About conditions of forming recurrence curves and the assessment of long-term and current (expected) seismic hazards

The blocked structure of geophysical medium and the values of limit magnitudes and deformations in the an earthquake source (Sadovskii, 1979; Sadovskii and Pisarenko, 1991), which depend on the block structure, have been the basis for the Earth’s crust model. The model includes 18 orders (1–18) of the structures whose dimensions are connected with one another in a nonrandom way and reflect the coefficient of similarity k = √10 characteristic of tectonic blocks, interblock boundaries, and earthquake sources (Sadovskii, 1979; Bugaev, 1999; Bugaev and Spivak, 2002). The relation of sizes of constituent structures of the model is described by a geometric progression: with a growth in the order of structure by 1, a ratio of sizes of the N _i and N _{i + 1} structures increases by k times and vice versa. If length logarithms of structures are considered, their properties are described by an arithmetic progression with the difference d = 0.5. This made it possible to formalize the size assessment of n first constituent structures of the earth’s crust model (with due account of a size of the maximum structure of a region) and determination of their frequency with allowance for a period of long-term geodynamic and seismic stability of the territory under study. The model was used (i) for estimating possible limits of change in the observed earthquake recurrence curves taking into account the dependences, reflecting a relation of limit magnitudes and the size of earthquake source, and conditions of its preparation and manifestation, and also (ii) for forecasting the earthquake recurrence curves based on geodynamic data (extension of a zone, strain rate in a zone, frequency distribution of extents of active faults or zones). The comparison of forecasted and observed data for the site of placement of the Fukushima-1 nuclear power plant before and after the March 11, 2011 earthquake with M _W = 8.9 allowed us to establish the regional and local features of the seismic process and to estimate the long-term and current seismic dangers according to geodynamic and seismological data.