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 Φ(ω)=∑_k=1^N p_k exp(iωχ_k), where p_k 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.

(Communicated by Seiya UYEDA, M.J.A.)