What is this long mark undersea
It looks like a submarine canyon incised into the seafloor and shelf by repeated sediment &quot;flushing&quot;. Given the location, earthquakes probably flush huge quantities of sediments from the low angle plains to the north, through the canyons, and into the abyssal plain to the south. You might find these canyons in any tectonically active seafloor that sees heavy sediment loading from, say, a mountainous tropical region. This one's not that big, compared to New Zealand's Hikurangi Channel, at nearly 700 km.
There's another one off the SE coast of Hokkaido Island. What, precisely, locally controls the formation of these canyons is an interesting question.