arXiv subjects

E. S. Skordas

Publications and source records attributed to E. S. Skordas.

At least 19 recordsLinked to original sources

arXiv2026

Identifying the occurrence time of an impending mainshock: A very recent case

The procedure by means of which the occurrence time of an impending mainshock can be identified by analyzing in natural time the seismicity in the candidate area subsequent to the recording of a precursory Seismic Electric Signals (SES) activity is reviewed. Here, we report the application of this procedure to an Mw5.4 mainshock that occurred in Greece on 17 November 2014 and was strongly felt in Athens. This mainshock (which is pretty rare since it is the strongest in that area for more than half a century) was preceded by an SES activity recorded on 27 July 2014 and the results of the natural time analysis reveal that the system approached the critical point (mainshock occurrence) early in the morning on 15 November 2014. SES activities that have been recently recorded are also presented. Furthermore, in a Note we discuss the case of the Mw5.3 earthquake that was also strongly felt in Athens on 19 July 2019 (Parnitha fault).

physics.geo-ph↗

arXiv2024

Geoelectric field and seismicity changes preceding the 2018 Mw6.8 earthquake and the subsequent activity in Greece

A strong earthquake of magnitude Mw6.8 struck Western Greece on 25 October 2018 with epicenter at 37.515N 20.564E. It was preceded by an anomalous geolectric signal that was recorded on 2 October 2018 at a measuring station 70km away from the epicenter. Upon analyzing this signal in natural time, we find that it conforms to the conditions suggested (e.g., Entropy 19 (2017) 177) for its identification as precursory Seismic Electric Signal (SES) activity. Notably, the observed lead time of 23 days lies within the range of values that has been very recently identified (Entropy 20 (2018) 561) as being statistically significant for the precursory variations of the electric field of the Earth. Moreover, the analysis in natural time of the seismicity subsequent to the SES activity in the area candidate to suffer this strong earthquake reveals that the criticality conditions were obeyed early in the morning of 18 October 2018, i.e., almost a week before the strong earthquake occurrence, in agreement with earlier findings. Furthermore, upon employing the recent method of nowcasting earthquakes, which is based on natural time, we find an earthquake potential score around 80% just before the occurrence of this Mw6.8 earthquake. Here, we also report the recording of more recent SES activities including a very recent one which just appeared at Pirgos measuring station on 13 October 2023.

physics.geo-ph↗

arXiv2018

Macroscopic crack propagation in brittle heterogeneous materials analyzed in Natural Time

Here, we analyze in natural time $χ$, the slow propagation of a macroscopic crack in brittle heterogeneous materials through sudden jumps and energy release events which are power law distributed with universal exponents. This macroscopic crack growth is currently believed to exhibit similar characteristics with the seismicity associated with earthquakes. Considering that the crack front is self affine and exhibits Family-Vicsek universal scaling, we show that the variance $κ_1 (\equiv\langle χ^2\rangle -\langle χ\rangle^2)$ of natural time is equal to 0.0686, which almost coincides with the value $κ_1\approx 0.07$ obtained from the seismicity preceding major earthquakes. This sheds light on the determination of the occurrence time of an impending mainshock.

physics.geo-ph↗

arXiv2017

Non-uniform scaling of the magnetic field variations before the $M_w$9.0 Tohoku earthquake in 2011

Applying Detrended Fluctuation Analysis (DFA) to the geomagnetic data recorded at three measuring stations in Japan, Rong et al. in 2012 reported that anomalous magnetic field variations were identified well before the occurrence of the disastrous Tohoku $M_w$9.0 earthquake that occurred on 11 March 2011 in Japan exhibiting increased "non-uniform" scaling behavior. Here, we provide an explanation for the appearance of this increase of "non-uniform" scaling on the following grounds: These magnetic field variations are the ones that accompany the electric field variations termed Seismic Electric Signals (SES) activity which have been repeatedly reported that precede major earthquakes. DFA as well as multifractal DFA reveal that the latter electric field variations exhibit scaling behavior as shown by analyzing SES activities observed before major earthquakes in Greece. Hence, when these variations are superimposed on a background of pseudosinusoidal trend, their long range correlation properties -quantified by DFA- are affected resulting in an increase of the "non-uniform" scaling behavior. The same is expected to hold for the former magnetic field variations.

