Abstract
This paper reviews the precursory phenomena of the 2011 MW9 Tohoku earthquake in Japan that emerge solely when we analyze the seismicity data in a new time domain termed natural time. If we do not consider this analysis, important precursory changes cannot be identified and hence are missed. Natural time analysis has the privilege that enables the introduction of an order parameter of seismicity. In this frame, we find that the fluctuations of this parameter exhibit an unprecedented characteristic change, i.e., an evident minimum, approximately two months before Tohoku earthquake, which strikingly is almost simultaneous with unique anomalous geomagnetic field variations recorded mainly on the z component. This is consistent with our finding that such a characteristic change in seismicity appears when a seismic electric signal (SES) activity of the VAN method (from the initials of Varotsos, Alexopoulos, Nomicos) initiates, and provides a direct confirmation of the physical interconnection between SES and seismicity.
Highlights
Natural time analysis, introduced in the beginning of 2000s (Varotsos et al 2001b, 2002a, b, 2003a, b), uncovers unique dynamic features hidden behind the time series of complex systems
When analyzing electrocardiograms in natural time aiming at the distinction between sudden cardiac death individuals (SD) and healthy (H) individuals (e.g., Varotsos et al 2007), we make use of the entropy fluctuations and the aforementioned complementarity holds in the following sense (Varotsos et al (2011a); see pp. 410–413 of Varotsos et al (2011c)): If in the frame of the latter complexity measures an ambiguity emerges in the distinction between SD and H, the former complexity measures give a clear answer
Label b200,min b250,min b300,min b300;min b200;min shown to be valid for another case, i.e., the MW9 Tohoku EQ in the following sense: By combining the results obtained from the natural time analysis of the seismicity data (Skordas and Sarlis 2014; Sarlis et al 2013, 2015) and the anomalous geomagnetic field variations reported by Xu et al (2013) and Han et al (2015, 2016), we find that bW,min of seismicity is almost simultaneous with the latter anomalous variations that are characteristic of a strong seismic electric signal (SES) activity; in addition, these two phenomena are linked in space since a spatiotemporal study of bW,min of seismicity by Sarlis et al (2015) led to a region that lies within a few hundred kilometers from the MW9 Tohoku EQ epicenter
Summary
Natural time analysis, introduced in the beginning of 2000s (Varotsos et al 2001b, 2002a, b, 2003a, b), uncovers unique dynamic features hidden behind the time series of complex systems. The two different geophysical observables under discussion, i.e., SES activity and minimum of fluctuations of the order parameter of seismicity, were shown by Varotsos et al (2013) to be linked in space as follows: Using a narrow 5° 9 5° spatial window sliding through the (21° 9 21°) area N4265E114265, we investigated the earthquake events that would occur in an almost two months period. Sarlis et al (2013) analyzed in natural time the seismicity of Japan from January 1, 1984, to March 11, 2011, using sliding natural time window of lengths W comprising the number of events that would occur in a few months within the area N4265E114285 They set a threshold MJMA = 3.5 to assure a safe data completeness which leads on the average to a number of the order of *102 earthquakes per month. The latter behavior is usually observed when a strong earthquake is included in the corresponding catalog excerpt invariance in a similar fashion as discussed in Sect. 2, i.e., by considering both the larger area N4265E114285 and the smaller area N4265E114265. (Recall that Sarlis et al (2013) considered only the larger area in their study.) We find that the results of this investigation, which are depicted in Fig. 4 and summarized in Table 1, remain unchanged upon varying the area studied, verifying the criterion of spatial invariance
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