Seismic Electric Signals: An additional fact showing their physical interconnection with seismicity

Natural time analysis reveals novel dynamical features hidden behind time series in complex systems. By applying it 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 by Uyeda and coworkers 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. ► Two different geophysical observables appear simultaneously before major earthquake. ► At the initiation of SES activity, a change of seismicity emerges in natural time. ► SES activity and seismicity change in natural time are also linked closely in space.