The tsunami which hit eastern Japan was barely noticed on the Chinese seaboard, but the earthquake nevertheless set in train the disaster plans laid out by the Chinese authorities.
The National Marine Environment Forecasting Centre 国家海洋预报台 in Beijing is one of two dozen administrative agencies subordinate to the State Oceanic Administration 国家海洋局, which itself is subordinate to the Ministry of Land and Resources 国土资源部. Under the SOA’s comprehensively-named “Storm Surge, Wave, Tsunami and Sea-Ice Disaster Contingency Plan”, a level 4 blue alert was issued for the Chinese coast. It was quickly calculated that a wave up to 60cm high would reach the coast of Zhejiang and Fujian around 10pm, with a wave of 30cm expected to reach Jiangsu and Shanghai at around 2am the following morning. In fact, the wave was much smaller when it did hit.
The SOA’s Disaster Contingency Plan ranks the danger from tsunamis on a scale of 1 to 4 (extremely serious, serious, quite serious, and moderate), with colours to go with each (red, orange, yellow, blue). A red alert is issued if there is expected to be a wave of 3m above the normal tidal range, causing serious damage and a threat to life and property over 300km or more of coastline. The orange alert is sounded if the wave is 2-3m high; the yellow if it’s 1-2m high with damage expected to buildings and shipping; while the blue alert is for waves less than 1m high with minor damage expected.
Could a tsunami cause the same kind of devastation in China as it did in Japan? Good question. One thing that Zhejiang and Jiangsu have going for them is that the East China Sea is hundreds of miles broad and quite shallow (most is barely 200m deep), so tsunamis tend to dissipate their energy before they hit land. Another tick in the “good news” column is that the seabed off Jiangsu and Zhejiang is not particularly earthquake prone, and with a quake of magnitude 7 generally needed to trigger a dangerous tsunami it’s unlikely that either province will be hit any time soon. The tendency for homes to be built largely from wood, as they are in Japan, isn’t as common nowadays in China as it once was, and one would expect to see more houses survive (notwithstanding the possibility that concrete houses, if jerry-built, might collapse nonetheless).
On the down side, coastal Jiangsu is very heavily populated and as flat as a pancake, so a tsunami like Japan’s would travel many miles inland and affect hundreds of thousands or even millions. Shanghai, too, is at sea level, and a tsunami would happily submerge Chongming Island and the exposed districts of Pudong even if it didn’t propagate up the Huangpu to swamp the city itself. The Qiantang River, famous for the tidal bore created by its bottleneck, would concentrate and magnify a tsunami and aim it directly at Hangzhou, while the nuclear power station at Qinshan on the estuary would be right in the firing line.
Further south, Fujian and Canton are hilly, with coastal communities concentrated in bays and inlets which would be vulnerable to tsunamis. Also, the relative proximity of the coast to the deep water of the South China Sea and the earthquake zones around Taiwan places these provinces at a higher tsunami risk (a large quake in 1771 caused a tsunami that devastated the nearby Ryukyu Islands, though records on how it affected mainland China are absent). There are two nuclear power stations on the Canton coast, at Daya Bay and Ling’ao, both of them at risk of tsunami damage if there is ever a large quake in the Taiwan strait. If they did ever meet the same fate as Fukushima, Hong Kong and Shenzhen are only a stone’s throw away...