Global Love wave overtone measurements

Love wave phase velocities for fundamental and higher models are difficult to measure because the different modes cannot easily be separated. Following Yoshizawa and Kennett (2002), we generate suites of path specific one-dimensional shear wave velocity profiles using the Neighbourhood Algorithm of Sambridge (1999a). From this family of O(10⁴) models both fundamental and higher mode phase velocities with mutually consistent uncertainties are calculated. We have fully automated the method and analysed over forty thousand Love wave seismograms from the GDSN and GEOSCOPE global networks from 1994-2004. Our phase velocity measurements agree remarkably well with previous studies, but we have been able to enlarge the available dataset dramatically. We present global Love wave phase velocity maps (up to the fifth overtone) with unprecedented resolution due to the improved path coverage. Comparing these maps to existing tomographic models, we discern evidence of significant anisotropy in the lower mantle around a depth of 1000 km in the Pacific.