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

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 degreesN-52 d egreesN latitude, 5 degreesE-15 degreesE 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 class ical 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 feat ure in this analysis is the azimuth-dependent pattern of the residuals, ind icating some evidence of velocity anisotropy in the upper mantle. The time- term method achieves about 55 per cent variance reduction relative to the s traight-line fit. Third, two modified 'anisotropic' time-term methods provi de an average P-n velocity of 8.09 km s(-1), with a further data variance r eduction of 64 and 20 per cent relative to the straight-line fit and the cl assical time-term method, respectively. The estimated anisotropy level is a bout 3.5-4 per cent, with maximum and minimum velocities of 8.24-8.27 kin s (-1) and 7.95 km s(-1). Our estimated maximum velocity direction of similar to N25 degreesE coincides with those of previous anisotropic studies on th e uppermost mantle in this region based on seismic refraction data. The res ults from the present study thus support the idea that P-n-wave anisotropy is a large-scale lithospheric feature over much of central Europe.