Earthquake and its defiance of predictability

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Syed Fattahul Alim
Bangladesh is very ill-prepared against any major earthquake in case one strikes the land with its potentially devastating fury. Historical records show that in the past major earthquakes struck this country from time to time.
The highlands like the 'Madhupur Gar'near Dhaka was created in the wake of a big earthquake that shook the country in 1762. The quake also created some lowland such as 'Haors'in Sylhet. The Teesta river, on the other hand, changed its course as an outcome of the 1787's earthquake. More than a hundred years later in 1891, a major earthquake destroyed 75, 000 kilometres of Khasia hills, which is now the present-day Meghalaya of India.
Another major earthquake in 1896 changed the course of the Brahmaputra River. In recent times, too, some smaller quakes rocked different parts of the country.
Although Bangladesh is extremely vulnerable to seismic activities, the nature and the level of this activity is yet to be defined. In Bangladesh complete earthquake monitoring facilities are not available. Accurate historical information on earthquakes is very important in evaluating the seismicity of Bangladesh in close coincidences with the geotectonic elements. Information on earthquakes in and around Bangladesh is available for the last 250 years. The earthquake record suggests that since 1900 more than 100 moderate to large earthquakes occurred in Bangladesh, out of which more than 65 events occurred after 1960. This brings to light an increased frequency of earthquakes in the last 30 years. This increase in earthquake activity is an indication of fresh tectonic activity or propagation of fractures from the adjacent seismic zones.
Very recently, on July 26, 2008, a 5.6 degree quake rocked Dhaka, and other parts of the country. A minor earthquake on August 23 2008 measuring 3.0 on Richter scale hit Chittagong and Rangamati creating panic in the districts. Earlier, on 7 November 2007, a 6.0 degree quake shook the South-eastern region with its centre at Roninpara, about 70km from Chittagon. It cracked open a fault in the hills of Chittagong and Khagrachhari districts.
The series of earthquakes shows that the country, the seventh most populated country on earth with its very high density of population, is dangerously exposed big seismic event anytime in the future. If these are a precursor of more powerful quakes in the days ahead, then Bangladesh must be prepared for a calamity of unprecedented scale with its colossal human and material cost.
But why do the earth's crust moves and shakes at all with such fatal consequence for the human settlements?
Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicentre refers to the point at the ground level directly above the hypocenter or focus.
It is at the epicentre where the shock of the earthquake is first experienced.
Most of the destructive earthquakes originate within two well-defined zones or belts namely, the circum-Pacific belt and the Mediterranean-Himalayan seismic belt.
The most disappointing aspect of earthquake is that even modern science cannot predict earthquakes. There are some rules of thumb, which are used to alert people of any possible shocks, but those, too, are not adequate for taking advanced measures to evacuate people from the quake-hazard-prone areas.
But recently some advance in this field is being claimed to have been made. Though tentative, Kenneth Chang of the New York Times reports on the following scientific researches on the predictability of the earthquakes.
'Almost all earthquakes are small. A small segment of a fault, miles underground, jerks a little, the rumble imperceptible at the surface. But with a few quakes, the fault continues breaking, the ground jumps several feet and the world shakes in cataclysm.
How does a rupture go from an inch a year to 3,000 miles per hour in a few seconds?" asked Ross S. Stein, a geophysicist at the United States Geological Survey. No one knows.
This gap in knowledge makes earthquake prediction a frustrating and chancy exercise, and complicates the effort to calculate the risk that a human construction like a water reservoir or a geothermal power plant could inadvertently set off a deadly quake.
Last month, Giampaolo Giuliani, a technician who works on a neutrino experiment at the Gran Sasso National Laboratory in Italy, issued an urgent warning that a large earthquake was about to strike the Abruzzo region. The prediction was based on measurements he had made of high levels of radon gas, presumably released from rocks that were being ground up by the stresses of an incipient quake.
On April 6, a magnitude 6.3 earthquake hit L'Aquila in central Italy, killing almost 300 people. Mr. Giuliani claimed vindication for his prediction, which had been discounted by officials.
But earthquake experts like Dr. Stein are sceptical. Scientists studied radon as a possible earthquake warning signal as far back as the 1970s, and while they found convincing cases of radon releases before some earthquakes - for example, levels of radon in groundwater were 10 times normal before the earthquake that hit Kobe, Japan, in 1995 - the correlations were not strong enough or clear enough for useful predictions.
