HYPOCENTRAL PARAMETERS AND VELOCITY ESTIMATION IN THE WESTERN SWISS ALPS BY SIMULTANEOUS INVERSION OF P-WAVE AND S-WAVE DATA

The extraction of tectonic information from seismicity data requires a ccurate hypocenter locations. Hypocenter and velocity-depth parameters may be obtained by simultaneously inverting P- and S-wave arrival tim es. The success and efficiency of such an inversion procedure depends on data quality and the suitability of the velocity model representati on. To ensure high data quality, a robust outlier detection scheme bas ed on Wadati diagrams has been adopted, and the effects of certain sys tematic errors and the inclusion of a priori constraints have been inv estigated. When the true velocity structure includes variable nu(p)/nu (s) ratios, incorporating S-wave data on the assumption of a constant nu(p)/nu(s) ratio may lead to more inaccurate hypocenter determination s than using P-wave data alone. By means of a synthetic example, the b enefits of decoupled P- and S-wave models are demonstrated. The simult aneous inversion procedure has been applied to new earthquake travel-t ime data recorded in the western Swiss Alps. The derived velocity-dept h functions provide a reasonable average of the upper crustal structur e in the investigation area, and based on accurate hypocenter informat ion, a nearly vertical active fault zone has been confirmed north of t he Rhone valley. Markedly different epicenter distribution patterns ar e observed on either side of the Rhone valley.