1. a Centro de Matemática da Universidade do Porto , Rua do Campo Alegre 687 , 4169-007 Porto , Portugal
2. b Faculdade de Economia, Universidade do Porto , Rua Dr. Roberto Frias, 4200-464 Porto , Portugal
3. c CNRS , Laboratoire Cassiopée , Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4 , France
4. d International Institute of Earthquake Prediction Theory and Mathematical Geophysics , 84/32 Profsoyuznaya St, 117997 Moscow , Russian Federation