Global variations of the Earth’s seismic regime in the years 1965–2005

Abstract

Significant variations of the Earth’s seismic regime in the period 1965‐2005 were revealed on the basis of a new methodical approach. The frequency of large earthquakes during an 11-yr interval (1982‐1993) was three or more times lower than in the preceding period. This was accompanied by their intense activation in the entire range of seismic source depths. 1 The seismic regime is traditionally displayed as recurrence graphs of earthquakes of various magnitudes. The evolution of seismic processes is commonly characterized by the total number and total energy of all earthquakes over a certain time interval. Such an integral presentation of the seismic regime smoothes away specific natural features of its spatiotemporal evolution and thus hampers their investigation [1, 2]. The results obtained in our work are based on a principally new method designed for the study of the seismic regime: the sequence of seismic events is analyzed in magnitude intervals that display the hierarchical nature of the block structure of the geological medium. We investigated successions of large earthquakes on the Earth in the years 1965‐2005 differentiated by the magnitude interval M = 7.0 ± 02, 7.5 ± 0.2, 8.0 ± 0.2, and 8.5 ± 0.2. This interval overlaps a wide energy range of M from 6.8 to 8.7 (Fig. 1). The last interval also includes the two largest earthquakes with M = 8.8. The total number of events was more than 600. The interval width of ± 0.2 M includes errors in the determination of this value. The step of 0.5 M also has a physical sense because of its relation to the hierarchical structure of the block‐fault medium [3‐6]. Hereinafter, magnitude M corresponds to the M s value determined from surface seismic waves. 1 When this paper was being printed, the region was subjected to two earthquakes with centers located in a breach within the central Kuril arc (November 15, 2006, M = 8.3; January 13, 2007, M = 8.2). Over the 40-yr period beginning from 1965, none of seismic events was missed in the world catalogs of earthquakes of the considered range of magnitudes and source depths. All these data are ordered to the highest extent in the ANSS catalog [7] used in this study. Figure 2 shows cumulative graphs of the temporal accumulation rate of seismic events of the magnitude intervals mentioned above. The abscissa shows ordinal numbers N in the sequence of earthquakes, while the ordinate shows years Y of their origin. Let us define events with hypocenters at a depth h of no more than 70 km and more than 70 km as shallow-focus and deepfocus earthquakes, respectively. The solid and thin lines approximate the data points related to shallow-focus and deep-focus earthquakes, respectively. The linear approximation is characterized by a high correlation coefficient (0.9 or higher). Only a fragment of the graph for shallow-focus earthquakes with M = 7.0 ± 0.2 is shown on the right side, because their number is too great. The horizontal scale of this graph is compressed approximately 2.5 times for the purpose of more convenient comparison. The thin line approximates the whole data array of these events over the 40-yr period, whereas the solid line is drawn similarly to other graphs.