1. Department of Mathematics and Computer Science of the University of Łódż, Banacha 22, Łódż 90-238, Poland
2. Gran Sasso Science Institute, Crispi 7, 67100 L’Aquila, Italy
3. Institute of Earthquake Prediction Theory and Mathematical Geophysics RAS, Profsoyuznaya 84/32, 117997 Moscow, Russia