DETERMINING THE FOCAL MECHANISM OF AN EARTHQUAKE IN THE TRANSCARPATHIAN REGION OF UKRAINE

D. Malytskyi; O. Muyla; A. Pavlova; O. Hrytsaj

Authors

D. Malytskyi Subbotin Institute of Geophysics NAS of Ukraine 3-b Naukova Str., Lviv, 79060 Ukraine
O. Muyla Subbotin Institute of Geophysics NAS of Ukraine 3-b Naukova Str., Lviv, 79060 Ukraine
A. Pavlova Subbotin Institute of Geophysics NAS of Ukraine 3-b Naukova Str., Lviv, 79060 Ukraine
O. Hrytsaj Subbotin Institute of Geophysics NAS of Ukraine 3-b Naukova Str., Lviv, 79060 Ukraine

Keywords:

Abstract

In the paper, the theory of seismic wave propagation in anisotropic media is presented, with the use of the matrix method when determining the mechanisms of local earthquakes. These issues are of importance in seismic studies in the Carpathian region where the number of seismic stations is insufficient. The work is aimed at the development of a methodology for calculating displacements on the surface of anisotropic medium and determining the mechanisms of local earthquakes with the use of the graphic method. The purpose of the graphic method consists in making it possible to use seismic records at the stations with indistinct polarities of first P-wave motions. The ratio between the amplitudes of direct P- and S-waves is used as an auxiliary parameter. The research results are illustrated with examples of using the records of 04.04.2013 event near Nyzhnye Selyshche (φ=48,1977, λ=23,4663, h=1,73 km, ML=2). Comparative analysis is carried out of seismograms calculated with the matrix method and those recorded at a seismic station, which confirms the effectiveness of the methodology for determining the seismic source parameters. Based on the graphic method, spectral and geometric parameters of the seismic source have been obtained: seismic moment, radius of shear dislocation, slip area, mean fault slip, stress drop, energy and magnitude. Scientific novelty of the work consists in developing a method of calculating the displacement field in anisotropic media with the use of the matrix method and in amending the graphic method so as to ensure determining the mechanisms of local earthquakes in the Carpathian region where the number of seismic stations is limited. Practical application of the work is in determining the source parameters of local earthquakes, based on the developed approaches, which is of crucial importance in local seismological studies. The version of the matrix method developed in this work for calculating seismic wave propagation in anisotropic media can be used for determining the tensor of seismic moment with the number of seismic stations being limited.

References

Aki K., Richards P.G., (2002). Quantitative Seismology, 2nd edition. University Science Books.

Baumbach M., Bormann P., (2011). Determination of source parameters from seismic spectra. Potsdam.

Crampin S., (1977). A review of the effects of anisotropic layering on the propagation of seismic waves. Geophys. J. R. astr, Soc., 49, 9-21.

Crampin S., (1981). A review of wave motion in anisotropic and cracked elastic media. Wave Motion, 3, 343-391.

Cronin V., (2004). A Draft Primer on Focal Mechanism Solutions for Geologists. Baylor University.

Fryer G.J., Frazer L.N., (1984). Seismic waves in stratified anisotropic media. Geophys J. R. and Soc. 78, 691-710.

Fryer G.J., Frazer L.N., (1987). Seismic waves in stratified anisotropic media.-II. Elastodynamic eigensolutions for some anisotropic systems. Geophys J. R. and Soc., 73-101.

Hardebeck J.L., Shearer P.M., (2003). Using S/P Amplitude Ratios to Constrain the Focal Mechanisms of Small Earthquakes. Bulletin of the Seismological Society of America, 93, 6, 2434-2444.

Keith C.M., Crampin. S., (1977a). Seismic body waves in anisotropic media: propagation through a layer. Geophys. J. R. astr. Soc., 49, 200-223.

Malytskyy D., Hrytsai O., Muyla O., (2013). The features of the construction of the focal mechanisms of local earthquakes using the example of an event near Beregovo 23.11.2006. Visnyk KNU, Geologiya, 60, 37-42 (in Ukrainian).