Determining source parameters from waveforms of small earthquakes in the Carpathian region of Ukraine
Keywords:small earthquakes, seismic moment tensor, focal mechanism, matrix method
In the paper, a method is presented for moment tensor inversion of only direct P- and/or S-waves registered at only one station. Lesser sensitivity of direct waves, if compared to reflected and converted waves, to path effects modeling significantly improves the method’s accuracy and reliability. Choosing to invert only the direct P-waves, calculated by matrix method, instead of the full field, enables to reduce the effects of the half-space model inaccuracy, reflected and converted phases being much more distorted by it. Point-source approximation is considered, with known location and origin time. Wave propagation in the medium modelled as horizontally layered heterogeneous elastic structure is calculated by matrix method, enabling to isolate only direct waves. Based on forward modeling, a numerical technique is developed for the inversion of observed waveforms for the components of moment tensor M(t), obtained by generalized inversion. The proposed inversion method is applied to three small earthquakes from the East Carpathian region to retrieve their moment tensors from waveforms registered at only one station. The resulting focal mechanisms are compared between the stations and with determined from polarities of first arrivals. It should also be pointed out that all three mechanisms determined here by the inversion of waveforms indicate northward thrusting, which occurs in a good agreement with predominantly NNE orientation (~60°) of principal compression stresses within the region revealed by different methods as well as with main features of local tectonics. The mechanisms also are compared with focal mechanisms estimated from first P-wave polarities. A conclusion is drawn out that the method will be useful when focal mechanisms can’t be obtained by other methods, the problem typical for the regions with low seismicity and insufficient number of seismic stations.
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