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Kazakhstan Seismic Monitoring System

#K-1049


Development of the Database and Data Analysis Techniques for the Large-Aperture Borovoye Array, as Part of the Kazakhstan Seismic Monitoring System

Tech Area / Field

  • ENV-SEM/Seismic Monitoring/Environment

Status
3 Approved without Funding

Registration date
30.06.2003

Leading Institute
National Nuclear Center of the Republic of Kazakstan / Institute of Geophysical Researches, Kazakstan, Kurchatov

Supporting institutes

  • Institute of Dynamics of the Geosphere, Russia, Moscow\nInstitute of Physics of the Earth, Russia, Moscow

Collaborators

  • NORSAR, Norway, Kjeller\nColumbia University / Lamont-Doherty Earth Observatory, USA, NY, Palisades\nGeoForschungsZentrum Potsdam, Germany, Potsdam\nOrfeus Data Center, The Netherlands, De Bilt

Project summary

The objective of this project is to expand capabilities of the Kazakhstan seismic monitoring system, as part of the International Monitoring System of the Comprehensive nuclear Test Ban Treaty Organization (IMS/CTBTO). This objective will be reached by analysis of seismic data that were accumulated during the whole period (more than 30 years) of operation of the Borovoye large aperture seismic array, by development of new data processing techniques and by investigation of the deep structure that affects propagation of seismic waves on a regional scale.

CTBT with a membership of more than 150 nations will establish an International Monitoring System (IMS), which consists of four monitoring sub-systems: seismic, hydroacoustic, infrasound and radionuclide. Data obtained from these networks will be made accessible to all participating countries, thus giving them equal opportunities to accomplish their monitoring purposes. Development of national monitoring systems plays a decisive role in establishment of IMS and makes violation of CTBT less probable.

The global scope of CTBT creates numerous scientific and technological problems that should be solved in order to establish a reliable monitoring system. The CTBT requires that almost any seismic event practically without limitation of its magnitude should be detected and identified. This means that possible illegal nuclear tests should be recognized among natural seismic events (earthquakes, mine breakouts, avalanches, etc) and industrial events like chemical blasts. Excitation of seismic waves is very different in different regions, and the wavefields are further modified by local wave propagation effects. This requires a detailed study and calibration of the excitation and propagation of seismic waves on a regional scale.

As a result of ISTC Project K-063 the modern seismological observational system equipped with telecommunication and data processing tools, which includes seismic stations of large aperture array Borovoye (BRVK stations and small arrays CHK, ZRN, VOS) and Kurchatov station, was created. Also, under the K-063 Project a unique archive of digital data, which had been acquired on BRVK during 1967 – 1990 was created. The archive consists of more than 1000 recordings of the known nuclear explosions and more than 30000 recordings of other seismic events. Most of the recordings were not analyzed, and the parameters of the related events still are not determined.

The accomplishment of the project will allow:

1. To prepare bulletin of small seismic events in Kazakhstan and neighboring regions for the time period of 1985-2000. Then the related database can be used for diagnostic purposes outside the known test sites.

2. To determine deviations of travel times and back azimuths of seismic waves recorded at the Borovoye array relative to their standard values.

3. To map velocity heterogeneity and anisotropy beneath and near every station of the Borovoye array in order to evaluate their disturbing effects and the related corrections to the recorded wavefields.

4. To estimate effectiveness of event location with L1 norm as a cost function.

5. To accomplish an acquisition, processing, transmitting and archiving of Borovoye large aperture array data in real-time.

Three scientific organizations participate in the Project: Institute of Geophysical Research of the National Nuclear Center of the Kazakhstan Republic, Institute for Dynamics of the Geospheres of the Russian Academy of Sciences, and Institute for Physics of the Earth of the Russian Academy of Sciences. All three organizations previously participated in joint projects and have high-qualified personnel.

Task 1. The creation of database of small seismic events in Kazakhstan and nearby territories.

Task description and main milestones.

