Just to tag onto Petra's message, having experienced many quakes myself I must say there is a dramatic difference in the experience when being outdoor vs. indoors. Back in that time I'm assuming they were outdoors and traveling by horse. Therefore it would be much much more difficult to experience magnitude 4 and less quakes when traveling and living outside. So it makes sense that they may have been experiencing the 5+ aftershocks for a couple of days only and as the quakes diminished in size it was harder for them to detect.

I've been on epicenter of 4 magnitude quakes indoors where you def. know you are having quake, and then I've gone outside only to find neighbors already outside totally oblivious. And as Petra pointed out they they prob. lost interest in keeping notes. I guess my point in this post is that IMO its difficult to figure a magnitude for these historic quakes based on settler or explorer reports. It could have been a six, maybe a seven or perhaps even a 5 that they were sitting right on top of.