Analysis of electrical activity and seismicity in the natural time domain for the volcanic-seismic swarm activity in 2000 in the Izu Island region, Japan

Seismicity and geoelectric potential changes, possibly associated with the seismic swarm activity in 2000 in the Izu Island region, Japan, are analyzed in the framework of the natural time χ, which has been recently introduced as an index of the kth event χ = k/N, where N is the total number of events. The Izu 2000 swarm activity lasted for about 2 months with some 7000 shocks with magnitude M ≥ 3 and five M ≥ 6 shocks, and was preceded by a pronounced electrical activity with innumerable signals that started 2 months prior to the swarm onset. It is shown, first, that the seismicity subsequent to the electrical activity approaches to a critical stage a few days before the occurrence of the first M ≥ 6 shock and, second, that the electrical signals also have the properties characteristic to the critical stage. Despite the big differences in time scale and numbers of electric signals and earthquakes, these features are found similar to those in Greece. The present results suggest that both in Greece and Japan, the electrical activity as well as the subsequent seismicity may have a self-similar structure and exhibits similar dynamic evolution toward critical stage.