Here is Shinji Toda’s calculation of the Coulomb stress imparted by the mainshock ruptures to the surrounding crust as a result of the combined M=6 and M=7 shocks. Regions in which strike-slip faults are brought closer to failure are red (the ‘trigger zones’); regions now inhibited from failure are blue (the ‘stress shadows’). Most of the aftershocks (the translucent green dots) lie in regions brought closer to failure, from which we infer that further strike-slip mainshocks are possible in the red lobes.