Petra,
You say in your post that it is hard to imagine such a large
aftershock, but right here on this board, in the far-field
aftershock forecast which was posted on June 23, 2001, it was
stated:

"Since it has been more than 50 years since the last event of
this size in the region and the area has a history of large
aftershocks, a second event of Mw>=7 is HIGHLY LIKELY within
the next 10 days within 200 km of this epicenter. The area should
be on high seismic alert throughout this time."
(emphasis added is mine).

Apparently, not only did the author of this far-field aftershock
forecast imagine such an aftershock, but published this information
two weeks ago on this board.

This should be a clue to those who do not believe it is
possible to predict large events, that under certain circumstances,
it is indeed possible.
I am sorry for the people of Peru who have had to endure these
earthquakes, but this warning was available through the internet
for the past two weeks, and local authorities should have been
prepared for a second great shock in the area.