Investigation of seismicity dynamics in the Baikal region

Abstract

For the first time, the methods of fractal theory were applied to the study of seismicity dynamics of the Baikal region and its three areas. Estimates of the Hausdorff dimensionality ( D ) show self-similarity of the seismic process in 1967‐2002 with a 10% significance level. The most prominent Hurst index ( H ) and D variations correspond with the aftershock series of strong earthquakes, while regions of high H and D values are congruent with the concentration zones of earthquake groups, suggesting a high influence of shock clusters on the regional seismicity. In general, the Baikal region and its three areas are characterized by a persistent seismic process with long-term memory. Modern theoretical and numeric simulations of seismicity are developing as a new basis for the study of spatiotemporal and energy structure of seismicity and prediction of strong earthquakes [1]. These models provide for an expansion of principal features of seismicity diagnostics by the joint analysis of models and phenomenology: scaling, similarity, self-similarity, and predictability on different averaging scales. It is well known that invariance relative to multiplicative scaling is governed by the self-similarity of spatiotemporal processes [2]. Self-similarity in application to random sets is not a rigorous concept, because parts in such a case must not be inevitably and exactly similar to the whole and is sufficient if parts and the scaled down whole have the same distributions. Notwithstanding their fundamental importance, such procedures of the diagnosis of a natural seismicity pattern were practically never applied for investigating the structure of seismicity dynamics (seismic process) in the Baikal region. Analysis of the seismic regime made it possible to identify periods of high seismic activity in the region [3]. During the instrumental observation period, the strongest earthquakes (energy class K e ≥ 14 ) generally followed geodynamic activation in some parts of the region and represented responses to the geophysical medium excitation owing to the stress field inversion during the self-organization of the Baikal rift system (BRS) [4, 5]. Excitation of the active geophysical medium was accompanied by an increase in the number of seismic event groups and shocks in earthquake swarms. The abundance of aftershocks and swarm events in the BRS lithosphere ([6, 7]) predetermines a priori the complicated spatiotemporal relationship of the seismic process with earthquake clusters and complicates analysis of dynamics of the regional seismicity, because estimation of the influence of shock groups of various energy classes on its variations becomes the priority issue. The most hazardous (in terms of seismicity) area of the Baikal region is represented by the rift zone extending as a depression system from northern Mongolia along Lake Baikal to southern Yakutia [4]. Earthquake epicenters are mainly concentrated as sublatitudinal and NE-oriented bands, while local high-density shock groups are most often related to aftershock and swarm shock activity. Figure 1 shows the distribution of the annual number of representative K e ≥ 8 earthquakes registered in 1967‐2002 in the Baikal region ( ϕ = 48.0°–60.0° N, λ = 96.0°–122.0° E), along with the southwestern (area 1, ϕ = 48.0°–54.0° N, λ = 96.0°– 104.0° E) and northeastern (area 3, ϕ = 54.0°–60.0° N, λ = 109.0°–122.0° E) flanks and the central part (area 2, ϕ = 51.0°–54.0° N, λ = 104.0°–113.0° E) of the BRS. These large geological areas can be considered two