The 39,910-ton vessel, the “Crimson Polaris,” was carrying wood chips when it ran aground on Wednesday morning in Hachinohe harbor.
It managed to free itself, but due to poor weather was unable to move far and ended up anchoring about 4 kilometers (2.4 miles) out from the port.
The vessel, which was at the end of a voyage from Thailand, split in two early on Thursday, the Coast Guard said.
An oil slick 5.1 kilometers long by about 1 kilometer wide was visible later in the day, but containment measures were being taken by patrol boats.
The vessel’s two parts have not moved and are being closely monitored by patrol boats, and by late afternoon there had been no apparent major change in its situation, a Coast Guard spokesman said.