Seismic Electric Signals Activities Research Articles

Estimating the Epicenter of an Impending Strong Earthquake by Combining the Seismicity Order Parameter Variability Analysis with Earthquake Networks and Nowcasting: Application in the Eastern Mediterranean

The variance κ1 of the natural time analysis of earthquake catalogs was proposed in 2005 as an order parameter for seismicity, whose fluctuations proved, in 2011, to be minimized a few months before the strongest mainshock when studying the earthquakes in a given area. After the introduction of earthquake networks based on similar activity patterns, in 2012, the study of their higher order cores revealed, in 2019, the selection of appropriate areas in which the precursory minima βmin of the fluctuations β of the seismicity order parameter κ1 could be observed up to six months before all strong earthquakes above a certain threshold. The eastern Mediterranean region was studied in 2019, where all earthquakes of magnitude M≥7.1 were found to be preceded by βmin without any false alarm. Combining these results with the method of nowcasting earthquakes, introduced in 2016, for seismic risk estimation, here, we show that the epicenter of an impending strong earthquake can be estimated. This is achieved by employing—at the time of observing the βmin—nowcasting earthquakes in a square lattice grid in the study area and by averaging, self-consistently, the results obtained for the earthquake potential score. This is understood in the following context: The minimum βmin is ascertained to almost coincide with the onset of Seismic Electric Signals activity, which is accompanied by the development of long range correlations between earthquake magnitudes in the area that is a candidate for a mainshock.

Self-organized criticality and earthquake predictability: A long-standing question in the light of natural time analysis

After the Bak-Tang-Wisenfeld seminal work on self-organized criticality (SOC), the following claim appeared by other workers in the 1990s: Earthquakes (EQs) cannot be predicted, since the Earth is in a state of SOC and hence any small earthquake has some probability of cascading into a large event. Here, we discuss that such claims do not stand in the light of natural time analysis, which was shown at the beginning of the 2000s to extract the maximum information possible from complex systems time series. A useful quantity to identify the approach of a dynamical system to criticality is the variance of natural time χ, which becomes equal to 0.070 at the critical state for a variety of dynamical systems. This also holds for experimental results of critical phenomena such as growth of ricepiles, seismic electric signals activities, and the subsequent seismicity before the associated main shock. Another useful quantity is the change of the dynamic entropy under time reversal, which is minimized before a large avalanche upon analyzing the Olami-Feder-Christensen model for EQs in natural time. Such a minimum actually occurred on 22 December 2010, well before the M9 Tohoku EQ in Japan on 11 March 2011, being accompanied by increases of both the complexity measure of the fluctuations and the variability of the order parameter of seismicity (which was minimized two weeks later). These increases conform to the seminal work on phase transitions by Lifshitz and Slyozov and independently by Wagner as well as to more recent work by Penrose et al. In addition, the evolution of the complexity measure of the fluctuations reveals a reliable estimation of the occurrence time of this M9 EQ.

Fluctuations of the entropy change under time reversal: Further investigations on identifying the occurrence time of an impending major earthquake

A procedure has been developed in a previous publication (Skordas E. S. et al., EPL, 128 (2019) 49001) for the identification of the occurrence time of the Tohoku earthquake of magnitude that occurred in Japan on 11 March 2011 based on natural time analysis of seismicity. Using the complexity measure that quantifies the fluctuations of the entropy change of seismicity under time reversal, we show here that, in the longer scales, the complexity measure of the entire Japanese region starts increasing from 22 December 2010 (the date at which is minimized) reaching a maximum close to the appearance of a Seismic Electric Signals activity (evidenced from the recording of anomalous magnetic field variations on the z-component) in the beginning of January 2011; then it gradually diminishes until just before the mega earthquake. On the other hand, around two days before its occurrence, the complexity measure in the candidate epicentral area exhibits an abrupt increase. This difference reveals, well in advance, that the M7.3 earthquake on 9 March 2011 was a foreshock.

Study of the temporal correlations in the magnitude time series before major earthquakes in Japan

AbstractA characteristic change of seismicity has been recently uncovered when the precursory Seismic Electric Signals activities initiate before an earthquake occurrence. In particular, the fluctuations of the order parameter of seismicity exhibit a simultaneous distinct minimum upon analyzing the seismic catalog in a new time domain termed natural time and employing a sliding natural time window comprising a number of events that would occur in a few months. Here we focus on the minima preceding all earthquakes of magnitude 8 (and 9) class that occurred in Japanese area from 1 January 1984 to 11 March 2011 (the day of the M9 Tohoku earthquake). By applying Detrended Fluctuation Analysis to the earthquake magnitude time series, we find that each of these minima is preceded as well as followed by characteristic changes of temporal correlations between earthquake magnitudes. In particular, we identify the following three main features. The minima are observed during periods when long‐range correlations have been developed, but they are preceded by a stage in which an evident anticorrelated behavior appears. After the minima, the long‐range correlations breakdown to an almost random behavior possibly turning to anticorrelation. The minima that precede M≥7.8 earthquakes are distinguished from other minima which are either nonprecursory or followed by smaller earthquakes. Furthermore, it is discussed whether Tsallis statistical mechanics, in the frame of which it has been suggested that kappa distributions arise, can capture the effects of temporal correlations between earthquake magnitudes.

Application of the Huang-Hilbert transform and natural time to the analysis of seismic electric signal activities.

