Geologist is crying wolf on earthquake threat facing HK

PUBLISHED : Thursday, 09 February, 2012, 12:00am
UPDATED : Thursday, 09 February, 2012, 12:00am

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Next time you go the beach, try a small experiment. Choose a flattish patch of damp sand below the high-tide mark but above the level of the sea. Then tap gently on the surface of the sand with the sole of your foot.

In an instant, the modest vibration of your tapping will be enough to turn what appeared to be firm, solid sand into a thick gloopy liquid that your foot will easily sink into.

What you will have just witnessed is a phenomenon known as liquefaction, in which a shock causes an apparently solid substrate to lose all its shear strength and turn into a liquid.

Liquefaction doesn't just happen on the beach. It also takes place on a much larger scale, for example when the vibrations from an earthquake hit land reclaimed from the sea.

If the shockwaves from a sizable earthquake were ever to hit Hong Kong, much of the land reclaimed from the sea in recent decades would turn into a similar gloop. That means the ground under Exchange Square and the IFC, as well as the Admiralty site on which the Hong Kong government's new headquarters is built, would instantly dissolve into the harbour.

No great loss, you might think. But many of Hong Kong's residential towers are also built on reclaimed land, including much of Tseung Kwan O, that would be similarly vulnerable.

The damage inflicted by an earthquake wouldn't stop at liquefaction. A biggish one would trigger a spate of landslides on the city's crowded hillsides, many worse than the 1972 Po Shan Road slip in Mid-Levels, which destroyed a 12-storey apartment block, killing 67 people.

And not even buildings on solid, flat ground would be safe. Of the thousands of residential blocks built between the 1950s and 1980s, many were thrown up hurriedly using substandard materials. Some used concrete mixed using seawater, which greatly reduces its strength. And none was built to withstand seismic shocks. As the 2010 collapse of a 55-year-old block in To Kwa Wan demonstrated, some of these old buildings are already crumbling. As a result, the devastation caused by an earthquake would be enormous.

According to Ian McFeat, a consultant engineering geologist with three decades of experience in Hong Kong, 'the risk is intolerable'.

McFeat points out that Hong Kong sits astride the Linhuashan fault zone, where small earthquakes with a magnitude of 3 or less, are a common occurrence. Given that a magnitude 5.7 earthquake hit Dangan Island, some 30 kilometres offshore in the Changle-Nanao fault zone in 1874, McFeat maintains that the risk of a large earthquake striking Hong Kong in the near future is far greater than commonly believed.

What's more, he also argues that Hong Kong is in real danger of being swamped by a tsunami generated by an earthquake in the Manila Trench subduction zone, which runs parallel to the northwestern shores of the Philippines, just across the South China Sea. 'It could happen tomorrow,' McFeat warns. 'The effects will be catastrophic.'

But while McFeat is certainly right about the damage a large earthquake would inflict on the city, there are good reasons to think that the probability of a big quake hitting Hong Kong may not be as great as he believes.

Of the two fault zones that run almost beneath our feet, the Linhuashan fault was formed in the Jurassic era, and has seen no major seismic activity for the last 100 million years. The Changle-Nanao zone is even older. Sure, tiny earthquakes are relatively frequent. But that is an encouraging sign that any seismic stresses are being dissipated, and are not building up to a big earthquake.

According to a 2008 study by seismologists from Ove Arup and the University of Hong Kong, the probability of a 'moderate' earthquake greater than magnitude 6 occurring within 100 kilometres of Hong Kong over the next 50 years is less than 10 per cent. They reckoned the chance of a 'large' quake greater than magnitude 7 to be just 1.5 per cent.

Nor is a tsunami a pressing concern. The biggest earthquake ever recorded along the Manila Trench was measured at magnitude 7.4 in 1934, and the seismologists at Arup put the 'maximum credible' shallow-crust earthquake - the ones that do the most damage - at magnitude 8.

That could cause a tsunami. But don't worry too much. The 2006 magnitude 7.1 Hengchun earthquake on the northern extension of the Manila Trench south of Taiwan did indeed trigger a tsunami, which reached Hong Kong. It was 11cm high.

That doesn't mean McFeat is wrong to sound the alarm. The chance of a major earthquake affecting Hong Kong may be low, but our urban geography and poor building standards mean the damage inflicted if one did hit would be huge.

But this is a long-term problem. When you go down to the beach to do your liquefaction experiment, you won't be swept away by a tsunami.