1. R&D Seismology and Acoustics, Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730 AE De Bilt, The Netherlands, assink@knmi.nl
2. Faculty of Civil Engineering and Geosciences, Department of Geoscience and Engineering, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands
3. Also at Faculty of Civil Engineering and Geosciences, Department of Geoscience and Engineering, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands.