Updating Magic Universe
Are earthquakes predictable? Frankly, no.
News that seven Italian geophysicists may face manslaughter charges for not telling people to leave their homes before last year’s earthquake in L’Aquila brings seismic forecasting into forensic focus. And it prompts me to look again at what I say about it in Magic Universe, to see if it needs updating.
You can read the news here: http://www.lifeinitaly.com/content/prosecutors-probe-experts-who-said-laquila-quake-unlikely
On 31 March 2009, the experts now under investigation said that six months of low-intensity seismic activity in the Abruzzo region did not foretell a major event. At 2.32 am on 6 April a 6.3 magnitude earthquake killed many people in their beds. With a 5.5 aftershock on 7 April, the total death toll was 308.
The most pointed remarks in the news report, from both sides of the argument, are these:
L’Aquila Mayor Massimo Cialente recalled his frustration at receiving no clear reply to his repeated questions and the apparent lack of concern on the part of some present.
“I well remember the words of Enzo Boschi who said, ‘What do you expect? An earthquake in L’Aquila is bound to happen at some point’,” said Cialente, who said he had been angered and worried by the answer.
Boschi is president of Italy’s Istituto Nazionale di Geofisica e Vulcanologia, and what he said was measured and honest. If it ever comes to trial, he and his colleagues can point out that far more deadly earthquakes of recent years weren’t predicted either: 2004 in the Indian Ocean near Sumatra (causing the Boxing Day tsunami), 2005 in Kashmir, 2008 in Sichuan, China, or 2010 in Haiti.
And they can call on expert witnesses from around the world – starting perhaps with my favourite seismologist, Hiroo Kanamori of Caltech. What people call the Richter scale of earthquake magnitudes is really the Kanamori scale nowadays. Thanks to him and his fellow geophysicists world-wide, the causes of earthquakes are well understood. But after decades of intense personal effort, Kanamori concluded in 1997 that the timing of earthquakes is probably impossible to predict.
Here’s a passage that explains his verdict, from the story in Magic Universe called “Earthquakes: Why they may never be accurately predicted, or prevented”.
Too many false alarms
As a young geophysicist, Hiroo Kanamori was one of the first in Japan to embrace the theory of plate tectonics as an explanation for geological action. He was co-author of the earliest popular book on the subject, Debate about the Earth (1970). For him, the terraces of Izu and Boso [on opposite sides of Tokyo Bay] were ample proof of an unstoppable process at work, such that the earthquake that devastated Tokyo and Yokohama in 1923, and killed 100,000 people, is certain to be repeated some day.
First at Tokyo University and then at Caltech, Kanamori devoted his career to fundamental research on earthquakes, especially the big ones. His special skill lay in extracting the fullest possible information about what happened in an earthquake, from the recordings of ground movements by seismometers lying in different directions from the scene. Kanamori developed the picture of a subducted tectonic plate pushing into the Earth with enormous force, becoming temporarily locked in its descent at its interface with the over-riding plate, and then suddenly breaking the lock.
Looking back at the records of a big earthquake in Chile in 1960, for example, he figured out that a slab of rock 800 by 200 kilometres suddenly slipped by 21 metres, past the immediately adjacent rock. He could deduce this even though the fault line was hidden deep under the surface. That, by the way, was the largest earthquake that has been recorded since seismometers were invented. Its magnitude was 9.5.
When you hear the strength of an earthquake quoted as a figure on the Richter scale, it is really Kanamori’s moment magnitude, which he introduced in 1977. He was careful to match it as closely as possible to the scale pioneered in the 1930s by Charles Richter of Caltech and others, so the old name sticks. The Kanamori scale is more directly related to the release of energy.
Despite great scientific progress, the human toll of earthquakes continued, aggravated by population growth and urbanization. In Tangshan in China in 1976, a quarter of a million died. Earthquake prediction to save lives therefore became a major goal for the experts. The most concerted efforts were in Japan, and also in California, where the coastal strip slides north-westward on the Pacific Plate along the San Andreas Fault and a swarm of related faults.
Prediction was intended to mean, not just a general declaration that a region is earthquake prone, but a practical early warning valid for the coming minutes or hours. For quite a while, it looked as if diligence and patience might give the answers.
Scatter seismometers across the land and the seabed to record even the smallest tremors. Watch for foreshocks that may precede big earthquakes. Check especially the portions of fault lines that seem to be ominously locked, without any small, stress-relieving earthquakes. The scientists pore over the seismic charts like investors trying to second-guess the stock markets.
Other possible signs of an impending earthquake include electrical changes in the rocks, and motions and tilts of the ground detectable by laser beams or navigational satellites. Alterations in water levels in wells, and leaks of radon and other gases, speak of deep cracks developing. And as a last resort, you can observe animals, which supposedly have a sixth sense about earthquakes.
Despite all their hard work, the forecasters failed to give any warning of the Kobe earthquake in Japan in 1995, which caused more than 5000 deaths. That event seemed to many experts to draw a line under 30 years of effort in prediction. Kanamori regretfully pointed out that the task might be impossible.
Micro-earthquakes, where the rock slippage or creep in a fault is measured in millimetres, rank at magnitude 2. They are imperceptible either by people or by distant seismometers. And yet, Kanamori reasoned, many of them may have the potential to grow into a very big one, ranked at magnitude 7 to 9, with slippages of metres or tens of metres over long distances.
The outcome depends on the length of the eventual crack in the rocks. Crack prediction is a notoriously difficult problem in materials science, with the uncertainties of chaos theory coming into play. In most micro-earthquakes the rupture is halted in a short distance, so the scope for false alarms is unlimited.
‘As there are 100,000 times more earthquakes of magnitude 2 than of magnitude 7, a short-term prediction is bound to be very uncertain,’ Kanamori concluded in 1997. ‘It might be useful where false alarms can be tolerated. However, in modern highly industrialized urban areas with complex lifelines, communication systems and financial networks, such uncertain predictions might damage local and global economies.’
Does this need updating? I’ve taken a look at what’s been said since Kanamori reached his downbeat conclusion. There’s an unlimited supply of novel suggestions about how to try to predict earthquakes. They range from observing snakes or toads to watching from space for tell-tale fluctuations in the density of ions in the ionosphere, 100 km or more above the ground. Various agencies and individuals claim that they have a special art.
But nothing has occurred to refute Kanamori’s fundamental point from materials science: that the rock breakage that takes you from magnitude 2 to magnitude 7 falls in the realm of chaos theory. For an analogy consider a thunderstorm. You may be fairly sure that a cumulonimbus cloud will generate lightning. You may even guess what tall trees and pointed structures are most vulnerable. But to predict when seems a rash ambition.
More to the point, perhaps, is to ask why human beings take pride in rebuilding stricken cities and continuing to live on the fault lines. San Francisco, Tokyo and Istanbul are just the most obvious of the great cities in grave danger.
N. Calder, Magic Universe, pp. 220-1, Oxford UP, 2003
H. Kanamori et al. Nature, 390, pp. 461-4, 1997