Synchronization of microseismic variations within a minute range of periods

摘要

Records obtained at the Petropavlovsk, Yuzhno-Sakhalinsk, Magadan, Yakutsk, and Obninsk IRIS broadband stations before the Kronotski (Kamchatka) earthquake are investigated with the use of various programs of processing and analysis of time series. Intervals of a stable manifestation of one or several periods of microseisms (synchronization intervals) inferred from data of different stations do not necessarily coincide in time. No anomalous meteorological effects are recorded in these intervals. Geodynamic phenomena on global and regional scales influence the duration and intensity of the synchronization intervals. As distinct from other stations, the Petropavlovsk record revealed asymmetric variations of the relaxation type that arose five days before the Kronotski earthquake and three days before the onset of intense foreshock activation. The amplitude of variations at this station exceeds the level of other stations by an order of magnitude, which indicates that the source of this earthquake was located in the Pacific seismically active region. The number of predominant periods at the Petropavlovsk station decreases toward the time moment of the Kronotski earthquake, and the polymodal spectrum becomes unimodal, primarily due to the loss of shorter periods: a period of 37 min is most clearly expressed 1 h before the earthquake. The synchronization intervals of variations, as well as the foreshock activation, are indicators of the unstable state of a seismically active region.