The funny shape of the East China Sea "co-development" zone

Source: FYJS , Yomiuri

The funny shape of the co-development zone in East China Sea recently nogotiated between China and Japan has puzzled many. I have speculated some reason in the choice of location (e.g. as far from Diaoyu / Senkaku as possible, largely on the disputed zone but includes a bit of the area west of Japan claim line to show goodwill, etc.) But the shape is still mysterious.

One theory (according Yomiuri - in its report also said explicitly now that Longjing Asunaro is not within the area) is that this area is chosen so that it satisfies two conditions
1) as far north as possible (as I speculated)
2) but avoid any potential dispute with Korea (!!!)

One may ask, what does Korea have anything to do with this. The map above shows (in white outline) the area stipulated by the February 1974 agreement between (South) Korea and Japan, as their codevelopment zone. Coordiantes of the polygon corners are:

• Point 1 32°57.0'N 127°41.1'E
• Point 2 32°53.4'N 127°36.3'E
• Point 3 32°46.2'N 127°27.8'E
• Point 4 32°33.6'N 127°13.1'E
• Point 5 32°10.5'N 126°51.5'E
• Point 6 30°46.2'N 125°55.5'E
• Point 7 30°33.3'N 126°00.8'E
• Point 8 30°18.2'N 126°05.5'E
• Point 9 28°36.0'N 127°38.0'E
• Point 10 29°19.0'N 128°00.0'E
• Point 11 29°43.0'N 128°38.0'E
• Point 12 30°19.0'N 129°09.0'E
• Point 13 30°54.0'N 129°04.0'E
• Point 14 31°13.0'N 128°50.0'E
• Point 15 31°47.0'N 128°50.0'E
• Point 16 31°47.0'N 128°14.0'E
• Point 17 32°12.0'N 127°50.0'E
• Point 18 32°27.0'N 127°56.0'E
• Point 19 32°27.0'N 128°18.0'E
• Point 20 32°57.0'N 128°18.0'E
• Point 1 32°57.0'N 127°41.1'E

The problem is, in 1974, China was still a closed country, far behind the bamboo curtain. This area overlaps with the Chinese claimed line (based on the Ocean trough, and 200 nautical miles -- shown in the map above). To make real progress and not complicate the problem it thus makes sense to defer any discussion that may involve another new interest party (which is also highly nationalistic).

• update: according to Mark Valencia's study, China had protested loudly in 1974.
• "On February 4, 1974, China protested loudly:
• 
  The Chinese Government holds that, according to the principle
  that the continental shelf is the natural extension of the continent,
  it stands to reason that the question of how to divide the continental
  shelf in the East China Sea should be decided by China and the
  other countries concerned through consultations. But now the
  Japanese Government and the South Korean authorities have
  marked off a so-called . . . “joint development zone” . . . behind
  China’s back. This is an infringement on China’s sovereignty"

p.s. It is a bit puzzling for me to understand why Japan had agreed to such a zone, as the south portion of the polygon extends far south of Cheju Island (or even Huang yan reef, which is the weaker version of 'okinotori' for Korea). It actually extended south of Kyushu and as far south as the latitude of Okinawa.

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传中日顾虑韩国因素 放弃开发龙井油田

【星岛网讯】中日双方日前正式就共同开发东海油气田问题达成共识，有消息称，由于顾虑韩国的关系，双方未把龙井（日本取名翌桧）油田列为共同开发的对象。只就龙井油田南侧海域的达成共识。　　

日本《读卖新闻》报道，中日双方在东海油田开发谈判中，把龙井油田排除在共同开发的对象之外，双方也将不单独开发，事实上是暂时放弃对这个油田的开发。　

报道指，中日两国由于分别顾虑到龙井靠近中国和韩国的中线海域以及接近日韩大陆架共同开发区，若进行开发有可能和韩国发生冲突而避开对这个海上油田的开发。　　

日本政府18日发表和中国达成的协议，只对龙井油田的南侧海域进行共同开发。日本在当天并未明示理由，只表示和中国的谈判结果是如此。　　

日本和中国的谈判中曾要求在跨越日本主张的日中东海中线一带的龙井（翌桧）、春晓（日本名白桦）、断桥（楠）和天外天（木坚）四个油田共同开发，结果只就共同开发龙井南侧海域以及日方出资参加中国已着手开发的春晓达成协议。　　

日本主张以日中两国海岸等距离的中线为两国的东海分界线，中国则坚持大陆架所及的琉球海沟是双方的分界线，成为谈判的最大争议点。这次以将争议点搁置方式达成共同开发的协议，中国并重申未承认日本主张的中线。---

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日中、東シナ海ガス田「翌檜」の開発断念…韓国に配慮し

Note Yumiuri had a strange (and wrong) label of Sino-Korea median line。 (1) a China-Korea line would run North-South wise instead of East-West wise, (2) there is no median line agreed at between China and Korea. what is relevant is the 200 nautical line as indicated in the first map of this post.

　日中の東シナ海ガス田協議で、翌檜(あすなろ)（中国名・龍井）を共同開発の対象としないことで合意していたことが２０日、分かった。
　日中ともに単独開発も行わず、翌檜は事実上放棄される。翌檜は〈１〉中国と韓国の境界の基準となる「中間線」〈２〉日韓大陸棚共同開発区域――に近接しており、開発すれば韓国と摩擦を生じかねないと判断、韓国に配慮した。
　日中交渉筋によると、両政府は翌檜の共同開発を一時検討。しかし、ガスを含む地層が、中韓の「中間線」をまたぎ日韓大陸棚共同開発区域に広がっている可能性があることがわかった。開発すれば、韓国が「資源を吸い取られる」と主張する懸念があった。一連の協議には韓国は加わっておらず、日中両政府は、翌檜の開発断念で一致した。
　政府が１８日に正式発表した東シナ海ガス田開発に関する日中合意では、翌檜は共同開発の対象から外され、翌檜南側の海域が共同開発の対象となった。理由について、政府は「交渉の結果としか言えない」と説明していた。
　東シナ海で日本が中国に共同開発を求めた４ガス田のうち、白樺（中国名・春暁）は日本が出資する形の共同開発で合意した。
（2008年6月20日14時43分 読売新聞）

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Another Yomiuri East China Sea Map for Martin J. Frid… Wait, It Gets Better