Under the K-063 Project a large amount of digital archive data of nuclear explosions and earthquakes recorded on BRVK station during 1967 – 1990 was reformatted into modern format. All data now are stored in the CSS 3.0 format. However, phase characteristics of seismic channels are not present in this database because, unfortunately, the results of pulse calibrations made on the station are lost. So, one of the task is to reconstruct phase characteristics of the channels.

The database of earthquakes with a magnitude more than 4.5 was created under the K-063 contract. At the same time weak seismic signals, recorded on regional distances are of great interest for international monitoring studies. The archive of Borovoye station stores about 30 000 records of weak signals, which, possibly, are associated with earthquakes, occurred within Kazakhstan and nearby Russian territories. It is proposed to make associations of the archive records of Borovoye station with regional catalogues (“Earthquakes of Kazakhstan and Central Asia” annuals).

Task 2. The estimation deviations of travel time and azimuths of seismic signals recorded on large aperture array system.

Task description and main milestones.

The arrival time, polarization and spectral composition of seismic phases are critical parameters for correct determination of seismic event hypocenter.

The large aperture seismic array “Borovoye”, placed on Kokchetav massif, was constructed in the year of 1980 and is operated till the present time. It consists of three subarrays; each subarray has 6 elements, placed in a circle with diameter 6 kilometres. In the centre of each subarray there is one three-component element. The distance between sub-arrays is about 110-140 kilometres.

It is proposed to study azimuth and slowness residuals, basing on the methods of frequency-wavenumber (f-k) analysis. However, the usual coherence analysis may be not suitable for elements separated by such a large distance. In this connection, it is proposed to study azimuth and slowness deviations basing on correlation analysis of seismic phases. To determine corrections depending on azimuth (it is possible, that such corrections would be also time dependent) it is necessary to analyse a substantial amount of data. Such database does exist for the Borovoye large aperture array for the time period 1980 – 2000, so it is quite possible to determine needed corrections.

Task 3. Mapping velocity heterogeneity and anisotropy beneath and near every station of the Borovoye array.

Task description and main milestones.

Velocity heterogeneity and anisotropy beneath the Borovoye array are practically unknown.

P and S receiver functions will be used to quantify stratification of the crust and upper mantle beneath the array. The receiver functions will be determined for many azimuths and then azimuthally filtered in order to separate effects of anisotropy (2nd azimuthal harmonic) and lateral heterogeneity (1st harmonic). Shear wave splitting due to anisotropy will be determined as a function of frequency and depth. The effects of splitting on polarization of regional shear phases will be evaluated and, if necessary, corrected. The analysis of secondary phases in the receiver region will be used to refine the raw recordings in order to identify the depth phases and to determine the depths of the events with a maximum accuracy. This is important for distinguishing the near -surface explosions from earthquakes at a finite depth. Inelastic attenuation in the region will be evaluated, as well as the related effects in the travel times. Variations of slowness of main seismic phases will be determined as functions of distance and azimuths. This will allow accurate determinations of back azimuths and distance of the recorded events.

Task 4. The development of new techniques for determination of seismic event hypocenter.

Task description and main milestones.

In seismological practice hypocenter coordinates are usually determined with sum of squares of first arrival time residuals as a cost function. One of the most important steps, when applying those methods is an excluding of rough measurements. This process is even more important for nuclear explosions monitoring systems where slowness and azimuths of signals are also participate in location determination along with signal arrival times. In this case, the data under process has unequal accuracy and analyst can’t control this inequality. In this connection there is need to develop an alternative approach, without excluding rough measurements. The new approach proposed in the Project bases on the L1 norm that is, under certain conditions, robust with respect to data outliers.

Task 5. The support of telemetry system on Borovoye large aperture array.

Task description and main milestones.

The telemetry system was installed on Borovoye large aperture array under K-063 Project. Now several seismic institutes and centers (Lamont Observatory, IRIS) receive data from Borovoye array in real-time. The next step is to support the operation of telemetry system, to establish a near real time processing of array data and to compile a seismic bulletin, basing on data, received in real-time.


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