Erhard Wielandt
Institute of Geophysics, University of Stuttgart
Richard-Wagner-Strasse 44, D - 70184 Stuttgart
e-mail: ew@geophys.uni-stuttgart.de
The original text was submitted as a contribution to the IASPEI
International Handbook of Earthquake and Engineering Seismology
edited by W. H. K. Lee, H. Kanamori and P. C. Jennings
This version has been slightly updated.
A postscript version can be downloaded from
ftp.geophys.uni-stuttgart.de/pub/ew/postscripts