1. University of Tübingen, Institute of Geosciences, Sigwartstr. 10, D‐72076 Tübingen, Germany. Emails:
2. Formerly University of Tübingen, Institute of Geosciences, Sigwartstr. 10, D‐72076 Tübingen, Germany; presently Swiss Federal Institute of Technology, Institute of Geophysics, ETH‐Hönggerberg, CH‐8093 Zürich, Switzerland.