NATO Advanced Research Workshop, Dubrovnik, September 19-21, 2007 Modifications to seismic hazard due to urban environment: increase or decrease ? Constraints from numerical and physical modelling Pierre-Yves BARD, Philippe GUEGUEN, Jean-Louis CHAZELAS, Marc KHAM and Jean-François SEMBLAT bard@obs.ujf-grenoble.fr NATO ARW, Dubrovnik, September 19-21, 2007