmatic intrusion into the upper crustal layers, which is based on the data of satellite interferometry. [ABSTRACT FROM AUTHOR]

Microseismic sounding along the profile in the Ossetian sector of the Greater Caucasus revealed two domains with characteristic properties and morphology deep beneath the mountain system. One subvertical domain is marked with low velocities and the other, also subvertical, has high velocities. The high-velocity zone is largely located beneath the northern limb and axial part of the Greater Caucasus mega-anticlinorium, whereas the low velocity zone projects on the southern limb. Almost throughout the entire structure of the block part of the northern limb of mega-anticlinorium, the top of the high-velocity zone beneath it is consistently horizontal at a depth of ∼10 km. This pattern is violated by the apparent steep rise of the top of the high-velocity zone to the surface in the southern direction, which starts approximately from the main thrust. Beneath the southern limb, the top boundary can also be guessed at a depth of ∼10 km, although less reliably. The roots of the low-velocity zone stretch to a depth of ∼50-60 km and narrow with the depth. The weak regional seismicity quite distinctly maps onto the high-velocity zone. In the depth interval of 10 to 25 km, weak seismicity abruptly drops northwards at the transition to the low-velocity zone. The independent magnetotelluric data show that electric resistivity of the low-velocity zone significantly exceeds the resistivity of the hosting rocks. The model of a medium filled with isolated fractures with mineralized fluid is suggested for the low-velocity zone. According to a series of features, the low-velocity zone tends to float up; in particular, there is a high lateral correlation between the most elevated part of the mountain relief, morphology, and age of the rocks, on one hand, and the position of the low-velocity zone, on the other hand. [ABSTRACT FROM AUTHOR]

The dispersion curves of the Rayleigh wave group velocities are constructed along 60 interstation seismic paths in Central Europe based on the cross-correlation function of seismic noise. Together with the previous data (Yanovskaya and Lyskova, 2013), this information was used for reconstructing the three-dimensional distribution of S-wave velocities in the upper mantle of the Carpathian region. In the present work, the previous results are refined by expanding the data set by the additional seismic paths that intersect the Carpathian region and by modifying the procedure for constructing the locally averaged dispersion curves so as to obtain a more compact resolution. The results of the study suggest the complex, multidirectional character of the plate motion in the region. [ABSTRACT FROM AUTHOR]

The low-frequency seismic noise recorded by the broadband IRIS stations in 1994-2012 is studied in the period range of 40 to 360 s. It is shown that for samples of a few months in length, the power spectra of noise at stations spaced apart a few thousand kilometers and operating in different meteorological and seismotectonic conditions are overall similar, which indicates that the sources of the noise are global. At the same time, the slope of the spectra changes with the increase in the period in the subintervals of 40-90, 120-200, and 200-360 s, which points to the difference of the sources generating the seismic noise. The amplitude of the noise at the stations located a few thousand km apart from the Sumatra earthquake of December 26, 2004, M = 9.2, and from the Tohoku earthquake of March 11, 2011, M = 9.0, increased after these events. This indicates the global character of the aftermath of these seismic catastrophes. After the Kronotskoe earthquake of December 5, 1997, which was weaker ( M = 7.9), the noise grew only at the PET station located within 300 km of the epicenter. According to the records at the PET station, this earthquake was preceded by the increased noise level observed in 1994-1997. After 1999, the seismic noise declined and remained low up to the end of the studied interval with a duration of 14 years. Our results show that the low-frequency seismic noise generated by the sources in the atmosphere of the Earth is contributed by the processes taking place in the lithosphere. [ABSTRACT FROM AUTHOR]

