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Earth’s Crust Structure of Black Sea

#G-1930


Investigation of Horizontal-Heterogeneous of the Earth's Crust Structure and Upper Mantle of Black Sea Water Area by Surface Waves Tomography Methods and Ultrasonic Seismic Modeling

Tech Area / Field

  • OBS-NAT/Natural Resources and Earth Sciences/Other Basic Sciences
  • ENV-MRA/Modelling and Risk Assessment/Environment
  • ENV-SEM/Seismic Monitoring/Environment
  • INF-SIG/Sensors and Signal Processing/Information and Communications
  • INF-SOF/Software/Information and Communications

Status
3 Approved without Funding

Registration date
15.03.2011

Leading Institute
Tbilisi State University / Geophysics Institute, Georgia, Tbilisi

Collaborators

  • Indiana University Purdue University Indianapolis / Department of Mathematical Sciences, USA, IN, Indianapolis\nFriedrich-Schiller-Universitat / Institut fur Geowissenschaften, Germany, Jena\nSaint Louis University / Department of Earth and Atmospheric Science, USA, MO, St Louis\nUniversity of Warsaw / Institute of Geophysics, Poland, Warsaw

Project summary

The Project aim. Within the Project we are planning to organize the observations on the Network of broadband stations on the perimeter of Black Sea. Data will income from the available network stations and also as the project works will advance, from new installed stations.

The main purpose of the project – using time series of surface waves recorded by broadband stations located on the perimeter of Black Sea: 1st – to construct „basic“ tomographical model of earth crust in the Black Sea based on the method of surface-wave tomography; 2nd – to determine velocity profiles for S-waves of different pairs of stations and traces, crossed the sea basin; 3rd – precise relation of epicenters with seismo active faults in the sea, and also identify major type of mechanisms of earthquakes on specifed faults; 4th – to organize monitoring of seismic behavor in earth crust by means of operative online processing of seismic information in the computer centre.

Current status in the research works. Geophysical investigations, carried out in the end of last century, were oriented on using the methods suitable for conditions of Black Sea water area. Between them the more spread was the method of seismo prospecting with using of longitudinal seismic waves (CMRW and RWM), gravimetrical and geothermal methods. Seismo prospecting was carried out only for shelf areas. Expensive works of deep seismic sounding (DSS) were carried out at several profiles above deep-water part of Black Sea. Considering specificity of carried out at the surface of water basin seismo prospecting works it was impossible to get velocities of S-waves in Earth’s crust. Determination of velocities can only be done by means of the surface seismic waves crossing basin of the sea on the various traces with implementation of the method of surface wave tomography. In tomographical calculations of 1988 year the seismic data only from 3 stations: Simferopol, Anapa, Sochi (in analog mode) were used. Records of seismic wave were digitized manually. Notwithstanding the gap in the station’s (trace’s) number and low data quality there was success in determination of low-velocity sediment layer with thickness of 2-3 km in western and eastern parts of Black Sea basin. The results of these calculations gives only smooth picture of region structure. North-West and the whole South-East of basin stayed the “white spot” in the calculations. Improving data quality and increasing the station’s number will detail the information about Earth crust structure and upper mantle in Black Sea region. The broadband stations operating for today in Russia, Turkey and Bulgaria (each country has 2 stations) send information about parameters of fixed earthquakes to the International Seismic Monitor Center IRIS. Cooperative works on data processing for solving of concrete tasks of seismology are not being carried out. The solving of this task is planned within Project.

The project’ influence on progress in this area. In case of successful realization of the works planned under the Project obtained results will have an effect on progress in the field of geophysics and, particularly, seismology.

  1. For the first time for the given region the tomographic "basic” model interpreting horizontal-heterogeneous block structure of Earth crust of region will be constructed.
  2. The high-velocity vertical profile of earth crust for S-waves will be received. At the present the data about this informative characteristic of rocks for Black Sea are missing.
  3. New stations added into the Network and installed on the perimeter of the Black Sea basin will make possible to solve such fundamental geophysical and applied tasks, as control of earth crust rock properties on the fixed traces “epicenter-station”, prognosis of oil and gas content in upper horizons of Earth crust, identification of long-term earthquake precursors, estimation of risk level of tsunami origination in active faults, estimation of risk for coastal regions of Black Sea in the case of strong earthquakes with magnitude M7 and so on.

The participants’ competence. Participants of the Project are developers of a method of the surface-wave tomography (Prof. T. B. Ianovskaya, the St.-Petersburg University) and techniques of the three-dimensional physical modeling surface-waves in horizontal- heterogeneous mediums (Institute of Geophysics SA Georgia).

Expected results and their application. On the termination, planned under the Project research-engineering and organizational works the next results will be received.

