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=1N pk exp(iωχk), 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.
(Communicated by Seiya UYEDA, M.J.A.)
2008 The Japan Academy