The coherent behavior of four parameters characterizing the global field of low-frequency (periods from 2 to 500 min) seismic noise is studied. These parameters include logarithmic variance, kurtosis (coefficient of excess), width of support of multifractal singularity spectrum, and minimal normalized entropy of the distribution of the squared orthogonal wavelet coefficients. The analy)sis is based on the data from 229 broadband stations of GSN, GEOSCOPE, and GEOFON networks for a 16-year period from the beginning of 1997 to the end of 2012. The entire set of stations is subdivided into eight groups, which, taken together, provide full coverage of the Earth. The daily median values of the studied noise parameters are calculated in each group. This procedure yields four 8-dimensional time series with a time step of 1 day with a length of 5844 samples in each scalar component. For each of the four 8-dimensional time series, the frequency-time diagram of the evolution of the spectral measure of coherence (based on canonical coherences) is constructed in the moving time window with a length of 365 days. Besides, for each parameter, the maximum-frequency values of the coherence measure and their mean over the four analyzed noise parameters are calculated as a measure of synchronization that depends on time only. Based on the conducted analysis, it is concluded that the increase in the intensity of the strongest ( M ≥ 8.5) earthquakes after the mega-earthquake on Sumatra on December 26, 2004 was preceded by the enhancement of synchronization between the parameters of global seismic noise over the entire time interval of observations since the beginning of 1997. This synchronization continues growing up to the end of the studied period (2012), which can be interpreted as a probable precursor of the further increase in the intensity of the strongest earthquakes all over the world. [ABSTRACT FROM AUTHOR]

The evidence attesting to the possibility of the largest spectral maximum in seismic noise with a peak frequency of 0.14-0.22 Hz to form as a result of low-frequency dissipation of elastic wave energy in rocks is presented. According to this mechanism, elastic wave energy in rock, which can be considered as a two-component medium (solid matrix + pore water), is dissipated in the form of low-frequency pulses whose energy is the lower the smaller the coefficient of porosity. It is assumed that weak seismic events continuously occur in the frequency range above 6 Hz, and their dissipated energy is a source of the low-frequency noise. [ABSTRACT FROM AUTHOR]

The structure of low-frequency seismic noise in the range of periods from 2 min to 500 min is studied from the data of continuous seismic monitoring at 77 seismic stations of the F-net broadband network in Japan from the beginning of 1997 to May 15, 2012. A new statistical characteristic of seismic noise is suggested, namely, the minimal normalized entropy En of the distribution of squared orthogonal wavelet coefficients. This parameter of seismic noise is analyzed in conjunction with the multifractal statistics-the support width of the singularity spectrum, Δα, and the generalized Hurst exponent, α*, which were extensively used by the author in the previous works for analyzing the low-frequency seismic noise. The method for constructing the maps of spatial distribution of Δα, α*, En, and their aggregated normalized value over the time windows with a given length is proposed. The maps are constructed by averaging the succession of the elementary charts, each of which corresponds to a day of observations. It is shown that, for the islands of Japan, the reduction in Δα and α* and the increase in En outline the area of the forthcoming mega earthquake of March 11, 2011, with M = 9 (Tohoku earthquake). According to the analysis of about a year's worth of data after this event, the region south of Tokyo (Nankai trough) is still dominated by decreased Δα and α* and increased En. This gives grounds to hypothesize that this region remains at a high level of seismic threat since the accumulated stresses were incompletely released by the Tohoku earthquake. Drawing an analogy to the behavior of the coefficient of correlation between Δα and α*, we may suppose that there is an increased probability of a strong earthquake occurring in the second half of 2013 or the first half of 2014. Constructing the averaged maps of the distributions of seismic noise parameters and their aggregated value in a moving time window is suggested as a new method for dynamical assessment of seismic hazards. [ABSTRACT FROM AUTHOR]

The surface wave tomography from ambient seismic noise recorded at stations in Western Europe (WE) and on the East European Platform (EEP) revealed the structure of the crust and upper mantle in the transitional zone from the Precambrian platform to the younger geological units in Western Europe. The Tornquist-Teisseyre Line separating these structures is clearly traced as a transition zone from the high velocities beneath EEP to the low velocities beneath WE in the crust and upper mantle, which extends to a depth of 150-170 km. Below 200 km the relationship between the velocities beneath EEP and WE becomes the opposite. A similar relationship between the velocities in the upper mantle down to a depth of 300 km is observed on the southern boundary, where EEP borders on the northern segment of the Alpine-Himalayan seismic belt. [ABSTRACT FROM AUTHOR]

