On the reality of the 56-year cycle and the increased probability of large earthquakes for Petropavlovsk-Kamchatskii during the period 2008–2011 according to lunar cyclicity
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A 56-year cyclicity in the occurrence of large Kamchatka earthquakes has been previously detected. This is another manifestation of the tendency for the timing of large Kamchatka earthquakes to be synchronized to the cycles related to the period To of rotation of the lunar nodes found by V.A. Shirokov in 1974. He identified cycles of 18.6 years = To and 6.2 years = To/3, while the period of the 56-year cycle is 3To. The genuineness of that phenomenon had to be revised in connection with the occurrence of a large (Mw = 7.8) earthquake in Kamchatka at the end of 1997, in violation of the 56-year cyclicity. It turned out that, even though the 56-year cycle has become less distinct after the 1997 event, the cyclicity itself has remained statistically significant. A byproduct is an updated forecast of earthquake hazard for Kamchatka. The update is necessary in view of the approaching hazardous period of 2008–2011. It is found that, assuming the validity of these empirical tendencies, the expected rate of large earthquakes off Kamchatka for the period of August 2008 to October 2011 will be four times as high as the long-term mean. We derive the first-ever estimate of future hazard in terms of felt intensity for specified soil conditions (the so-called average soil) at a specified site (the town of Petropavlovsk-Kamchatskii). For these soil conditions, the estimated probability of at least one shock of intensity VII or greater during the period specified above is equal to 0.39 ± 0.15. The expected rate of single events or sets of events with Mw ≥ 7.6 in Kamchatka during this period is 0.76 ± 0.25.
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