Complex sentence: The catalog of items included 200 shocks of magnitude 3.0 or greater and four shocks of magnitude 5.0 or greater and was mostly complete, except for the first hour of activity, when the larger aftershocks were overshadowing the smaller events.
Split into simpler sentences: The catalog of items included 200 shocks of magnitude 3.0 or greater and four shocks of magnitude 5.0 or greater and was mostly complete. During the first hour of activity, the larger aftershocks were overshadowing the smaller events.