I tried to look at the on-line seismograms for several Colorado events and quickly
discovered that they do not generally contain enough detail to determine first
motion, at least for these events. The USGS provides a page of arrival time data
(see link below). All the arrivals from the larger event were emergent eP or eS.
This means there were no sharp arrivals, but they were smeared out. This is rather
odd, as usually a number of the first arrivals are sharp. It is almost like the
source was muffled somehow. Just tends to raise more questions than it answers.

• Arrival time data for largest Colorado earthquake