physics.geo-ph↗

arXiv2017

Minima of the fluctuations of the order parameter of seismicity worldwide

It has been shown [Phys. Rev. E 84, 022101 (2011); Chaos 22, 023123 (2012)] that earthquakes of magnitude $M$ greater or equal to 7 are globally correlated. Such correlations were identified by studying the variance $κ_1$ of natural time which has been proposed as an order parameter for seismicity. Here, we study the fluctuations of this order parameter using the Global Centroid Moment Tensor catalog for a magnitude threshold $M_{thres}=5.0$ and focus on its behavior before major earthquakes. Natural time analysis reveals that distinct minima of the fluctuations of the order parameter of seismicity appear {within} almost five and a half months{-on average-} before all major earthquakes of magnitude larger than 8.4. This phenomenon corroborates the finding [Proc. Natl. Acad. Sci. U.S.A. 110, 13734 (2013)] that similar minima of the seismicity order parameter fluctuations had preceded all major shallow earthquakes in Japan. Moreover, on the basis of these minima a statistically significant binary prediction method for earthquakes of magnitude larger than 8.4 with hit rate 100\% and false alarm rate 6.67\% is suggested.

physics.geo-ph↗

arXiv2017

Identification of Seismic Electric Signals upon significant data loss

When monitoring geophysical parameters, data from segments that are contaminated by noise may have to be abandoned. This is the case, for example, in the geoelectrical field measurements at some sites in Japan, where high noise -due mainly to leakage currents from DC driven trains- prevails almost during 70\% of the 24 hour operational time. We show that even in such a case, the identification of seismic electric signals (SES), which are long-range correlated signals, may be possible, if the remaining noise free data are analyzed in natural time along with detrended fluctuation analysis (DFA).

physics.geo-ph↗

arXiv2017

Comment on LiH as a Li+ and H- ion provider

A first-principle study of the formation and migration of native defects in LiH, a material of interest in hydrogen storage and lithium-ion batteries, has recently been published. Their results are found here to be of key-importance to deduce useful results for the pressure dependence of the ionic conductivity of LiH.

cond-mat.mtrl-sci↗

arXiv2017

On a universal relation for defects in solids

We show that the defect data parameters related to various defect processes, e.g., formation, migration, dielectric relaxation parameters, obey a universal law. In particular, the defect entropies scale with the defect enthalpies irrespective of the process considered. A concrete example is given here for SrF$_2$ by considering the dielectric relaxation parameters (R$_1$ relaxation mechanism) for crystals doped with trivalent ions of Ce, Eu and Gd, parameters for the anion Frenkel formation as well as for the migration of anion vacancy and the anion interstitial motion.

cond-mat.mtrl-sci↗

arXiv2017

Natural Time Analysis of Seismicity in California: The epicenter of an impending mainshock

Upon employing the analysis in a new time domain, termed natural time, it has been recently demonstrated that a remarkable change of seismicity emerges before major mainshocks in California. What constitutes this change is that the fluctuations of the order parameter of seismicity exhibit a clearly detectable minimum. This is identified by using a natural time window sliding event by event through the time series of the earthquakes in a wide area and comprising a number of events that would occur on the average within a few months or so. Here, we suggest a method to estimate the epicentral area of an impending mainshock by an additional study of this minimum using an area window sliding through the wide area. We find that when this area window surrounds (or is adjacent to) the future epicentral area, the minimum of the order parameter fluctuations in this area appears at a date very close to the one at which the minimum is observed in the wide area. The method is applied here to major earthquakes that occurred in California during the recent decades including the largest one, i.e., the 1992 Landers earthquake.

physics.geo-ph↗

arXiv2017

Application of the Huang-Hilbert transform and natural time to the analysis of Seismic Electric Signal activities

The Huang-Hilbert transform is applied to Seismic Electric Signal (SES) activities in order to decompose them into a number of Intrinsic Mode Functions (IMFs) and study which of these functions better represent the SES. The results are compared to those obtained from the analysis in a new time domain termed natural time after having subtracted the magnetotelluric background from the original signal. It is shown that the instantaneous amplitudes of the IMFs can be used for the distinction of SES from artificial noises when combined with the natural time analysis.

physics.data-an↗

arXiv2017

Heterodiffusion coefficients in α-iron

The diffusion of tungsten in α-iron is important for the application of ferritic-iron alloys to thermal power plants. These data, over a wide temperature range across the Curie temperature, have been recently reported. We show that these diffusion coefficients can be satisfactory reproduced in terms of the bulk elastic and expansivity data by means of a thermodynamical model that interconnects point defects parameters with bulk qualities.

cond-mat.mtrl-sci↗

arXiv2017

A tentative model for estimating the compressibility of rock-salt AgCl_{x}Br_{1-x} alloys

Ab initio detailed calculations of the elastic properties of AgCl_{x}Br_{1-x} alloys recently appeared using density-functional perturbation theory and employing the virtual crystal approximation or by means of the full potential linearized augmented plane wave method. Here, we suggest a simple theoretical model that enables the estimation of the isothermal compressibility of these alloys in terms of the elastic data of end members alone. The calculated values are in satisfactory agreement with the experimental ones. The present model makes use of an early suggestion that interconnects the Gibbs energy for the formation and/or migration of defects in solids with bulk properties.

