Shallow seismic surface waves analysis across a tectonic fault

When performing a seismic survey of a shallow medium, we record wave motion which can be excited by a sledge hammer blow on the ground surface. The recorded wave motion is a complex combination of different types of waves, propagating directly from the source to the receiver, reflecting from velocity boundaries, passing through multiple layers or forming dispersive surface waves. We can use all of these wave types to identify the structure of the medium. In the presented contribution we deal with interpretation of surface waves. In contrast with body waves, the surface wave velocity is frequency-dependent. This property is called dispersion, and the dependence of the velocity on the frequency is known as the dispersion curve. The measured dispersion of the surface waves can be used to assess the structural velocity distribution in the layered medium, through which the waves propagate. We analyze surface waves recorded within the geophysical survey of the paleoseismological trench site over the Hluboka tectonic fault, Czech Republic, Central Europe. The surface waves in frequency range 15 - 70 Hz were recorded by the three component geophones with the active (sledge hammer) source. Group velocities are analyzed by the program SVAL which is based on the multiple filtering technique. It is a standard method of the Fourier transform-based frequency-time analysis. The spectrum of each record is multiplied by weighting functions centered at many discrete frequencies. Five local envelope maxima of all quasiharmonic components obtained by the inverse Fourier transform are found and their propagation times determined. These maxima are assigned to different modes of direct surface waves as well as to possible reflected, converted and multipathed modes. Filtered fundamental modes at pairs of geophones are correlated and phase velocities of surface waves are computed from the delays of propagation times of all quasiharmonic components. From the dispersion curves the shear wave velocity profiles is inverted by isometric method, which is a fast inverse algorithm that combines features of several standard methods, particularly the simplex method, Newton's least squares method and simulated annealing. These results are subsequently compared with the results of the seismic tomography, resistivity tomography and finally with the paleoseismological results from the trench.