The dynamics of seismic noise before the catastrophic earthquake of March 11, 2011 in Japan is studied in a minute period range using the records of broadband IRIS stations. It is found that the dispersion of the noise and the number of asymmetric pulses recorded within 500 km from the epicenter of the earthquake have drastically increased 1.5 months before the event. This is the highest enhancement ever recorded by these stations during the same interval of the year for the past 15 years since the IRIS network started stably operating. No increase is revealed in the noise recorded beyond 1200 km from the epicenter. The pattern of decay in amplitudes of the anomalous noise with increasing distance from the epicenter to the corresponding stations likely indicates that the sources of the noise were located close to the source of the earthquake. The noise contains both regular and chaotic components. [ABSTRACT FROM AUTHOR]

A probable physical mechanism of tidal modulation of intensity of the endogenous seismic noise is proposed. The mechanism associates this phenomenon with modulation of the size of the region over which the recorded noise is acquired due to nonhysteresis amplitude-dependent absorption in the Earth's rocks. The two most important cases, namely dry and fluid-saturated rocks, are considered. In both cases, internal elongated strip-like contacts (even in minor quantities) are found to be of fundamental importance. The proposed mechanism provides an explanation for a variety of features of high-frequency seismic noise modulated by tides, which were revealed in the long-term observations on the Kamchatka Peninsula: (i) the modulation depth on the order of the first few percent; (ii) stabilization of the modulation phase before a strong earthquake; (iii) a frequently observed near jump-like change in the phase to the opposite-sign phase after the earthquake; (iv) the subsequent period of a relatively unstable phase; and (v) temporary predominance of the modulation component on the second harmonic of the fundamental tidal frequency in the vicinity of the time when the earthquake occurred. [ABSTRACT FROM AUTHOR]

method is proposed for describing low-frequency microseismic noise from the network of broadband seismic stations in a large seismically active region of the Japan Islands. The median values of daily estimates from each station for seven parameters (three characteristics of the multifractal singularity spectra of the waveforms, their spectral exponents and the smoothness indices, the logarithmic variance and the linear predictability index) are used for the description. These parameters are determined for consecutive daily time intervals from the beginning of January 1997 through the end of February 2010. Since these parameters are taken as the median values of the estimates from each station, they are, actually, the integral statistics of the microseismic field. The present paper is the continuation of two previous works [Lyubushin, 2009; 2010], where the effects of synchronization in the low-frequency microseismic field on a large time scale were analyzed on the data from the F-net stations. In the present work, the number of different 'behavior modes' of the microseismic field are sought as the number of clusters in the optimal partition of the cloud of 7-dimensional vector parameters, estimated within a moving time window with a width of 2 years. A new characteristic of the geophysical field is introduced, namely, the notion of the cluster exponent, which is the power exponent in the dependence of the value of the compactness function of a cloud of vector parameters on the number of clusters in the optimal partition of this cloud. Previously, a relatively rapid increase was revealed in the level of synchronization of the microseismic field, which started in the middle of 2002 and lasted for approximately one year. The level of synchronization remains high up to now. During the past 4 years (taking into account the width (2 years) of the time window within which the estimates were made), the cluster exponent exhibits a long trend which is similar to the shorter trend before the Hokkaido event ( M = 8.3) that occurred on September 25, 2003. These facts, together with the pattern of variations in the coefficient of correlation between two multifractal parameters of the field, suggest a hypothesis of the enhancement of the seismic hazard in the region of the Islands of Japan from the second half of 2010. [ABSTRACT FROM AUTHOR]