Experimental study of the anisotropy of longitudinal and transverse waves from local earthquake records

The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δts1-s2) between the arrival times of shear waves with different polarizations. The velocities of P-waves vary with azimuth from 5.3 to 6.27 km s-1 and the velocities of S-waves vary from 3.15 to 3.5 km s-1. The delay times ΔtS1 — S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S1 and S2 propagating at different velocities, and the character of the distribution of ΔtS1 — S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.