Complex sentence: An average of 5 cm of Plinian ashfall and coignimbrite ashfall covered Sea of Japan and northern of Japan about 1.5 million km 2, and the ash layer named "Baegdusan-Tomakomai ash" (B-Tm).
Split into simpler sentences: An average of 5 cm of Plinian ashfall and coignimbrite ashfall covered about 1.5 million km 2 of the Sea of Japan and northern Japan. This ash layer has been named the "Baegdusan-Tomakomai ash" (B-Tm).