I thought this might be a piece of ingenious Chinese self-consolation a.k.a. a lie, but it was true! as the WaPo confirmed in an article titled Japan says Abe's quotes about China in Post interview were 'misleading':
The existence of the Okinawa Trough complicates descriptive issues in the East China Sea. According to Professor Ji Guoxing of the Asia-Pacific Department at Shanghai Institute for International Studies,
China's interpretation of the geography is that"...the Okinawa Trough proves that the continental shelves of China and Japan are not connected, that the Trough serves as the boundary between them, and that the Trough should not be ignored ...."
Japan's interpretation of the geography is that "...the trough is just an incidental depression in a continuous continental margin between the two countries ... and that any legal effect of the trough should be ignored ...."
Enthusiasts for ambiguity will be overjoyed to learn that the truth is somewhere in between! as can be gleaned from Okinawa Trough: Origin of a Back Arc Basin, a joint study of the seismic and magnetic characteristics of the geology of the trough by US and Taiwanese oceanographers.
The rock composing the bed of the Okinawa Trough is originally continental crust, so score one for Japan!
However, the Okinawa Trough itself was formed by ongoing, violent geologic action associated with the subduction (descent) of the Pacific oceanic plate into the Ryukyu Trench on the east side of the Ryukyu Islands. The heat and ruckus associated with the melting of billions of tons of oceanic basalt bowed up the continental crust into an arc, which then collapsed into the Okinawa back arc or trough.
This action was accompanied by the shattering of the continental crust structure of the local seabed, and intrusion into the seabed of the Okinawa trough of igneous rock melted off the Pacific plate as it disappeared into the earth's crust.
more of the melted Pacific plate wells up into it.
So the Okinawa Trough represents the border of the Eurasian continental shelf and the beginning of a transition zone of violent, high-speed geologic events associated with the Pacific plate's subduction and the creation of the Ryukyu Islands out of volcanic activity, leaving the Senkakus by themselves on the continental shelf i.e. Chinese side of the equation.
But some of the Senkakus are also volcanic, presumably created by localized upwelling from the Ryukyu Trench subduction process...and some are "rocky outcroppings".
Call it a tie!
"There is no doubt that the Senkaku Islands are an inherent part of the territory of Japan in light of historical facts and based upon international law. The Senkaku Islands are under the valid control of Japan," it said.
Embarrassing for Japan, which has few claims to the Senkakus on the basis of propinquity, geology, or history, and has to rest its case based upon the acquisition of the islands during the less-than-fondly recalled extension of the Rising Sun brand into East Asia during the 1895-1945 period and the somewhat muddled circumstances of their retention when the Japanese empire was dismantled--and for the argument that China has no defensible claim on the Senkakus and therefore never attempts to advance them in international venues.
Nevertheless, historical, geographic, and geologic claims butter no parsnips when it comes to the hot-button principle of sovereignty.
Unless we're talking about their interesting geology, of course...