Is this a run-on sentence?
According to one theory, aviator Amelia Earhart may have crash landed on a remote island in the Pacific Ocean rather than crashing into the water as many historians believe.

 - yes
 - no
This is not a run-on sentence. It is not formed from two sentences that have been run together without appropriate punctuation.
According to one theory, aviator Amelia Earhart may have crash landed on a remote island in the Pacific Ocean rather than crashing into the water as many historians believe.