  1. The “basic” tomographical model of horizontal-inhomogeneous earth crust structure in the Black Sea basin will be constructed.
  2. Earth crust velocity profiles for different traces (“epicenter-seismic station” and “seismic station-seismic station”) will be determined with application of surface wave tomography.
  3. The relation of epicenters with seismo active faults and major type of earthquake mechanism will be explored.
  4. The estimation of seismic risk and ecological damage for eastern coast of Black Sea in the case of strong earthquakes with magnitude M7 originated in central part of sea and mechanism of type “thrust” and “slip” will be done.
  5. Organization of computer center in order to process seismic data and monitor the state of aggregation of earth crust structure at the end of Project will be realized.
  6. The features of kinematic and dynamic characteristics of surface waves in horizontal-inhomogeneous mediums will be estimated with application of 3D seismic modeling.
  7. Organized in West Georgia broadband digital seismic stations formed the triangle of observation points, will be served besides tomographical calculations for identification of possible exploration of nuclear weapons on the territory of Small Caucasus (Iran, Turkey).

Meeting the ISTC goals and objectives. In the Project the experts having knowledges in the field of various kinds of weapons are engaged; the Project has a peace orientation and corresponds to purposes of ISTC. Conformity with these purposes is reached by involving participants of the Project into world scientific community with presenting the given under Project results at the international seminars and conferences.

Scope of activities. The following activities will be implemented under the Project:

  • construction of tomographical models and determination of high-velocity profiles for different traces “epicenter-station” in the region of Black Sea;
  • cataloging of earthquakes occurred in last years with using of digital seismic records.
  • organization of additional 2 station on the coast line of Black Sea;
  • carrying out of a series of experiments on ultrasonic seismic 3D modeling, corresponding to the most typical geological structures in Earth’s crust;
  • the organization of the centre of the computer data processing incoming from a net of stations in Tbilisi;

Role of Foreign Collaborators/Partners In quality of Collaborators of the given Project have shown desire to act: Prof. Marek Grad, University of Warsaw (Poland); Doc. of Science Elizabeth Its, University of Mineopolis (State Indiana, USA). According to the letters from Collaborators they highly appreciate the content and the purposes of the Project and are ready to exchange the scientific information and if it’s necessary consult the Project managers on the project’s thematic.

Technical approach and methodology. Surface waves are the main source of information about structure of medium along the traces “epicenter - station” and they expand knowledge about body waves. Surface waves are the most sensitive to distribution of share waves’ velocities in medium, less sensitive to density and velocity of P-waves. Jointly interpretation of P and S-waves allows us to make some conclusions about physical condition of medium (partial melting, micro cracking, fluid saturated). In order to receive data for tomographical reconstruction of velocities of seismic waves in the Earth it is not enough to have only observations from seismic stations. It concerns rock and sea regions with complex geology-geographic relief where it is too difficult to use tradition methods of seismic prospecting or they are much expensive. So using for this purpose cinematic characteristics of surface waves are the main instrument.

Surface seismic waves originate due to interference of body waves: S- and P- waves. They disperse thanks to stratification of lithosphere, changing from initial impulse into oscillation train with changing time periods. Dispersion of velocity of oscillation propagation, i.e. its dependence period, is defined on the whole by thickness and velocity which characterize medium layers. If we know distribution of velocities of seismic waves with the depth, we can calculate dispersive curve for surface wave and on the contrary, - with the help of dispersive curve velocity section can be restored. In horizontal-homogeneous medium surface waves in the form of oscillation train with changing periods spread radially from the epicenter by linear trace along the Earth’s surface. Basic energy of surface wave is transferred inside the layer with the thickness of the order of wave length, and thus velocity of surface wave of fixed period is defined by averaged velocity of share (and partially longitudinal) waves in side such layer. It makes possible to restore velocity of share wave with the depth according to data about dependence of velocities of surface waves upon time period.

Depending on the period of surface waves the following problems can be solved: from 1 sec. to 5 sec. period – study of sedimentary cover; from 5 sec. to 10 sec periods – study of sedimentary basins; from 10 to 35 sec. periods – study of the Earth’s crust; more than 35 sec. periods – investigation of upper mantle.

If weak horizontal heterogeneities exist in medium, the surface wave propagates so, as if its velocity at each point were the same as in horizontal-homogeneous medium, characterized by velocity section under this point. This is the base for restoration of three-dimensional distribution of velocity according to data of surface wave dispersion. By means of two-dimensional tomography of surface wave velocities (phase or group), which were measured at various intersecting traces, two-dimensional distribution of surface wave velocity is restored for each fixed period, and correspondingly, local dispersive curves are defined. Then, at each point of the area, inversion of local dispersive curve in vertical velocity section is realized for transverse waves

Experiments on seismic modeling will be carried out on original, developed in Institute of Geophysics SA Georgia, Ministry of Education and Science of Georgia, a technique of three-dimensional ultrasonic modeling of surface waves in environments with horizontal heterogeneity. With this technique and proper selection of modeling materials, keeping the main criteria of similarity it will be possible to produce 3D models with horizontal heterogeneties, similar to the most often occurring in earth crust.

The main aim of seismomodeling experiments – to establish at what frequencies it is possible to consider model with horizontal heterogeneity as model with weak laterals variations of structure and accordingly what tomographic methods to use, and when if it is necessary to introduce corresponding corrections.


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