Migrating Seismicity in the Lithosphere of the Baikal Rift Zone: Spatiotemporal and Energy Distribution of Earthquake Chains

摘要

Quasi-linear sequences of the epicenters of strong earthquakes, identified in many seismically active regions, are phenomenologically viewed as "migrations" of earthquake sources. Following this analogy, an earthquake chain is understood in this paper as a set of seismic events of different energies, leading to a quasi-linear one-way change in the position ("migration") of the successive epicenters of shocks on the surface of the lithosphere zone under study. Based on this statement, a formalized method for an azimuthal analysis of seismicity is developed, which allows one to identify and isolate earthquake chains from arrays of seismological data presented in the standard format of a catalog of earthquakes. As this method is tested on a catalog of earthquakes and a catalog of simulation events, all model chains of events and a large number of earthquake chains and simulation events are identified. It is indicated by isolating chains in a random field of simulation events that some of these earthquake chains can be formed by a random spatiotemporal combination of shocks. Migrating seismicity in the lithosphere of the Baikal rift zone (BRZ) is studied by applying the method of identification and isolation of earthquake chains to materials from the "Catalog of Earthquakes of the Baikal region". According to data on 52,700 earthquakes with a representative energy class K-p >= 8, occurring in the Baikal region from 1964 to 2013, there are 2143 earthquake chains identified and isolated within an angular sector of azimuthal analysis q= 10 degrees (+/- 5 degrees from azimuth a to the epicenter). As the angular sector of azimuthal analysis increases to q= 20 degrees (+/- 10 degrees), there is an approximately twofold increase in the number of chains (M= 4245). As shown by the analysis and comparison of spatiotemporal distribution of earthquakes and earthquake chains, the spatiotemporal and energy distribution of chains of seismic events is formed by earthquake distribution in the BRZ. Beyond this zone, the chains are small in number. The established relationship between the distributions of earthquakes and earthquake chains in space, in time, and by energy indicates that in the epicentral field of seismicity of the BRZ, both "seismicity migration" chains and randomly formed chains ("pseudomigration" chains) are identified. The migrating seismicity of the BRZ is studied according to the seismicity statistics by using the criteria developed within the framework of the simulation base model of migrating seismicity to the results obtained. An index of seismicity migration activity (ISMA) that reflects a seismicity migration/ pseudomigration ratio at the set level of significance is used to obtain distribution maps over the territory and graphs of changes over the years for this index. The maps show that small areas ISMA > 1 are seen in close proximity to rifting attractor structures (RASs), while seismicity migration in the rest of the BRZ is not statistically obvious. The graphs show that periods ISMA > 1 at three levels of significance are observed three to four years after the activation of RASs, which makes it possible to estimate the phase propagation velocity of a slow deformation perturbation of about 250-300 km/year.