One instance of confusing radon signals occurred in 1979. Two detectors in Southern California, 20 miles apart, measured unusually high levels of radon beginning in the summer. The radon levels then decreased in October, shortly before three earthquakes struck.
One earthquake, of magnitude 6.6, occurred 180 miles to the southeast, and the two smaller ones, of magnitudes 4.1 and 4.2, were 40 miles away. In addition, a radon detector close to one of the smaller quakes did not observe high radon levels, although it did observe a radon drop a few days earlier.
That left scientists puzzled about how they could construct a prediction out of the rising and falling radon levels. Data on other gases like carbon dioxide and on electromagnetic emissions that have sometimes been detected before earthquakes are also confusing.
"You can't hang your hat on it unless it's a reliable precursor and it happens before most earthquakes and it doesn't happen at other times," said Susan Hough, a seismologist at the geological survey.
To complicate matters, Mr. Giuliani's prediction was off in time and place. He had predicted that the quake would hit a week earlier in a town 30 miles away. Had officials acted on his prediction, said Richard M. Allen, a professor of geophysics at the University of California, Berkeley, "you would have evacuated the wrong town and you would have evacuated the wrong town at the wrong time."
While prediction remains elusive, scientists have learned that human activity can set off an earthquake. In December 2006, a geothermal energy project in Basel, Switzerland, started injecting water three miles into the ground. Some tiny tremors were expected, but the water was shut off when one of the quakes reached a still minor magnitude of 2.7. A few hours later, a larger quake, at magnitude 3.4, shook Basel, causing minor damage to buildings.
A couple of months later, there were two more magnitude 3 earthquakes. Researchers at the Swiss Federal Institute of Technology in Zurich calculate that the area will experience a slightly greater number of small earthquakes over the next 20 to 40 years as a result of the brief geothermal project, which remains halted.
The worry is that one of these small earthquakes could cascade into a big earthquake like the one that badly damaged Basel in 1356. Conversely, the small earthquakes could instead be relieving stress along a fault, reducing the likelihood of a larger quake.
"With the current knowledge, I can't really tell you," said Jochen Woessner, one of the Swiss scientists.
Geologists do not know how the pieces of the Earth's crust that usually squeeze together tightly with high friction slip past each other smoothly during a large earthquake, as if sandpaper suddenly changed to Teflon. "It looks like friction is more a complicated beast than anyone would have imagined," Dr. Stein said.
A core dug up from the San Andreas fault in California revealed the presence of talc, which could be acting as a lubricant during an earthquake. But from one core, scientists cannot tell whether this is typical of rocks around earthquake faults.
At a meeting of the Seismological Society of America last week in Monterey, Calif., a lively debate continued about whether big earthquakes are fundamentally different from small earthquakes or whether a big earthquake is just a small earthquake that did not stop. If big earthquakes are different, then it might be possible to detect them in the first few seconds of seismic waves and send out a warning. People would not have time to evacuate, but they might have enough time before the heaviest shaking to move to a safer location in a doorway or under a desk.
Reservoirs are also believed to induce some earthquakes. Most seismologists believe that a magnitude 6.5 earthquake in India in 1967 that killed about 200 people was set off by the weight of water in a reservoir that had been filled a few years earlier. A reservoir cannot generate an earthquake by itself, but it can act as a trigger to release accumulated tectonic stresses and hasten an earthquake by years or centuries.
More controversial is the assertion by some scientists that a magnitude 7.9 earthquake in Sichuan province in China last year that killed about 80,000 people was set off by the 320 million tons of water in a nearby reservoir.
Leonardo Seeber, a research scientist at the Lamont-Doherty Earth Observatory at Columbia University, is not sure about the Sichuan earthquake, but he believes that scientists and officials need to take more account of the risk of induced earthquakes.
For example, Dr. Seeber wonders whether a swarm of magnitude 4 earthquakes a couple of weeks ago around the Salton Sea in Southern California, close to one end of the San Andreas, might have been caused in part by a nearby geothermal power plant.
Extraction of oil from the ground may have set off other earthquakes, Dr. Seeber said. In the coming years, the proposed strategy to reduce global warming by capturing carbon dioxide from power plants and pumping it into the ground could create new earthquake risks.
So far, experiments in this kind of carbon sequestration have focused on whether it will work to keep carbon dioxide out of the air for centuries. But Dr. Seeber said this technology "has huge implications for triggering earthquakes'."