A ‘Black Swan’ is an event that has a very low probability of occurring but a very high impact if it does. This is the nature of major natural disasters such as Typhoons, for which Hong Kong is probably the best prepared place in the Western Pacific. Earthquakes however are another matter as they arrive with virtually no warning at all.
The risk can be determined based on earth plate tectonics, a fairly recent but proven science and there is one megathrust earthquake a 9.0+r on the Richter scale waiting to happen fairly close by. It’s that we don’t quite know when.
The Manila Trench is the second deepest place on the planet. There is a reason for this: the Philippines tectonic plate is colliding with the Eurasian plate at the Philippine Mobile Belt, the tectonic boundary between the Eurasian Plate and the Philippine Sea Plate. At that point the Eurasia plate is subducting or sliding underneath it. Every half to a millennia or so it slips and that is not good for us. The Manila Trench was classified as the highest megathrust earthquake tsunami source at the USGS (United States Geological Survey), post Indo 2003 tsunami workshop in 2006.
The inference in the papers tabled at the workshop is a major Manila Trench event every 500 – 1000 years, a blip in geological time but not so good if it comes in our lifetimes. There was a 7.8r in 1990, the last recorded big shaker, but no really big one – 8.5/9.5r – has been recorded since modern records began by the Spanish when they invaded the Philippines. That was 460 years ago…
As this is a subduction fault when it slips it will produce an upthrust hence as it is offshore it causes a tsunami. This is quite unlike say the San Andreas in California that is a lateral slip fault which slides in opposite directions with no upthrust. Lots of shaking and damage, but even when the ‘big one’ hits (est. 8.2r) it will still release only 6% of the energy generated by a 9.0 megathrust subduction quake, such as here. The one in North America to watch is the Cascadia subduction fault on the Pacific coast, which will be the really big one. Those are the ones that make mountains — or massive tsunamis.
If the prediction for the Manila Trench event is the same as the last two megathrust Pacific quakes, a 9.0-9.5. The rupture may be up to 1000 km long, but the effect will be a lot worse than the Japan/Indonesia events. This is due to massive wave reflections it will generate back and forth across the relatively shallow South China Sea as most of the tsunami’s energy will be trapped within the SCS, rather like a big lake with a bit of leakage at either end, so most of the energy can’t escape. It will be the tsunami that will keep on giving — or taking, like a classic Wimbledon volley in the finals and will subside relatively slowly as it runs out of energy.
The effects have been modeled by the Earth Observatory in Singapore. Have a look at the Manila Trench 9.0 in this link; it is scientifically as accurate as can be predicted:
Major ruptures can travel at up to 10,000 km/h, with the tsunami generated moving at 1,000 km/h in open water until it hits the shallows, where it slows down and then heaps up and causes the most damage. The 2018 Sulawesi earthquake and tsunami for example was a 7.5r event. It was offshore and due to a shallow bay nearby, a small open water tsunami reached up to 7m, killing over 4,300 people.
A peer reviewed Malaysian paper predicts a 12M + open water tsunami from a 9.0r event in the Manila Trench. When it hits the shallows of the PRD, HK harbour and Mirs Bay there is only one way it can go –up — as in Japan where it reached 40+m in height as it channeled into the bays and 30m from the 2003 Indonesian event that killed people as far away as Sri Lanka and India.
What a sight it would be as it comes through Lei Yue Mun if you were there. Regrettably it would probably be the last thing you would witness.
The same will happen in Manila Bay and Subic Bay. Shanghai gets inundated too a few hours later as the wave gets driven up the Taiwan Strait. So that all of China’s commercial centres (the wall of water could flood the entire PRD including GZ and Shenzhen) would be substantially damaged within 24 hours.
This is a risk that has now been recognised by the PRC. On 8 May 2018 the South China Sea Tsunami Advisory Centre was inaugurated in Beijing to monitor such an event probably as a response to the data from their survey ships in the SCS.
The last major tsunami that hit Hong Kong was in 1906. 10,000 people died. Although caused by a typhoon storm surge, the effects were similar to that expected if the Manila Trench becomes active.
Of course it could all end on September 9 this year, if the rock below arrives in Hong Kong, in which case don’t worry about earthquakes and tsunamis…
https://www.theregister.co.uk/2019/07/17/wasteroid_qv89_earth/
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