Isotropic and anisotropic P n velocity inversion of regional earthquake traveltimes underneath Germany

Summary This paper investigates the uppermost mantle velocity beneath Germany using regional earthquake traveltime data. 2149 P n traveltimes corresponding to 220 events recorded at 70 stations covering the region of 47°N–52°N latitude, 5°E–15°E longitude result in a satisfactory ray‐path distribution. Three methods with increasing degree of complexity are used to analyse the P n traveltime data: a straight‐line fit; the classical time‐term method; and a modified time‐term method including azimuthal anisotropy. First, from the straight‐line fit to the data set, an average P n velocity of 7.98 km s −1 is inferred. Second, the classical time‐term method yields a mean uppermost mantle velocity of 7.99 km s −1 . The most important feature in this analysis is the azimuth‐dependent pattern of the residuals, indicating some evidence of velocity anisotropy in the upper mantle. The time‐term method achieves about 55 per cent variance reduction relative to the straight‐line fit. Third, two modified ‘anisotropic’ time‐term methods provide an average P n velocity of 8.09 km s −1 , with a further data variance reduction of 64 and 20 per cent relative to the straight‐line fit and the classical time‐term method, respectively. The estimated anisotropy level is about 3.5–4 per cent, with maximum and minimum velocities of 8.24–8.27 km s −1 and 7.95 km s −1 . Our estimated maximum velocity direction of ∼N25°E coincides with those of previous anisotropic studies on the uppermost mantle in this region based on seismic refraction data. The results from the present study thus support the idea that P n ‐wave anisotropy is a large‐scale lithospheric feature over much of central Europe.