The Huang method is applied to Seismic Electric Signal (SES) activities in order to decompose them into their components, named Intrinsic Mode Functions (IMFs). We study which of these components contribute to the basic characteristics of the signal. The Hilbert transform is then applied to the IMFs in order to determine their instantaneous amplitudes. The results are compared with 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 these instantaneous amplitudes, when combined with the natural time analysis, can be used for the distinction of SES from artificial noises.

On the increase of the "non-uniform" scaling of the magnetic field variations before the M(w)9.0 earthquake in Japan in 2011.

By applying Detrended Fluctuation Analysis (DFA) to the time series of the geomagnetic data recorded at three measuring stations in Japan, Rong et al. in 2012 recently reported that anomalous magnetic field variations were identified well before the occurrence of the disastrous Tohoku Mw9.0 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. This explanation is strengthened by recent findings showing that the fluctuations of the order parameter of seismicity exhibited an unprecedented minimum almost two months before the Tohoku earthquake occurrence which is characteristic for an almost simultaneous emission of Seismic Electric Signals activity.

On the anomalous changes of seismicity and geomagnetic field prior to the 2011 [formula omitted] 9.0 Tohoku earthquake

In Xu et al. (2013), it has just been reported that approximately 2months prior to the Mw 9.0 Tohoku earthquake that occurred in Japan on 11 March 2011 anomalous variations of the geomagnetic field have been observed in the vertical component at a measuring station about 135km from the epicenter for about 10days (4–14 January 2011). Here, we show that this observation is in striking agreement with independent recent results obtained from natural time analysis of seismicity in Japan. In particular, this analysis has revealed that an unprecedented minimum of the order parameter fluctuations of seismicity was observed around 5 January 2011, thus pointing to the initiation at that date of a strong precursory Seismic Electric Signals activity accompanied by the anomalous geomagnetic field variations. Starting from this date, natural time analysis of the subsequent seismicity indicates that a strong main shock was expected in a few days to one week after 08:40 LT on 10 March 2011.

Investigation of 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 main shock. This is based on the view that the occurrence of earthquakes is a critical phenomenon to which statistical dynamics may be applied. In the present work, analysing the time series of small earthquakes, the concept of natural time χ was used and the results revealed that the approach to criticality itself can be manifested by the probability density function (PDF) of κ1 calculated over an appropriate statistical ensemble. Here, κ1 is the variance κ1(= 〈χ2〉 − 〈χ〉2) resulting from the power spectrum of a function defined as , where pk is the normalized energy of the k-th small earthquake and ω the natural frequency. This PDF exhibits a maximum at κ1 ≈ 0.070 a few days before the main shock. Examples are presented, referring to the magnitude 6∼7 class earthquakes that occurred in Greece.

Investigation of 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 main shock. This is based on the view that the occurrence of earthquakes is a critical phenomenon to which statistical dynamics may be applied. In the present work, analysing the time series of small earthquakes, the concept of natural time chi was used and the results revealed that the approach to criticality itself can be manifested by the probability density function (PDF) of kappa(1) calculated over an appropriate statistical ensemble. Here, kappa(1) is the variance kappa(1)(=<chi(2)>-<chi>(2)) resulting from the power spectrum of a function defined as Phi(omega)= summation operator(k=1)(N) p(k) exp(iomegachi(k)), where p(k) is the normalized energy of the k-th small earthquake and omega the natural frequency. This PDF exhibits a maximum at kappa(1) asymptotically equal to 0.070 a few days before the main shock. Examples are presented, referring to the magnitude 6 approximately 7 class earthquakes that occurred in Greece.

Recent Seismic Electric Signals (SES) activities in Greece

Quite recently two intense Seismic Electric Signals activities were recorded at Pirgos station in western Greece. They have been followed by two strong earth-quakes with magnitudes 6.1 and 6.9 that occurred in western Greece and southern Greece; the latter is the strongest earthquake that occurred in Greece during the last two decades. The compatibility of these results with some relationship between selectivity and earthquake focal mechanism suggested by Uyeda et al. (1999) is discussed.

Some properties of the entropy in the natural time

We show that the entropy S , defined as S identical with chi ln chi - chi ln chi [Phys. Rev. E 68, 031106 (2003)] where chi stands for the natural time [Phys. Rev. E 66, 011902 (2002)], exhibits positivity and concavity as well as stability or experimental robustness. Furthermore, the distinction between the seismic electric signal activities and "artificial" noises, based on the classification of their S values, is lost when studying the time-reversed signals. This reveals the profound importance of considering the (true) time arrow.

Attempt to distinguish electric signals of a dichotomous nature.

Three types of electric signals were analyzed: Ion current fluctuations in membrane channels (ICFMC), Seismic electric signals activities (SES), and "artificial" noises (AN). The wavelet transform, when applied to the conventional time domain, does not allow a classification of these signals, but does so in the "natural" time domain. A classification also becomes possible, if we study <chi(q)>-<chi>(q) versus q, where chi stands for the "natural" time. For q values approximately between 1 and 2 the signals are classified and ICFMC lies between the other two types. For q=1, the "entropy" S identical with <chilnchi>-<chi>ln<chi> of ICFMC almost equals that of a "uniform" distribution, while the AN and SES have larger and smaller S values, respectively. The recent [P. Varotsos, N. Sarlis, and E. Skordas, Phys. Rev. E 67, 021109 (2003)] finding that, in short time scales, both SES and AN (which are shown to be non-Markovian) result in comparable detrended fluctuation analysis exponents alpha in (1.0,1.5) is revisited. Even a Markovian dichotomous time series, in short time scales, leads to similar alpha exponents.