Wednesday, 27 January, 2021

Seismic hazard

Seismic hazard assessment is the first link in the prediction of seismic situation. Depending on the research objectives and the scale seismic zoning can be general, detailed,  and microseismic. The Institute has the fairly comprehensive materials on earthquakes with K 14-15 (M 5.5-6.0) for the entire region  since 1865, and on instrumental observations since 1929; for K 12-13 (M 4.5-5,0) since  1951; for K 8-9 (M 3.0-3.5) since 1961. Based on these materials are created:  a map of the epicenters of strong earthquakes, including events with M 4.0 and reflecting in general features of their spatial distribution; a map of epicenters of small earthquakes, reflecting their more common, but not the same density in different areas, as well as their spatial relationship distribution with the latest morphostructures; the maps of epicenters and seismic activity, which show the degree of differentiation of the territory on these parameters; the maximum possible earthquake map (Kmax) for different seismic regions.

The study of the deep structure tectonosphere criteria to determine the degree of typing its seismic hazard is carried out. Speed, density and structural properties of the crust and upper mantle of the Tien Shan and adjacent areas were studied based on deep seismic soundings, regional seismological observations and seismogravitational modeling based on integrated interpretation of gravity and seismic wave fields. The results revealed a fundamentally important regional structural features of the earth's crust and upper mantle seismic and aseismic areas.

Within Kazakhstan, there are three types of crust differ in physical properties and confidence level are identified by the gravitational field. Type I corresponds to the Kazakh shield and Tarim plate, type II - the crust of the Turan plate, type III - orogenic Tien Shan and Altai Dzungaria.

The properties of the geophysical environment at different hypsometric levels (up to 300 km) tectonosphere Tien Shan and adjacent platform areas are studied. It is found  that the crust and mantle of the Tien Shan on the fundamental physical parameters (speed, density, geothermal, elastic strength and energy) are abnormally different from those in the platform areas. The mantle of the Tien Shan is significantly heterogeneous structure both laterally and vertically, marked structures with high and low values of velocity and density parameters in relation to the normal mantle.

A block diagram of the soles of the crust and mantle activity in Kazakhstan and the surrounding areas is created.  The generalized geodynamic model of tectonosphere of the Tien Shan seismogenic zone, covering focal zone of the strongest earthquakes is compiled;  it explains the increase in the total thickness of the crust in mountain areas due to the increased thickness of the granite-gneiss layer, which is fundamentally different from the previous model.

Morphologically identified and described in the North Tien Shan a deep destructive area between the Ili  and Issyk-Kul cavities, is fundamentally different to the physical parameters of the platform and orogenic crustal blocks. Available data allow to carry the North Tien Shan destructive zone to the category of mobile zones of the cortex, which has an excited endogenous regime and increased permeability. At the top of the cortex area is expressed by a series of upthrust-thrusts. Such tectonic position determines the high degree of seismic activity in the region. A map of the depth and power hierarchy of seismogenic zones of the south-east of Kazakhstan is developed. A map of the Alpine geodynamics of North and Central Asia in scale 1:8000000 is developed.

A map of the new active faults, reflecting the dynamics of tectonic movements in meridional tangential compression of the lithosphere is constructed. The identified network of faults (shifts, reverse faults, thrust upthrust and discharges), along with a conjugate morphostructures, presents  a single paragenesis corresponding geodynamic model meridional tangential compression of the crust. A map of the seismogenic zones of the Republic of Kazakhstan in the scale of 1:2,500000, is made, which was the basis of the seismic zoning.

In order to study the stress-strain state of the upper crust as a consequence of neotectonic movements, the technique of the geometrization and the calculation of the quantitative characteristics of new morphological structures is developed. Morphometric study of new structures is produced and for the first time for the south-east of Kazakhstan the quantitative parameters of the stress-strain state (the value of reducing the upper crust due to horizontal compression, the value of the total vector of tectonic movements, the angle of the uprising full vector, the horizontal and vertical deformation rate of the horizontal and vertical deformation of the latest stage - 30 106 years) are calculated. The scheme of zonation of  advanced structures in the degree of compression in the horizontal scale of 1:1,000,000 is made.

The study of seismic effects and the forecast maximum intensity tremors.

For the prediction of seismic effects, the  macroseismic materials collected during different periods in areas with different tectonic and geotechnical conditions are  studied;  the damping intensities with distance is investigated (determined the  regional coefficients in the equations of the macroseismic field);  a theoretical calculation for a given seismic source parameters and environment  is made. The macroseismic data are studied, the models of isoseismals are made  and the decay laws of seismic tremors at different distances from a certain magnitude of the earthquake are derived (the task of determining the coefficients in the known equations of the macroseismic field).

The map of summary isoseismals is  the total picture of the distribution on the surface of the observed shocks. To estimate the parameters of seismic effects on the expected strong earthquakes,  the relations between the main parameters of impacts to the characteristics of sources and the environment are established. To do this, the available global and regional dependences, describing the relationship of maximum amplitudes corresponding periods of duration, spectral width of the reaction to the established factors affecting the seismic vibrations - magnitude, type, focal mechanism, distance, type of soil conditions have been used.

Practical implementation in the assessment of seismic hazard in the Republic of Kazakhstan is carried through the development of seismic zoning maps of varying detail, the need for which arises in connection with the development of the national economy.  and development.

Due to the large scale civil and industrial construction in earthquake-prone areas of the Almaty industrial area, a detailed seismic zoning of the region to scale 1:500,000. The main map of the initial intensities and seismogenic zones provides an assessment of Mmax by 0.5 units, the expected seismic intensity contours with a step by 0.5 points, contour lines mean repetition frequency tremors of varying intensity. The package includes also a map of the total intensities due to geological conditions and a map of the maximum acceleration. The development can be used for selection of construction sites for all objects (except for very important), and for the prediction of seismorelated  processes as well.

Integrated seismic zoning map of the city of Almaty is based on new methodological developments of the Institute of Seismology, including two crucial points: 1 - to determine the seismic design, a new approach, the distinctive feature of which is the summation of intensity increments with smoothly decreasing initial intensities, 2 – the results of depth study of the structure of the basement and the sedimentary cover are used, which allowed for the first time in the world practice to identify and take into account in normative terms the position of fault zones, which are activating  by strong transition earthquakes.

As a result of scientific cooperation in the field of seismic hazard assessment of Kazakhstan, Kyrgyzstan and China,  a map of seismic zoning of the Tien Shan in scale 1:2000000 have been created.

The general seismic zoning map of Kazakhstan on the scale 1:2,500,000 is developed.  The main load is the contour separating the zone of maximum intensity of the shock in points and engineering characteristics, the probability of exceeding of which a given time interval is less than the threshold. This map for Kazakhstan is accompanied by a set of maps showing the necessary features of tectonics, deep structure, seismotectonics, seismic parameters, seismic effects, etc. To map is a list of settlements, indicating the background intensities within them.

Geological Survey of the United States of AmericaGeological Survey of the United States of America