1. Institute of Rock Structure and Mechanics of The Czech Academy of Sciences, v. v. i., V Holesovickach 94/41, 182 09 Prague 8, Czech Republic, wcislo@irsm.cas.cz, leo@irsm.cas.cz, malek@irsm.cas.cz, vlcek@irsm.cas.cz
2. Also at Charles University in Prague, Faculty of Mathematics and Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic.
3. Charles University in Prague, Faculty of Science, Albertov 6, 128 43 Prague 2, Czech Republic, fischer@natur.cuni.cz, gunther.kletetschka@natur.cuni.cz
4. Also at Charles University in Prague, Faculty of Science, Albertov 6, 128 43 Prague 2, Czech Republic.