Inversion of arrival-times in a region of dilatancy anisotropy

Shear-wave splitting has been identified in many three-component seismograms from two separate field experiments on a section of the North Anatolian Fault in North-West Turkey. These observations are consistent with shear-wave propagation through a zone of extensive-dilitancy anisotropy. A preliminary attempt has been made to confirm this interpretation by simultaneously inverting suites of arrival-times for hypocentral locations and for parameters describing an anisotropic halfspace. Although the inversion procedure is not globally convergent, it is possible to recognize the true solution by systematically varying the initial conditions. Applied to selected data sets, the inversion defines several anisotropic models that fit the data significantly better than a simple isotropic model, and display the anisotropy required by the shear-wave splitting. However, most of these anisotropic models are not superior when they are used to individually locate events in a much larger data set. However, for each experiment, there is a single model that produces clearly superior locations for the larger data sets than those of other anisotropic or simple isotropic models. Both models display similar velocity variations which are characteristic of propagation through distributions of biplanar cracks displaying orthorhombic symmetry. The principal axes of the two models are oriented in similar directions and are within 20° of the principal axis of regional stress derived from fault-plane solutions. The solutions indicate low velocities close to the tensional axis, as would be expected in extensive-dilatancy anisotropy.