USING DIFFERENTIAL WAVE-FORM DATA TO RETRIEVE LOCAL S-VELOCITY STRUCTURE OR PATH-AVERAGED S-VELOCITY-GRADIENTS

An algorithm is presented which uses the surface waveforms originating from one event recorded in two stations which are located close to ea ch other. The waveforms are inverted for local laterally homogeneous S velocity structure or for path-averaged horizontal S velocity gradien ts. In the first case the stations are situated on the same great circ le to the source. In the second case the two stations lie at approxima tely the same epicentral distance, but with a slightly different azimu th. In both cases, the frequency-dependent phase differences between t he two recordings are used as data for an inversion for S velocity val ues. No a priori knowledge about the source parameters and the medium between the source and the nearest receiver is needed, These factors a re estimated in the first step of the inversion (linear). In the secon d step of the inversion (nonlinear) the S velocity information is obta ined. Synthetic tests show that the algorithm is a powerful tool to im age structure with only two fundamental mode surface wave recordings a s data. In addition, they show that for the case where a horizontal gr adient is determined, higher modes can be included in the analysis to display gradients at larger depths, without the need to identify the i ndividual modes separately. The power of the method is illustrated wit h some real data examples that are all related to a pronounced tectoni c boundary in western Eurasia: the Tornquist-Teisseyre zone. The resul ts of the experiments with real data coincide well with previously obt ained models for this area.