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| SCIENCE TRIBUNE | Thursday,
July 4, 2002, Chandigarh, India |
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What determines magnitude, intensity of quakes? Few alternatives to animal testing A world is born in a wink |
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What determines magnitude, intensity of quakes?
A number of smaller size earthquakes take place before and after a big earthquake (i.e., the Main Shock). Those occurring before the big one are called Foreshocks, and the ones after are called Aftershocks. Magnitude Magnitude is a quantitative measure of the actual size of the earthquake. Prof Charles Richter noticed that (a) at the same distance, seismograms (records of earthquake ground vibration) of larger earthquakes have bigger wave amplitude than those of smaller earthquakes; and (b) for a given earthquake, seismograms at farther distances have smaller wave amplitude than those at close distances. These prompted him to propose the now commonly used magnitude scale, the Richter Scale. It is obtained from the seismograms and accounts for the dependence of waveform amplitude on epicentral distance. This scale is also called Local Magnitude scale. There are other magnitude scales, like the Body Wave Magnitude, Surface Wave Magnitude and Wave Energy Magnitude. These numerical magnitude scales have no upper and lower limits; the magnitude of a very small earthquake can be zero or even negative. An increase in magnitude (M) by 1.0 implies 10 times higher waveform amplitude and about 31 times higher energy released. For instance, energy released in a M7.7 earthquake is about 31 times that released in a M6.7 earthquake, and is about 1000 ( 31 31) times that released in a M5.7 earthquake.
Intensity
Basic difference Magnitude of an earthquake is a measure of its size. For instance, one can measure the size of an earthquake by the amount of strain energy released by the fault rupture. This means that the magnitude of the earthquake is a single value for a given earthquake. On the other hand, intensity is an indicator of the severity of shaking generated at a given location. Clearly, the severity of shaking is much higher near the epicenter than farther away. Thus, during the same earthquake of a certain magnitude, different locations experience different levels of intensity.
One often asks: Can my building withstand a magnitude 7.0 earthquake? But, the M7.0 earthquake causes different shaking intensities at different locations, and the damage induced in buildings at these locations is different. Thus, indeed it is particular levels of intensity of shaking that buildings and structures are designed to resist, and not so much the magnitude.
For instance, during the 2001 Bhuj earthquake, the area enclosed by the isoseismal VIII (Figure 2) may have experienced a PGA of about 0.25-0.30g. However, now strong ground motion records from seismic instruments are relied upon to quantify destructive ground shaking. These are critical for cost-effective earthquake-resistant design. Based on data from past earthquakes, scientists Gutenberg and Richter in 1956 provided an approximate correlation between the Local Magnitude ML of an earthquake with the intensity I0 sustained in the epicentral area as: ML ˜ I0 + 1. (For using this equation, the Roman numbers of intensity are replaced with the corresponding Arabic numerals, e.g., intensity IX with 9.0). There are several different relations proposed by other scientists. Authored by C.V.R.
Murty for Indian Institute of Technology Kanpur, and Building
Materials and Technology Promotion Council, New Delhi. |
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Few alternatives to animal testing As the whole debate over animal testing and unethical treatment of animals in labs threatens to impede scientific research, experts say there are few alternatives available. "It is true that some alternate methods such as tissue culture and cell culture can be used in certain studies but these cannot replace animals," says Dr Gajraj Singh, senior scientist at the Indian Veterinary Research Institute, Bareilly. While animal rights activists have been campaigning for invitro research for preventing "animal sacrifice", scientists have expressed their scepticism at the potential of "alternative techniques" and if they can totally replace animal testing. "For the so-called abolitionists who seek immediate end to all animal research and testing, the term alternative refers to those techniques which can entirely replace the use of animals. But to many biomedical researchers alternative techniques mean those which can be used in addition to the more traditional animal testing methods," says Singh adding that these are adjunct to the more commonly used animal models. "Invitro research is part of the regular testing procedure. Whenever a drug is tested, the first step is to carry out invitro tests. After that it is important that the drug is experimented on live animals otherwise it is impossible to know how the drug will affect humans," says Dr D.S. Gambhir, president-elect of the Cardiology Society of India, while reiterating that animal testing remains an "integral" part of the research. "However, alternate techniques can focus on specific biological functions and in many cases reduce the number of animals used," says Singh. Critics of animal testing argue that such studies do not accurately predict the effect on humans as there are physiological differences between the species. Experiments on drug metabolism, drug toxicity and drug-drug intercatiently. But then there are very few options if we have to refrain from testing on humans, says Gambhir. "Simply conducting invitro research cannot suffice. Experimenting on animals with the assumption that they will show similar results in humans is our best bet," he says. While the strict imposition of guidelines on animal testing by the Committee for the Purpose of Control and Supervision of Experiments on Animals (CPCSEA) has brought the scientific community and animal rights activists to a point of confrontation, researchers do concede that demands of the CPCSEA are justified. "The CPCSEA is correct in asking that the research animals should be properly bred and taken care of. If the experiments are carried out on sick animals, the results can definitely be compounded which will affect the outcome of the study," says Gambhir.
