Complex sentence: The earthquake caused by the 2016 nuclear test was similar to the 5.1 magnitude earthquake that accompanied North Korea's previous 2013 nuclear test, which was estimated by South Korea to have a yield of 6--9 kilotons of TNT.
Split into simpler sentences: The earthquake caused by the 2016 nuclear test was similar to the 5.1 magnitude earthquake that accompanied North Korea's previous 2013 nuclear test. The 2013 nuclear test was estimated by South Korea to have a yield of 6--9 kilotons of TNT.