Complex sentence: A magnitude 5.5 earthquake occurred near El Reno on April 9,1952 at 10:29 a.m. CST (16:29 UTC), and at the time, it was the strongest earthquake ever recorded in Oklahoma.
Split into simpler sentences: A magnitude 5.5 earthquake had occurred near El Reno on April 9,1952 at 10:29 a.m. CST (16:29 UTC). At the time, it was the strongest earthquake ever recorded in Oklahoma.