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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS009 Oral Presentation 2026
Natural time analysis of seismicity in Japan and California
Prof. Seiya Uyeda
Approach to critical state of seismicity a few days before mainshocks was indicated in areas other than
Greece. Time series of seismicity before 2000 Izu-island (M6 class), 1995 Kobe (M7.3), 2004 Niigata
Chuetsu (M6.8), 2003 Tokachi-oki (M8.0) in Japan, and 2004 Parkfield earthquakes (M5.9) in California.
The 2000 swarm activity in Izu-island region, lasting for about two months with some 7,000 shocks with
magnitude M≥3 and five M≥6 shocks, was preceded by a pronounced seismic electric signal (SES)
activity with innumerable signals that started a few months prior to the swarm onset. It has been shown
first that the seismicity subsequent to the electrical activity approached to the 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 a critical stage. These features were similar to those found earlier in Greece.
Seismicities before 1995 M7.3 Kobe, 2003 M8.0 Tokachi-Oki, and 2004 M6.8 Niigata Chuetsu
earthquakes (EQs), in Japan, have been analyzed in the natural time-domain. For the Kobe and
Tokachi-oki EQs, the occurrence time of the impending mainshocks were assessed with accuracy of the
order of a few days, when the computation had started 1-2.5 months before the mainshocks. In the
present cases where there was no SES data, the starting dates of computation were sought by trial and
error approach. For the Chuetsu event, the results were uncertain. The same type of analysis has been
made on seismicity after 2003 San Simeon EQ and before 2004/09/28 Parkfield EQ in California. In
circular and elliptic areas with different sizes around San Andreas Fault, magnitude 2.2-2.6 EQs
(Catalog: NCEDC) were shown to reach critical point 5 days before the mainshock of the Parkfield EQ,
when the calculation was started on 2004/05/01.
Keywords: natural time, critical state, seismicity