Complex sentence: In January 2016, North Korea claimed to have successfully tested a hydrogen bomb though only a magnitude 5.1 seismic event was detected at the time of the test, a similar magnitude to the 2013 test of a 6-9 KT atomic bomb.
Split into simpler sentences: In January 2016, North Korea claimed to have successfully tested a hydrogen bomb. This was notably of similar magnitude to the 2013 test of a 6-9 KT atomic bomb.