With all the significant aftershocks after the 8.8 Chile aftershock, the accumulation of more data, and more analysis, the USGS poster has already changed.
But in my desire to present the different pieces of the earlier version of the poster, below is the fault plane solution (aka focal mechanism solution):
The solution above is that of a nearly north-south striking fault. If the fault plane is dipping to the east, it is shallow; if it is dipping to the west, it is steep. Either would be consistent with the first motion analysis,but only the former makes more sense for the tectonic setting: the subduction of the Nazca Plate beneath the South Americn plate.
Arild Andresen of the University of Oslo has a very nice online tutorial on fault plane solutions.