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Seismicity, focal mechanisms and paleosesimic studies

#KR-2253


Seismicity, modern tectonically movements and seismic hazard of Kyrgyzstan

Tech Area / Field

  • OBS-NAT/Natural Resources and Earth Sciences/Other Basic Sciences

Status
3 Approved without Funding

Registration date
04.02.2016

Leading Institute
Kyrgyz-Russian Slavonic University, Kyrgyzstan, Bishkek

Collaborators

  • Earth and Environmental Sciences Rensselaer Politechnic Institute, USA, New York

Project summary

Project objective is to update existing seismic hazard assessment and seismic zoning of Kyrgyzstan (particularly in recreational area at Issyk-Kul) based on analysis of crustal seismicity, focal mechanisms, crustal stress and paleoseismic data. The existing seismic zoning maps of Kyrgyzstan do not rely much on regional stress or archeo- and paleoseismic data. The project aims to fill this gap. The plan is to go away from focal mechanism studies to the study of crustal stress and strain in the Tien Shan region.

Research background: The existing regional network of digital seismic stations allows for determining not only major source parameters, but also focal mechanisms of Tien Shan’s earthquakes with magnitude M≥3.0. A bulk of seismic data has been collected and systematized to date. A relatively comprehensive database of moderate and strong earthquakes in Kyrgyzstan is available for the period since 1770 [Januzakov, 1964], which means that macroseismic data on strong earthquakes is available for the past 245 years (from 1770 to 2015). Systematic instrumental observation of earthquakes began 88 years ago, when the first seismic station was built in 1927 in Frunze (Bishkek now). The earthquake catalog of Kyrgyzstan contains data on over 90,000 earthquakes with magnitude M≥1.0 that have occurred in the country over the past 60 years (from 1955 to 2105). Focal mechanism solutions are available for 5,000 earthquakes with magnitude M≥3.5 for the period starting in 1946. All this data needs to be significantly updated to improve seismic hazard assessment and earthquake forecasting in Kyrgyzstan.

Please note that owing to ISTC Project KR-2011 (2013-2015; project manager A.M. Muraliev, Doctor of Physical and Mathematical Sciences) we have already collected and systematized available seismic data on focal mechanisms of Tien Shan earthquakes. Focal mechanisms of strong earthquakes have been compiled into a database. This database can now be used to study crustal stress and strain in the Tien Shan region. This project proposes to use focal mechanism data for strain calculations to further improve seismic hazard assessment based on the newly produced data. The project allows for the transition from the collection and processing of actual data (magnitude and/or energy class of earthquakes, epicenter coordinates, focal depth, focal mechanisms, isoseismal lines, geological and geophysical properties of the medium which earthquakes occur in) to understanding of the seismic behavior and assessment of seismic hazards based on available knowledge about the stress and strain state responsible for strong earthquakes. To solve these issues, it is important to understand the relationships between stress and strain parameters in inpidual areas and the regional stress field generated by the interactions between the Indian and Eurasian plates.

The introduction of digital seismic stations in Kyrgyzstan (KNET, a broadband seismic network in 1991 and Norwegian digital seismometers in 2009-2010) allowed us to start computing dynamic parameters (seismic moment М0, stress drop Δσ, etc.) of seismic sources in the country. Seismic moment (М0), which was first introduced into seismology by K. Aki in 1966, is a very important energy parameter of the earthquake directly related to the stress-strain behavior of rocks. Seismic moment allows to calculate the magnitude of stress released at the earthquake source. Under this project, seismic moment and its correlations with magnitude M or energy class KR will be used to determine the integral contribution of each earthquake to the overall tectonic strain.

The strain rate tensor is a sum of seismic moment tensors scaled to shear time, rate and modulus [Kostrov, 1974]. Calculation results will be presented in the form of maps for the area under study. A matrix of the average focal mechanism and its eigenvalues and eigenvectors makes it possible to determine the directions of the principal axes of maximum compression and maximum tension. This matrix will be created for each seismically active zone in Kyrgyzstan. The analysis of principal strain directions helps understand strain patterns in the earth’s crust. It appears reasonable to look for a correlation between strain properties: earthquake size distribution on the one hand and the type of stress and strain state in rocks on the other.

Focal mechanisms of earthquakes describe the strain state, but not the stress state in the geological environment: orientations of the axes of maximum, minimum and intermediate principal extension. Assuming that strain and stress tensors are coaxial, we can obtain information on principal stress axes as well. However, the absolute values of principal stress tensors will remain unspecified. This method cannot produce data on the volumetric part of the stress tensor.

Paleoseismic data is collected from trenching activities and discoveries of paleoseismic displacements in rocks. To date, some experience has been gained in identifying paleoseismic displacements in the areas of known strong earthquakes (Kemin and Susamyr) and in the areas of supposed strong historical earthquakes. This method allows for estimating peak ground velocities (PGV) in strong earthquakes based on statistic field data. The method will be used to estimate PGVs as part of seismic hazard assessments.

Project impacts on the given research area: The proposed methods of transition from computing focal mechanism solutions based on instrumental seismic data to the study of the stress and strain state and use of stress-strain data in seismic hazard assessments are promising areas of research and will definitely facilitate progress in seismology in terms of both methodology and application. We will not only produce data on fault plane motion at the source of each inpidual earthquake from instrumental observations, but also, using this data, calculate and map strain and stress distributions in seismically active zones. In addition, of interest is the new method of calculating peak seismic loads. Reconnaissance surveys completed to date at the Naryn hydroelectric power plant construction site have confirmed the designed values of maximum seismic loads. Under this project, we plan to apply this experience in other priority areas in Kyrgyzstan as well.


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