Although it looks like some kind of 'explosion' i doubt it was, it looks more like a collision with another ship the image alone is not great and the black makings on the water line are not so scorched as an explostion would have caused,
Infact the black makings look more like rub marks that would have been caused by a tug during normal operation of docking. its safe to assume that if there was an explosion anything that could have caused that indentation surly would have ruptured the skin of the vessel. there would have been a smaller indentation where any device was on the hull then the large one which is the shock wave ( primary blast radius, where the device was, and secondary radius, what the device caused)
currently there are several theroes going around on the media. one is such an explosion the other is now claiming a freak wave. and i quote..
''Crew members reported seeing a flash and hearing an explosion during the incident shortly after midnight local time on Wednesday.
"There are some reports saying the tanker was hit by strong waves, but it's quite unlikely," said Masahiko Hibino, Mitsui OSK safety management official.
"We have not reached a conclusion, but we still suspect the tanker was hit by a blast."
At a press conference in Tokyo, Mr Hibino displayed images of the tanker which showed damage to the railings, the loss of a life vessel, a shattered window, and broken furniture and fittings and shattered glass inside a dry cabin.
Pictures of the outside of the vessel, published separately, show an indentation several metres across in the hull, but no obvious burn marks or signs of an explosion.''
all this leads me to believe one of a few things, it was a freak wave maybe throwing something heavy up into the side of the ship, or not uncommon possible a large whale smacking against the hull this has been known to happen on occasions to other ships, although its a bit of an odd claim I'm not sure if whales migrate around those waters?
This one i think is most likely whats happened...
smuggling on an oil tanker, drop ship hits the tanker during the transfer( smuggling operation aborted due to damage) then the crew need to make up a story about what happened. Around those waters given the situation in that area smuggling might be a real possibility . either in guns and ammunition, drugs, or people trafficking/illegal immigrants.