cond-mat.mtrl-sci↗

arXiv2016

Comment on "Modeling oxygen self-diffusion in UO2 under pressure by M.W.D Cooper et al., Solid State Ionics 282 (2015) 26-30"

The oxygen self-diffusion coefficient in UO${_2}$ has been recently studied [Cooper et al. Solid State Ionics 282 (2015) 26-30] over a range of pressures (0-10GPa) and temperatures (300-1900K) by combining molecular dynamics calculations with a thermodynamical model, the cBΩ model. A significant reduction in oxygen self-diffusion as a function of increasing hydrostatic pressure, and the associated increase in activation energy was identified. Here, we extend this study and find that the compressibility of the corresponding activation volume exceeds significantly the compressibility of the bulk material by almost one order of magnitude. This results is important since in the literature it is usually assumed that these two compressibilities are equal. The same holds when comparing the thermal expansion coefficient of this volume with that of the bulk solid.

cond-mat.mtrl-sci↗

arXiv2015

Estimating the compressibility of osmium from recent measurements of Ir-Os alloys under high pressure

Several fcc- and hcp-structured Ir-Os alloys have been recently studied up to 30 GPa at room temperature by means of synchrotron-based X-ray powder diffraction in diamond anvil cells. Using their bulk moduli -which increase with increasing osmium content showing a deviation from linearity- and after employing a thermodynamical model it was concluded that the bulk modulus for osmium is slightly smaller than that for diamond. Here, a similar conclusion is obtained upon employing an alternative model, thus strengthening the conclusion that osmium is the densest but not the most incompressible element. This is particularly interesting for Earth Sciences since it may be of key importance towards clarifying the anomalous elastic properties of the Earth's core.

cond-mat.mtrl-sci↗

arXiv2015

Fluctuation theorem and natural time analysis

Upon employing a natural time window of fixed length sliding through a time series, an explicit interrelation between the variability $β$ of the variance $κ_1$($=< χ^2 > - < χ>^2$) of natural time $χ$ and events' correlations is obtained. In addition, we investigate the application of the fluctuation theorem, which is a general result for systems far from equilibrium, to the variability $β$. We consider for example, major earthquakes that are nonequilibrium critical phenomena. We find that four (out of five) mainshocks in California during 1979-2003 were preceded by $β$ minima lower than the relative thresholds deduced from the fluctuation theorem, thus signalling an impending major event.

cond-mat.stat-mech↗

arXiv2012

Order parameter fluctuations in natural time and b-value variation before large earthquakes

Self-similarity may stem from two origins: the process' increments infinite variance and/or process' memory. The $b$-value of the Gutenberg-Richter law comes from the first origin. In the frame of natural time analysis of earthquake data, a fall of the b-value observed before large earthquakes reflects an increase of the order parameter fluctuations upon approaching the critical point (mainshock). The increase of these fluctuations, however, is also influenced from the second origin of self-similarity, i.e., temporal correlations between earthquake magnitudes. This is supported by observations and simulations of an earthquake model.

physics.geo-ph↗

arXiv2012

Effect of significant data loss on identifying electric signals that precede rupture by detrended fluctuation analysis in natural time

Electric field variations that appear before rupture have been recently studied by employing the detrended fluctuation analysis (DFA) as a scaling method to quantify long-range temporal correlations. These studies revealed that seismic electric signals (SES) activities exhibit a scale invariant feature with an exponent $α_{DFA} \approx 1$ over all scales investigated (around five orders of magnitude). Here, we study what happens upon significant data loss, which is a question of primary practical importance, and show that the DFA applied to the natural time representation of the remaining data still reveals for SES activities an exponent close to 1.0, which markedly exceeds the exponent found in artificial (man-made) noises. This, in combination with natural time analysis, enables the identification of a SES activity with probability 75% even after a significant (70%) data loss. The probability increases to 90% or larger for 50% data loss.

cond-mat.stat-mech↗

arXiv2010

Detrended fluctuation analysis of the magnetic and electric field variations that precede rupture

Magnetic field variations are detected before rupture in the form of `spikes' of alternating sign. The distinction of these `spikes' from random noise is of major practical importance, since it is easier to conduct magnetic field measurements than electric field ones. Applying detrended fluctuation analysis (DFA), these `spikes' look to be random at short time-lags. On the other hand, long range correlations prevail at time-lags larger than the average time interval between consecutive `spikes' with a scaling exponent $α$ around 0.9. In addition, DFA is applied to recent preseismic electric field variations of long duration (several hours to a couple of days) and reveals a scale invariant feature with an exponent $α\approx 1$ over all scales available (around five orders of magnitude).

cond-mat.stat-mech↗