Maritime Boundary Disputes in the East and South China Seas


The fundamental legal disputes in the East and South China Seas are about which state has the better claim to sovereignty over the disputed offshore islands. Such disputes are governed by the rules and principles of customary international law on the acquisition and loss of territory.1 A dispute over which state has the better claim to territorial sovereignty over offshore geographic features can only be resolved if the claimants either agree to settle the dispute through direct negotiations, or agree to refer the dispute to an international court or international arbitral tribunal.

The 1982 United Nations Convention on the Law of the Sea (UNCLOS)2 does not prescribe procedures for the determination of sovereignty. UNCLOS assumes that it is known which state has sovereignty over the land territory and islands. Instead, UNCLOS sets out what maritime zones may be claimed from land territory, as well as the rights and jurisdiction of states in such maritime zones.

UNLCLOS and Maritime Boundary Disputes

UNCLOS provides guidance on how to resolve maritime boundaries where there is an overlap of maritime zones between two states. A state is entitled to establish a territorial sea out to 12 nautical miles (M) measured from baselines along its coast. Article 15 of UNCLOS specifically provides that where the territorial seas of two states overlap, neither State is entitled to extend its territorial sea beyond the median line between the two States, unless special circumstances exist.

Under UNCLOS, every coastal state is entitled to claim an exclusive economic zone (EEZ) extending to 200 M from the baselines from which the territorial sea is measured. When the EEZs of two countries overlap, Article 74 of UNCLOS provides that delimitation shall be effected by agreement on the basis of international law to achieve an equitable solution. This provision in effect leaves it to international courts and tribunals to set out the international law governing the method of delimitation. The International Court of Justice (ICJ) has developed a method for delimiting the EEZ called the three stages approach,3 which requires (i) drawing a median line, (ii) making any adjustments to the median line based on relevant circumstances, and (iii) ensuring that the final result is equitable for both parties. The ICJ has used this method consistently, most recently in the delimitation between Peru and Chile in 2014.4

The EEZ regime gives the coastal state sovereign rights and jurisdiction to explore and exploit the natural resources of the seabed and subsoil as well as the water column. However, the continental shelf regime in Part VI of UNCLOS permits a coastal state to extend its continental shelf beyond 200 M. A coastal state can claim an ‘extended continental shelf’ only if it receives a recommendation from the Commission on the Limits of Continental Shelf (CLCS) determining the existence and extent of its outer continental shelf. Any claim to an extended continental shelf must be made in accordance with the CLCS’s recommendation.

Maritime Boundary Disputes in the East China Sea

The East China Sea is a semi-enclosed sea bordered by China to the west, South Korea to the north, Taiwan to the South and Japan to the east. Since the widest point between China and Japan in the East China Sea is only 345 M, there exists an area where each country’s EEZ projection overlaps.

Japan argues that the boundary should be the median line between the coasts of the two states. China, on the other hand, claims that it is entitled to an outer continental shelf beyond its EEZ that extends to the Okinawa Trough, located up to 277 M from its shores.5 Japan counters that it is just as entitled to the continental shelf in the East China Sea as China is because the Okinawa Trough does not constitute a break in the continental shelf (in which case would give a greater outer continental shelf to China) but is instead a continuous continental shelf between both States, meaning that the median line should be the boundary.6 This dispute could be resolved by the CLCS, which is a scientific committee that determines the existence and limits of outer continental shelf claims based on scientific evidence.

In 14 December 2012, China made a submission to the CLCS requesting a determination of its outer continental shelf in the East China Sea;7 however, Japan objected to China’s submission.8 The CLCS is unable to consider China’s submission because of Japan’s objection. Since no coastal state can unilaterally claim an outer continental shelf without the recommendation of the CLCS, the issue will remain unresolved until both China and Japan ask the CLCS to determine whether the continental shelf in the East China Sea consists of one continuous shelf or of two distinct continental shelves separated by the Okinawa Trough.

