I think you are at least partially right. The earthquake happened in March 2011, which coincides with the dramatic drop in sales. There was another big aftershock in April. A local dealer told me that summer that they could not get any more Zs for a while because production had been halted due to unavailability of parts. There was no mention of the factory being damaged, but parts suppliers for the Z were unable to meet delivery schedules. Maybe this, in combination with lack of promotion by Honda after normal production resumed, was the cause.