Complex sentence: The Phoenix Theater is an all-ages club located in Petaluma, California, USA, which has been in existence since 1905, and has been subject to many changes in both structure and purpose, mostly due to severe damage caused by several fires.
Split into simpler sentences: The Phoenix Theater is an all-ages club located in Petaluma, California which has been in existence since 1905. The club has been ever changing ever since in both structure and purpose, mostly due to severe damage caused by several fires.