The properties of slow seismic activity migration have been revealed by the space-time analysis of the total earthquake energy (LgEsum). Our study of seismic activity covers the fragments of the Central Asian, Pacific and Alpine seismic belts: the Baikal rift system (BRS, Russia), the San Andreas fault zone (California, USA), the Christchurch fault (New Zealand), the North and East Anatolian faults (Turkey), the Philippine subduction zone, and the central fragment of the Mid-Atlantic oceanic ridge. The chains of LgEsum clusters mark the propagation of the maximum stresses front in the weaker crust areas, the zones of fault dynamic influence, and the regions of conjugated tectonic structures. The migration process is characterized by a periodicity, changes in direction, and similar modular values of the migration rates within a single fault segment (or a fault zone), which is probably related to the mechanical and rheological crust and upper mantle properties. The data analysis shows that a strong earthquake source may occur at a location wherein the front of seismic activity propagates with periodical changes in direction, and such a source can develop within a period that is multiple of the migration fluctuations, probably associated with the influence of external periodic factors. The main periods of migration fluctuations (2-4 years, and 9-13 years, in different ratios) are present in the seismic regimes of different seismic belts. The migration rate, as well as the propagation velocity of the maximum stresses front, directly depends on the velocity of movements between the plates in the region.
The Baikal rift system faults, having developed in the recent rift formation period, are characterized by hydrothermal and seismic activity. Especially in the northeastern part, the level of fracturing affects the localization of thermal outlets and the distribution of earthquake swarms. The specific features of the hydrothermal outputs and seismicity interposition, depending on the fracture heterogeneity and water saturation of the lithosphere layers, have not been previously evaluated. The results of the statistical analysis of the hydrothermal data, presented herein, show that most of the springs are distributed in areas of increased fault density. Multiple less hot hydrotherms are associated with zones of maximum density in the inter-block space. The spatio-temporal analysis of seismicity showed that migrations of weak and moderate seismic activity propagate from earthquake swarms through these zones. Swarms initiate the deformation front by propagating in the quasiplastic layer of the upper mantle at a speed of tens of kilometers per year, which can increase the aqueous fluid pore pressure in the lower earth’s crust, facilitate the movement of the fluid upwards along the section, and cause a process of successive stress relaxation in zones of high fracturing and concentration of hydrothermal springs. Earthquake swarms occur in areas of about average fracture density, associated with deep faults framing consolidated blocks of the earth’s crust. The hydrotherms with high or average temperature, and with probable deep source chemical composition components are related with such zones.
The orientation of deformation process development during rifting controls the strike of the regional active faults, which determine the epicentral field structure features in the northeast part of the Baikal rift system (BRS), characterized by current high seismic activity. Rose charts were plotted for the number of faults of each strike range. For zones of epicenter concentrations, the polygons of seismic data projection were determined in accordance with the general strike of most of the active faults and their average length. Taking into account the anisotropy of the fracture network, the spatiotemporal analysis of the earthquake epicentral field was carried out using GIS technology. The seismic activity migration episodes as a result of crustal deformation are inherent to a non-stationary seismic process for the investigated area. Migrations, characterized by constant rate, are recurred in places of intense lithosphere fracturing and change the direction in potential M ≥5 earthquake and swarms occurrence sites. Ordered sequences of energy clusters most likely propagate at a depth of the fluid-saturated layer beneath the thickened granite batholith where the heat accumulated warms up the fluid and apparently reduces the viscosity of seismoactive layer. The existence of thickened plastic lithospheric layer and viscosity reduction of brittle crustal layer are conditions probably conducive to migration of seismic activity.
For the Baikal rift system (BRS) territory (Russian Federation) and the Barguzin-Baunt depression branch zone, active faults density is compared with the thermal and cold spring’s quantity and temperature to identify the geospatial statistical relationship of those parameters with a view to quantify the main factors connected with base principles study of modern hydrothermal formation. Statistical calculations show that the hot and cold spring’s number per unit area increases within the active faults number. It is established that an area with active faults density higher, than the mean value, is characterized by the maximum number of thermal springs. Correlation analysis showed that the number of modern mineral water springs per unit area and their temperature depend on the earth’s crust fragmentation degree: zones of reduced density are characterized by rare but hotter thermal springs, and zones of increased density are characterized by numerous and colder thermal vents, which probably connects with water dilution by underground cold waters in a more fractured geological environment.
Three-dimensional space-time diagrams of «logarithm of total energy released by earthquakes» parameter, lgE sum are constructed for regions with stable concentrations of earthquake epicenters in Cis-Baikal region for a period from 1964 to 2002. Based on analyses of such diagrams, areas of slow migration of seismic activity are defined. Estimated are distances, time and velocities of slow migration in the range of the first kilometers-first dozen of kilometers per year. Procedures of seismic data projection and construction of 3D diagrams are described in brief. A general scheme including contours of projection areas is proposed for the Pribaikalie (Fig. 1). Three space-time diagrams are presented as examples of application of the above mentioned procedures. They are constructed for the Middle and Southern Baikal basins and the western part of the NE flank of the Baikal rift system (Fig. 2). Integrated analytical results are presented for all the diagrams which record earthquake migration within the Baikal rift system. We also present a scheme of the zone of slow migrations ranked by dominating velocities (Fig. 3) and a diagram of the migration velocity range. We consider possible causes of slow migration of seismic activity at variable velocities: (1) slow deformation waves spreading in the crust, and (2) independent propagation of the deformation front along active faults. Regulations of migration of strong earthquakes can be useful for definition of timelines and locations of future strong seismic events.
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