Nevertheless, such a determination regarding the continental shelf in the East China Sea may not be relevant to the delimitation of the maritime boundary between the two countries. This is because the ICJ has ruled that, where the distance between two opposing coastal States is less than 400 M, the distance factor, and not the geophysical factor, should be considered in determining the boundary.9 This means that in the East China Sea, regardless of whether China has an outer continental shelf beyond 200 M, the boundary should be based on the distance factor, which would be the median line between China and Japan.10

Some writers argue that, under UNCLOS, China is entitled to more than the median line because China is the larger country.11 This statement is inaccurate for two reasons. First, nowhere in UNCLOS is it stated that a larger country is entitled to a larger maritime zone due to its size. Second, the ICJ has stated repeatedly that the size of the land mass of a coastal state is not a factor that can justify the adjustment of the median line.12 Rather, the determining factor is the length of the relevant coast. Thus, a median line adjustment may be in order if China can show that it has a longer relevant coast in the East China Sea than Japan.

The Senkaku/Diaoyu Islands are located on Japan’s side of what would be the probable median line in the East China Sea. Land generates maritime zones, not vice versa, and the effect of these islands on the EEZ boundary will depend on whether they are entitled to an EEZ of their own. This determination would be based on the interpretation of Article 121(3) of UNCLOS, which provides that islands that cannot sustain human habitation or economic life of their own are only entitled to a 12 M territorial sea. Such an action can be brought in front of an international court or tribunal.

If it were determined that the Senkaku/ Diaoyu Islands were not capable of sustaining human habitation, these islands would only be entitled to a 12 M territorial sea, which would have a minimal impact on the maritime boundary between China and Japan. However, a determination that the islands were capable of sustaining human habitation might create a larger EEZ area for whichever State were determined to have sovereignty over the islands.

Maritime Boundary Disputes in the South China Sea

In the South China Sea, China, Malaysia, the Philippines, Taiwan, and Vietnam have sovereignty claims to some or all of the islands in the Spratly Islands, while Brunei’s EEZ projection includes one or two reefs in the Spratly Islands. China, Taiwan and Vietnam also claim sovereignty over the Paracel Islands.

All the states bordering the South China Sea claim an EEZ measured from the baselines along their mainland coasts or, in the case of the Philippines and Indonesia, from archipelagic baselines.13 It is generally agreed that the archipelagic baselines employed by Indonesia and the Philippines around their main archipelago are consistent with UNCLOS.14 Although some of the straight baselines used by China, Malaysia, Taiwan and Vietnam may be of questionable legality15 and may have an impact on the areas subject to the regimes of internal waters and territorial sea, they will have a minimal impact on the outer limit of the EEZ claims measured from those baselines. Therefore, the questionable use of straight baselines is relatively unimportant in identifying the areas of overlapping maritime claims.

None of the Claimants have issued charts or coordinates of the baselines of the islands from which they measure the territorial sea, as required by Article 16 of UNCLOS. They also have not clarified which low-tide elevations can be used as basepoints because they are located within the 12 M territorial seas of an island.16 China has issued straight baselines around the Paracel Islands (‘Xisha Islands’ in Chinese), but such baselines are not consistent with UNCLOS17 because only ‘archipelagic States’ can draw straight baselines around mid-ocean archipelagos.18

None of the States bordering the South China Sea have issued official charts or lists of geographic coordinates showing the outer limit lines of their EEZs claimed from their mainland. 19However, the outer limits of the EEZ claims of Malaysia and Vietnam are shown on the maps contained in their submissions to the CLCS, together (interestingly) with the EEZ limits of the Philippines.20

Several of the states bordering the South China Sea are adjacent to each other and agreements will be required to clarify their adjacent EEZ boundaries. Brunei is reported to have worked out their adjacent boundaries with Malaysia in an exchange of letters,21 and China and Vietnam have reached a partial boundary agreement in the Gulf of Tonkin.22 The adjacent boundary between Malaysia and the Philippines is especially challenging because of the historic sovereignty claim of the Philippines to the Malaysian State of Sabah.23

Assoc Prof Robert C. Bekman is the Director of the Centre for International Law (CIL) and the head of its programme in Ocean Law and Policy. Leonardo Bernard is a research fellow at the Centre for International Law, specialising in maritime law. This is an extract from the article “Maritime Disputes in the South and East China Seas” published by the Centre for Strategic and International Studies (CSIS) in the The Future of the Seas in East Asia: Forging a Common Maritime Future for ASEAN and Japan, pp. 27 – 32, 2015. It is republished with permission.