東海 釣魚臺 大陸架 海域劃界 自然延伸原則 等距線原則 中間線原則 共同開發 島嶼效力 專屬經濟海域 East China Sea Diaoyutai Senkaku Islands Continental shelf Delimitation of the maritime boundaries Principle of natural prolongation Principle of equidistance Principle of median lines Joint exploitation Provisional arrangements Legal effect of the islands Exclusive economic zone