Seismic Electric Signals: A physical interconnection with seismicity
  By applying natural time analysis to the time series of earthquakes, we find that the order parameter of seismicity exhibits a unique change approximately at the date(s) at which Seismic Electric Signals (SES) activities have been reported to initiate. In particular, we show that the fluctuations of the order parameter of seismicity in Japan exhibits a clearly detectable minimum approximately at the time of the initiation of the SES activity observed almost two months before the onset of the Volcanic-seismic swarm activity in 2000 in the Izu Island region, Japan. To the best of our knowledge, this is the first time that, well before the occurrence of major earthquakes, anomalous changes are found to appear almost simultaneously in two independent datasets of different geophysical observables (geoelectrical measurements, seismicity). In addition, we show that these two phenomena are also linked closely in space. 
Panayiotis A. Varotsos, Nicholas V. Sarlis, Efthimios S. Skordas and   Mary S. Lazaridou
physics.geo-ph cond-mat.stat-mech