Abstract
The extraction of tectonic information from seismicity data requires accurate hypocenter locations. Hypocenter and velocity-depth parameters may be obtained by simultaneously inverting P- and S-wave arrival times. The success and efficiency of such an inversion procedure depends on data quality and the suitability of the velocity model representation. To ensure high data quality, a robust outlier detection scheme based on Wadati diagrams has been adopted, and the effects of certain systematic errors and the inclusion of a priori constraints have been investigated. When the true velocity structure includes variable 
p/s ratios, incorporating S-wave data on the assumption of a constant p/s ratio may lead to more inaccurate hypocenter determinations than using P-wave data alone. By means of a synthetic example, the benefits of decoupled P- and S-wave models are demonstrated. The simultaneous inversion procedure has been applied to new earthquake travel-time data recorded in the western Swiss Alps. The derived velocity-depth functions provide a reasonable average of the upper crustal structure in the investigation area, and based on accurate hypocenter information, a nearly vertical active fault zone has been confirmed north of the Rhone valley. Markedly different epicenter distribution patterns are observed on either side of the Rhone valley.