JAPANESE EARTHQUAKE BACKGROUND
The earthquake hitting Japan Friday afternoon is yet another reminder that the human species, despite all the advances in science and technology, is still relatively helpless in the face of the worst that nature can do. As is now well known, Japan sits astride the edge of two slowly moving plates, and as with other Pacific rim countries in the so called ring of fire this brings the near certainty of spasmodic Earthquakes. Japan is one of the better prepared nations for such eventualities having faced numerous earthquake disasters over the last few hundred years. The problem comes when the earthquakes are sufficiently large to show up the human efforts for self protection to be puny in comparison. Even the size of a quake is not the only factor in determining risk. For example if the quake is near the surface, and the geological structure under the dwellings in inherently unstable, there is likely to be far more damage. Some types of housing eg high-rise or unreinforced brick or block is far more vulnerable than houses or buildings specifically designed for high risk areas. A substantial Tsunami is typically more destructive than the original quake for those on or near the coast.
The range of earthquake intensity is very wide, and the standard earthquake scale is logrithmic. In other words the difference between a size 3 and 4 earthquake is nowhere near the difference between say an 8 and 9. For example according to University of Tokyo geophysicist Robert Geller, the 6.3 magnitude Earthquake that struck Christchurch and severely damaged the city centre was approximately 1000 times less severe in terms of energy release than the 8.9 quake that has just struck Japan. The US Geological Survey, later corrected this figure to say that the Japan Earthquake was actually of the order of 8000 times the energy release of the Christchurch Earthquake. It should also be noted that the Christchurch quake occured in a relatively small area while the Japan Earthquake represented a sudden and severe drop of the edge of one plate underneath the adjoining plate along a length of several hundred km. (Even so local geology caused the Christchurch Earthquake to be a world record in terms of upward and sideways acceleration, so the damage was correspondingly higher that would normally be expected given the energy total for such an earthquake)
In all the area of ground that shifted with the quake was reckoned to be the equivalent of about half the area of the North Island of New Zealand.
This earthquake 130 kilometres east of Sendai and 373 kilometres North East of Tokyo has sent substantial tsunamis fanning out across the Pacific. Nearby coastal areas encountered 10 meter high waves which swept houses and cars before them. Because the energy of the quake is spread over a wider and wider area as the tsunami wave front moves further from the earthquake centre, Pacific islands and other Pacific rim countries are in less danger the further away they are. Also since the tsunami pulse rate movement is known, time is given to evacuate people in the at risk areas when the land threatened is more than a few hundred km away. For example in New Zealand, knowing the speed of the Tsunami through the Pacific, a Tsunami wave pattern was predicted to arrive the next day at 6. 41am at North Cape.
Some precautions have been helpful. For example the nuclear power plants went into immediate shut down, although given the size of the quakes even that was not enough to prevent a subsequent build up in pressure for two of the plants. Under normal circumstances back-up cooling systems would be expected to kick in, cooling the core reactor. In this instance abrupt electrical failure prevented the cooling systems continued effective use and it remains to be seen how a rather minor nuclear disaster can still be avoided at the affected plants. As might be expected some of the media reports on the nuclear power plants have not been accurate from a science point of view and for those puzzled why I am not overly concerned, if you read the article entitled “Why I am not worried about Japan’s nuclear reactors” which you would find at http://morgsatlarge-blogorific.wordpress.com you would find a detailed and well-informed article on what has happened and what the likely consequences are to be. Similarly even although there are numerous shut down mechanisms for the oil refineries one of them still caught fire.
It should be stressed that even when even the buildings designed to withstand relatively strong earthquakes apparently held up when the quake struck, in the immediate aftermath of the quake, a good number of the buildings in the coastal vicinity could be seen being swept away by the Tsunami which followed.
That the earthquake struck where it did was not a surprise because the latest seismic risk map had already identified the area as being likely to suffer a 99% chance likelihood of a 7.5 or stronger earthquake within the next 35 years. Unfortunately scientists can not yet predict how much tension will be released when an earthquake strikes or when it will happen.
The other difficulty is that when such a large quake does strike – and this was the fifth biggest since 1900, the vulnerability of nearby areas depends on such matters like the time of day, the local geography (eg where buildings and roads are situated) the shape of bays etc. Certainly some of the loss of life in the present Quake situation would have been partly caused by the large flat areas in the Sendai vicinity of the quake, which meant after the Tsunami warning, some would have had to go as much as 10km before reaching high ground. One slightly mitigating factor with the accompanying Tsunami is that in Japan the flat land tends to be used more for farming and horticulture while people often live in the hills. Another helpful factor was that earthquake engineering meant that many modern buildings were more stable than would have been the case a few years ago. At the same time, there is a limit to how much protection can be achieved by good design.
Having talked with families who witnessed the world’s biggest 9.5 quake in Chile on May 5 1960, I was told one woman had seen the ground open, houses falling into the holes and the ground closing back over them with a total of about 1600 killed and about 2,000 000 left homeless. Near the epicentre of a large quake, even a building strengthened for earthquakes is going to afford much protection against the worst that nature can produce.
The tsunamis can be even more devastating. The 9.1 Indonesian Earthquake killed more than 220 000 people and devastated large areas of coastline round the Indian Ocean. Given the low lying Islands in the Pacific and the tendancy of many people to want to live on the coast, a clear understanding of what to do in case of a Tsunami is critical.
The Japan Earthquake might also be helpful in retrospect in that it is a reminder that very careful planning is necessary for those living in the earthquake prone areas. Nuclear power is cheap and efficient but it is also vulnerable. It is also incredibly expensive to fix when it goes wrong. Gas and electricity are essential in many areas but without the earthquake triggered shut off valves (which are still the exception rather than the rule) there are serious dangers when the rare big one strikes.