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P. A. Varotsos

Publications and source records attributed to P. A. Varotsos.

At least 19 recordsLinked to original sources
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↗
arXiv2019

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↗
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

Interconnection of point defect parameters in solids with bulk properties: Application to diamond

We show that the values of the defect entropy and the defect enthalpy for the vacancy formation in diamond, have a ratio which is comparable to the one predicted by a model suggested four decades ago. This model, which interconnects the formation Gibbs energy with the bulk elastic and expansivity data, has been also recently found of value in high $T_c$-superconductors as well as in glass-forming liquids.

cond-mat.mtrl-sci↗
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

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↗
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↗
arXiv2009

The fluctuations, under time reversal, of the natural time and the entropy distinguish similar looking electric signals of different dynamics

We show that the scale dependence of the fluctuations of the natural time itself under time reversal provides a useful tool for the discrimination of seismic electric signals (critical dynamics) from noises emitted from man made sources as well as for the determination of the scaling exponent. We present recent data of electric signals detected at the Earth's surface, which confirm that the value of the entropy in natural time as well as its value under time reversal are smaller than that of the entropy of a "uniform" distribution.

cond-mat.stat-mech↗
arXiv2008

Seismic Electric Signals and 1/f "noise" in natural time

By making use of the concept of natural time, a simple model is proposed which exhibits the $1/f^a$ behavior with $a$ close to unity. The properties of the model are compared to those of the Seismic Electric Signals (SES) activities that have been found to obey the ubiquitous $1/f^a$ behavior with $a \approx 1$. This comparison, which is made by using the most recent SES data, reveals certain similarities, but the following important difference is found: The model suggests that the entropy $S_-$ under time reversal becomes larger compared to the entropy $S$ in forward time, thus disagreeing with the experimental SES results which show that $S$ may be either smaller or larger than $S_-$. This might be due to the fact that SES activities exhibit {\em critical} dynamics, while the model cannot capture all the characteristics of such dynamics.

cond-mat.stat-mech↗
arXiv2008

Investigation of the seismicity after the initiation of a Seismic Electric Signal activity until the main shock

The behavior of seismicity in the area candidate to suffer a main shock is investigated after the observation of the Seismic Electric Signal activity until the impending mainshock. This makes use of the concept of natural time $χ$ and reveals that the probability density function of the variance $κ_1(=< χ^2 > -< χ> ^2)$ exhibits distinct features before the occurrence of the mainshock. Examples are presented, which refer to magnitude class 6.0 earthquakes that occurred in Greece during the first two months in 2008.

cond-mat.stat-mech↗
arXiv2007

Natural time and 1/f "noise"

Seismic electric signals have been found to obey the ubiquitous $1/f^a$ behavior [{\em Phys. Rev. E} {\bf 66}, 011902(2002)]. The newly introduced concept of natural time enables the study of the dynamic evolution of a complex system and identifies when the system enters the critical stage. On the basis of this concept, a simple model is proposed here which exhibits the $1/f^a$ behavior with $a$ close to unity. Furthermore, we present recent data of electric signals, which when analyzed in the natural time domain are found to exhibit {\em critical} dynamics and hence can be classified as seismic electric signals.

cond-mat.stat-mech↗
arXiv2006

Is time continuous?

Conventional time is modelled as the one dimensional continuum R^1 of real numbers. This continuity, however, does {\em not} stem from {\em any} fundamental principle. On the other hand, natural time is {\em not} continuous and its values as well as those of the energy, form {\em countable} sets, i.e., with cardinalities either finite or equal to \aleph_0, where this symbol stands for the {\em transfinite} number of natural numbers. For infinitely large number of events, the values of natural time form a {\em denumerable} set, i.e., its cardinality is exactly \aleph_0, while those of conventional time an {\em uncountable} set. This has a drastically larger cardinality, which in the light of the continuum hypothesis becomes equal to 2^{\aleph_0}.

cond-mat.other↗
arXiv2006

Additional information for the paper `Entropy of seismic electric signals: Analysis in natural time under time-reversal'

After the submission of the paper, three strong earthquakes with magnitude around 6.0-units occurred on October 17 and October 20, 2005, with epicenters in the Aegean Sea, at a distance {\em only} 100km from MYT station at which the intense signals $M_1$ to $M_4$ -analyzed in the main text- have been recorded. This confirms experimentally the proposed criterion we used for the classification of these signals as Seismic Electric Signals (SES). Moreover, we show that, if we follow the procedure described in [P.A. Varotsos, N. V. Sarlis, H. K. Tanaka and E. S. Skordas {\it Phys. Rev. E} {\bf 72}, 041103 (2005)], the analysis in the natural time of the seismicity after the SES initiation allows the estimation of the time window of the impending earthquakes with very good accuracy.

physics.geo-ph↗
arXiv2005

Similarity of fluctuations in correlated systems: The case of seismicity

We report a similarity of fluctuations in equilibrium critical phenomena and non-equilibrium systems, which is based on the concept of natural time. The world-wide seismicity as well as that of San Andreas fault system and Japan are analyzed. An order parameter is chosen and its fluctuations relative to the standard deviation of the distribution are studied. We find that the scaled distributions fall on the same curve, which interestingly exhibits, over four orders of magnitude, features similar to those in several equilibrium critical phenomena (e.g., 2D Ising model) as well as in non-equilibrium systems (e.g., 3D turbulent flow).

physics.geo-ph↗
arXiv2005

Entropy of seismic electric signals: Analysis in natural time under time-reversal

Electric signals have been recently recorded at the Earth's surface with amplitudes appreciably larger than those hitherto reported. Their entropy in natural time is smaller than that, $S_u$, of a ``uniform'' distribution. The same holds for their entropy upon time-reversal. This behavior, as supported by numerical simulations in fBm time series and in an on-off intermittency model, stems from infinitely ranged long range temporal correlations and hence these signals are probably Seismic Electric Signals (critical dynamics). The entropy fluctuations are found to increase upon approaching bursting, which reminds the behavior identifying sudden cardiac death individuals when analysing their electrocardiograms.

physics.geo-ph↗