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 κ
1 calculated over an appropriate statistical ensemble. Here, κ
1 is the variance κ
1(=<χ
2>−<χ>
2) resulting from the power spectrum of a function defined as Φ(ω)=∑
k=1N pk exp(
iωχ
k), where
pk is the normalized energy of the k-th small earthquake and ω the natural frequency. This PDF exhibits a maximum at κ
1≈0.070 a few days before the main shock. Examples are presented, referring to the magnitude 6∼7 class earthquakes that occurred in Greece.
(Communicated by Seiya UYEDA, M.J.A.)
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