Thursday,August 20, 1998
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Thursday,August 20, 1998 |
What makes earthquakes stop? Science notebook |
What makes earthquakes stop? SEISMIC Waves released by some earthquakes can release so much energy that the Earth actually rings like a bell. The scale of such events is matched by the devastation they cause. Only recently, 4,000 people died in the earthquake that hit Afghanistan. Such disasters have made earthquake prediction an urgent topic of research but, so far, the science has been relatively ineffectual. Research published in Physical Review Letters could, however, provide a breakthrough. It might seem perverse, but American researchers suggest that finding what makes earthquakes stop could prove more useful than discovering how they start. Earthquakes occur because the plates that make up the Earth’s crust are slowly moving relative to each other along the fault lines that divide the plates. This movement builds up stresses that eventually “go critical” and rip the rocky surface apart. Seismographs detect a million or more earthquakes around the Earth every year, but only 50,000 or so of these are large enough to be noticed by the population in a region. However, according to the model that is currently used to simulate and help predict earthquakes, there is nothing to stop every single tremor growing and growing until it shoots off the Richter scale. “The simple theory that everyone believes has it that the crack should go on forever,” says John Rundle, professor of geological science at the University of Colorado. “As it grows larger, its presence concentrates more stress on the crack’s tips, which drives it further on. So the question is, if you have a small earthquake, why doesn’t it grow into a magnitude 9 earthquake?” Magnitude 9 corresponds to the most powerful earthquakes on record (the Afghanistan quake was magnitude 6.9). The 1964 magnitude 9 earthquake that hit Alaska resulted in a crack in the Earth’s crust 1,000 km long. Such extreme occurrences are thankfully rare but, if current earthquake models are to be believed, they shouldn’t be. The existing models assume that these stresses are uniform in the rock around the area of a crack, but Rundle and his colleagues have now called this into question. They made a different assumption: that there must be large variations in the stresses, and these variations hold the earthquake back. “If a crack is growing in an area where the stress is high, then it’s going to grow easily,” Rundle says. “But if it goes through an area where the stress is low, it will slow down and maybe stop.” The researchers constructed a computer model to include these stress variations, and found that, for the first time, they could reproduce all the phenomena associated with real earthquakes. Rundle and his colleagues
can now model the effects of different types of fault,
and produce the many types of earthquake that are seen in
nature. The team is now applying its new tool to simulate
earthquakes seen in the past. Recognising the signature
of an impending disaster may enable them to predict
events up to six months in advance, Rundle says.—
(The Guardian) |
Science
notebook It is certain that the original home of humanity was in Africa. There is no record of the presence of the hominid family (which includes all upright-walking primates) anywhere outside this continent. The question that remains is at what stage did the migrations take place out of Africa. The earliest hominids, designated Australopithecus anamensis, have been dated about 4 million years (Myr) ago. Far better known is the succeeding species, famously represented by the Lucy skeleton from Ethiopia, formally called Australopithecus afarensis, and dated in the 3.9 to 3.0 Myr range. Lucy and her kind were upright-walkers, but carried many features from their ancestors.From the proportions of their limbs and from their hands and feet ,it is clear that they were expert tree- climbers. There brains were ape-sized. adept at climbing trees. They have been quite appropriately dubbed "bipedal chimpanzees". The picture becomes clearer from about 1.9 Myr ago when first good evidence comes from north Kenya of a species recognizably like ours. A good example of this is the skeleton known as Turkana Boy,discovered in 1984.The boy had an essentially modern body structure,indicative of modern gait.He had a brain double the size of the apes, but still about half the modern human average.For specificity,we may note that the average brain size is about 1400 milliliters for the modern humans and about 400 milliliters for the apes. This boy is now said to belong to a species of his own, Homo ergaster. It is is now believed that this species is a plausible ancestor for all subsequent humans. The better known species, Homo erectus, is now believed to be a local east Asian development (during 1-0.5 Myr) which did not evolve further. It is however not clear what brought Homo erectus to extinction. The next state of development on record is is the species H.heidelbergensis, of about 780,000 years (780 Kyr) ago. It gradually evolved into the Neanderthals, or H. neanderthalensis. These large-brained hominids are known only from Europe and western Asia, where they flourished in the period between about 200 Kyr and 30 Kyr, when they were "extinguished" in some way by Homo sapiens, that is the modern man. The fossil records show that from the earliest times Africa has been the centre from where new lineages have sprung. Whatever evolutionary developments occurred in both Europe and east Asia involved populations that were derived from Africa. More importantly,they were eventually supplanted by new arrivals from Africa. An intriguing question
remains.What is it that prompted homonids to spread far
and wide. Was it merely the requirement of food? It
cannot be so, because the populations were small and land
in plenty? Was it the desire to see the world, as strong
then as it is now. |
Building materials from marble dust Environmentally hazardous marble dust from marble slab cutting and polishing centres can be utilised as building materials, according to researchers of Central Building Research Institute (CBRI) in Roorkee. A study sponsored by the Building Materials Technology Promotion Council (BMTPC) at a number of centres in Rajasthan and Haryana has shown that tiles, bricks, colour washes, cellular concrete, masonry cement and gypsum-based boards for false ceilings can be developed using marble dust, CBRI researchers reported in a seminar in New Delhi. Marble dust, which is generally discarded as waste and dumped along road sides, contains marble particles of varying sizes and dolomite and calcite minerals. Powders from different places vary in their physical properties and chemical compositions which affect their utilisation potential. The fine dust was mixed with a predetermined amount of portland cement and water and pressed in a mould to form brick which gained sufficient strength after curing, they said. Though masonry cement prepared from marble dust could be used for obtaining masonry mortars, it was not recommended for concrete and reinforced concrete cement (RCC) works. Gypsum plaster-based blocks and boards which were popular for their lightness, fire-resistance and thermal insulating properties could also be produced by replacing a part of gypsum plaster with marble dust, the researchers reported. The dust could be used to prepare cellular concrete blocks which find application in framed structure and multi-storeyed buildings. When mixed with suitable pigment extenders, water soluble binders and preservatives, the dust perform as a colour wash also. All the building materials
developed were evaluated according to the relevant Indian
standards. |
Rock and Ram Breakthrough in
drug delivery “Space-age”
farming |
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