After a Chinese nuclear attack submarine was discovered by the Japanese navy while submerged near disputed islands in the East China Sea, military experts say it could be too easy to detect. 

The PLA Navy’s 110-metre Shang-class submarine surfaced in international waters with a Chinese flag on its mast on January 12 after it was followed by the Japan Maritime Self-Defence Force for two days.

Some military experts believe the vessel was forced to surface, but others say there is not enough information to back up that theory.

China’s defence ministry has not responded to inquiries from the South China Morning Post regarding the incident. 

What is known is that the submarine entered the contiguous zone less than 24 nautical miles from the contested Diaoyu Islands, which are known as the Senkakus in Japan. 

Relations between China and Japan have long been tense because of historical issues and their territorial disputes over the tiny, uninhabited archipelago – which lies between Taiwan and the southern Japanese island of Okinawa – that is controlled by Japan but also claimed by China.

It was the first time a Chinese navy submarine had come so close to the islands, leading to speculation that it was an apparent move by China to demonstrate its sovereignty claim. 

But the early and long exposure of its underwater trajectory, according to military experts, suggests the vessel is not as quiet as it should be. Japan’s defence ministry said anti-submarine ships and planes had been tracking the Chinese submarine since January 10.

China’s nuclear attack submarine has been in service since 2006, carrying out missions in the Indian Ocean and Western Pacific. Two of the submarines, type 093, were built in the 2000s, and at least two more – the upgraded type 093A – were commissioned in 2016, according to a report to the US Congress in 2017.

Japan did not say whether the submarine spotted near its waters was one of the earlier vessels or the upgrade, but experts say it was the newer type. That submarine is believed to have a vertical launch system for anti-ship YJ-18 cruise missiles, and was expected to be on par with the United States’ Los Angeles-class submarines – or at least much quieter than its notoriously noisy predecessor, the type 091 Han-class.

“This is such a shame for the navy,” said a Beijing-based military source, who requested anonymity, adding that the vessel was detected because it was “too noisy”.

The incident has also shown the strong anti-submarine capabilities of Japan, which has the technological backing of the US military, according to military commentator Zhou Chenming in Beijing.

“It’s not so bad that they’ve been exposed, it could push the Chinese to work harder on making the submarines quieter,” Zhou said. “As a strong military power China should be confident enough not to cover up its weaknesses and failures.”

It is also unusual that a nuclear submarine – which could stay underwater for months – surfaced in front of another navy, given that they usually strive to stay unseen and undetected.

“Once a submarine has been exposed and its unique acoustics have been recorded, it puts them at a great disadvantage,” said Li Jie, a researcher at the Naval Military Studies Research Institute in Beijing.

In 2004, a type 091 Han-class nuclear submarine was detected as it trespassed in Japanese territorial waters near the recent incident. But it remained submerged until it returned to Chinese waters, despite being chased by Japanese ships and planes dropping sonobuoys, which pick up underwater sounds and transmit them.

Macau-based military expert Antony Wong Dong believed the nuclear attack submarine was forced to surface, and said it was “dumb” of the Chinese navy to allow its features to be seen and photographed.

He also dismissed claims that the submarine was flying a Chinese flag to assert its claim to sovereignty over the Diaoyus, noting that it surfaced in international waters.

“If they wanted to claim sovereignty, why didn’t the submarine enter the territorial seas of the islands?” Wong said.

Flying a flag is the general practice when a submarine surfaces in international or foreign waters.

But Li said there were other possibilities to explain why it surfaced, such as the need for clearer communication, positioning or technical problems. Li also noted that the Chinese submarine did not enter Japan’s territorial waters so in tailing the vessel, the Japanese navy was actually breaching international law.

According to the Japanese defence ministry statement, a Chinese frigate was also seen on the second day near the disputed islands, but it was not clear whether the warship was still in the area when the submarine surfaced.

The Chinese navy plans to expand the fleet of nuclear attack submarines to six, according to the US Congress report, before it brings in the next generation type 095 – which it hopes will be substantially quieter when it is introduced in the 2020s.