"The scientists support the animal
rights movement. The controversy arises, when non-technical people,
who do not know about the animals, start dictating terms without
knowing the ground realities about the scientific, administrative and
financial limitations," says Singh. PTI |
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A world is born in a wink Astronomers have observed a star giving birth. They have pinpointed a distant sun where a new planet is being created around it. The discovery, a first for astronomers, has been hailed by scientists who believe it will shed crucial light on the frequency of solar systems throughout the galaxy. Researchers have recently pinpointed scores of planets in orbit round stars, raising hopes of finding life elsewhere in the universe. But the exact mechanism of the formation of planets has remained a mystery. However, observations of star KH15D by a group of students using a simple university telescope have raised hopes of a breakthrough. "This is going to add a whole new dimension to astronomy," said NASA physicist Steven Maran. The astronomers, from Wesleyan University, Connecticut, spotted KH15D — a young star the size of our sun, sited in the constellation Monocero — six years ago when a routine survey showed it was behaving oddly. For 32 days it shone brightly. Then it dimmed dramatically for the next 16. "Basically, the star is winking at us," said project leader Prof William Herbst. "Something is passing between it and Earth. However, it cannot be another star or a large planet. That would produce much briefer eclipses. These last for more than two weeks." Herbst concluded that a collection of smaller objects — dust grains, rocks and asteroids, strung out in a great clumpy arc — must be orbiting KH15D. As these swaths are exactly what astronomers would expect to find in a system giving birth to planets, a major programme of observations of KH15D was launched. According to theory, a star starts to spin as it is born, causing surrounding clouds of dust and grain to form a disc, just as a flat pizza is created when a chef whirls a piece of dough. The grains in the disc bash into each other, accrete into bigger and bigger lumps — and eventually become asteroids and finally planets. The trouble is astronomers have never observed this process. Hence their excitement about KH15D and its swaths of dust. Only lengthy observations would provide the information astronomers needed — which ruled out the use of large expensive telescopes that only allocate brief periods of observing to astronomers. So the Wesleyan team ran programmes, using the campus’s modest 24-inch telescope, for graduate and undergraduate students who maintained bouts of uninterrupted star-gazing. In addition, astronomers in Uzbekistan, Germany, Israel and Spain joined in to keep a watch on KH15D. The results, revealed at a conference in Washington a few days ago, produced several surprises. First, the eclipse was found to have lengthened from 16 to 18 days, indicating that the orbiting dust clouds were moving closer to the KH15D. Second, in the middle of each eclipse brief interludes of blue light emerged from KH15D which astronomers believe come from a large, single object, most probably a planet, that seems to have formed inside the star’s dust cloud. "We can’t tell exactly what size this object is yet, but we should be able to in a year or so," said Herbst. "However, something very unusual is happening around KH15D." This excitement is shared by other astronomers. "Theory predicts that, as a planet forms, it will be followed around in its parent disk by waves of denser material, and that seems to be what we are seeing here," said Edinburgh astronomer Dr Jane Greaves, of the Royal Astronomical Society. "Planet formation is a short-lived process, tiny compared to the lifetime of the star. We are only seeing this happen round KH15D because we are looking at exactly the right moment. That is why it is so exciting." The star is only three million years
old, compared to our own sun, which is billions of years old. For a
planet to have formed so quickly is surprising. However, the surface
of this other world will not provide a home for life-forms for a long
time. Earth was battered for hundreds of millions of years by orbiting
debris left over from its creation, making life untenable. And even
after Earth cooled down, and primitive life had appeared, it still
took another three billion years to evolve into intelligent beings.
Observer |
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SCIENCE & TECHNOLOGY CROSSWORD
Crossword:
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The
point on the fault where slip starts is the Focus or Hypocenter, and
the point vertically above this on the surface of the Earth is the
Epicenter (Figure 1). The depth of focus from the epicenter, called as
Focal Depth, is an important parameter in determining the damaging
potential of an earthquake. Most of the damaging earthquakes have
shallow focus with focal depths less than about 70km. Distance from
epicenter to any point of interest is called epicentral distance.
Most
of the energy released goes into heat and fracturing the rocks, and
only a small fraction of it (fortunately) goes into the seismic waves
that travel to large distances causing shaking of the ground en-route
and hence damage to structures. (Did you know? The energy released by
a M6.3 earthquake is equivalent to that released by the 1945 Atom Bomb
dropped on Hiroshima!!) Earthquakes are often classified into
different groups based on their size (Table 1). Annual average number
of earthquakes across the Earth in each of these groups is also shown
in the table; it indicates that on an average one Great Earthquake
occurs each year.
Intensity
is a qualitative measure of the actual shaking at a location during an
earthquake, and is assigned as Roman Capital Numerals. There are many
intensity scales. Two commonly used ones are the Modified Mercalli
Intensity (MMI) Scale and the MSK Scale. Both scales are quite similar
and range from I (least perceptive) to XII (most severe). The
intensity scales are based on three features of shaking - perception
by people and animals, performance of buildings, and changes to
natural surroundings. 
Table
2 gives the description of Intensity VIII on MSK Scale. The
distribution of intensity at different places during an earthquake is
shown graphically using isoseismals, lines joining places with equal
seismic intensity (Figure 2).
To
elaborate this distinction, consider the analogy of an electric bulb
(Figure 3). The illumination at a location near a 100-Watt bulb is
higher than that farther away from it. While the bulb releases 100
Watts of energy, the intensity of light (or illumination, measured in
lumens) at a location depends on the wattage of the bulb and its
distance from the bulb. Here, the size of the bulb (100-Watt) is like
the magnitude of an earthquake, and the illumination at a location
like the intensity of shaking at that location.
The
peak ground acceleration (PGA), i.e., maximum acceleration experienced
by the ground during shaking, is one way of quantifying the severity
of the ground shaking. Approximate empirical correlations are
available between the MM intensities and the PGA that may be
experienced (e.g., Table 3). 
Clues