The Tribune, Chandigarh, India

How the ground shakes

Large strain energy released during an earthquake travels as seismic waves in all directions through the Earth’s layers, reflecting and refracting at each interface. These waves are of two types — body waves and surface waves; the latter are restricted to near the Earth’s surface (Figure 1). Body waves consist of Primary Waves (P-waves) and Secondary Waves (S-waves), and surface waves consist of Love waves and Rayleigh waves. Under P-waves, material particles undergo extensional and compressional strains along the directions of energy transmission, but under S-waves, oscillate at right angles to it (Figure 2). Love waves cause surface motions similar to that by S-waves, but with no vertical component. Rayleigh wave wakes a material particle oscillate in an elliptic path in the vertical plane (with horizontal motion along direction of energy transmission).

P-waves are fastest, followed in sequence by S- Love and Rayleigh waves. For example, in granities, P- and S-waves have speeds ~4.8 km/sec and ~3.0 km/sec, respectively. S-waves do not travel through liquids. S-waves do not travel through liquids. S-waves in association with effects of Love waves cause maximum damage to structures by their racking motion on the surface in both verticle and horizontal directions. When P- and S-waves reach the Earth’s surface, most of their energy is reflected back. Some of this energy is returned back to the surface by reflections at different layers of soil and rock. Shaking is more severe (about twice as much) at the Earthsurface than at substantial depths. This is often the basis for designing structures buried underground for smaller levels of acceleration than those above the ground.

Measuring Instruments

The instrument that measures earthquake shaking, a seismograph, has three components — the sensor, the recorder and the timer. The principle on which it works is simple and is explicitly reflected in the early seismograph (Figure 3) — a pen attached at the tip of an oscillating simple pendulum (a mass hung by a string from a support) marks on a chart paper that is held on a drum rotating at a constant speed. A magnet around the string provides required damping to control the amplitude of oscillations. The pendulum mass, string, magnet and support together constitute the sensor; the drum, pen and chart paper constitute the recorder; and the motor that rotates the drum at constant speed forms the timer.

One such instruments is required in each of two orthogonal horizontal directions. Of course, for measuring vertical oscillations, the string pendulum (Figure 3) is replaced with a spring pendulum oscillating about a fulcrum. Some instruments do not have a timer device (i.e., the drum holding the chart paper does not rotate). Such instruments provide only the maximum extent (or scope) of motion during the earthquake; for this reason they are called seismoscopes.

The analog instruments have evolved over time, but today, digital instruments using modern computer technology are more commonly used. The digital instrument records the ground motion on the memory of the microprocessor that is in-built in the instrument.

Shaking of ground on the Earth’s surface is a net consequence of motions caused by seismic waves generated by energy release at each material point within the three-dimensional volume that ruptures at the fault. These waves arrive at various instants of time, have different amplitudes and carry different levels of energy. Thus, the motion at any site on ground is random in nature with its amplitude and direction varying randomly with time.

Large earthquakes at great distances can produce weak motions that may not damage structures or even be felt by humans. But, sensitive instruments can record these. This makes it possible to locate distant earthquakes. However, from engineering viewpoint, strong motions that can possibly damage structures are of interest. This can happen with earthquakes in the vicinity or even with large earthquakes at reasonable medium to large distances.

Characteristics

The motion of the ground can be described in terms of displacement, velocity or acceleration. The variation of ground acceleration with time recorded at a point on ground during an earthquake is called an accelerogram. The nature of accelerograms may vary (Figure 4) depending on energy released at source, type of slip at fault rupture, geology along the travel path from fault rupture to the Earth’s surface, and local soil (Figure 1).They carry distinct information regarding ground shaking; peak amplitude, duration of strong shaking, frequency content (e.g., amplitude of shaking associated with each frequency) and energy content (i.e., energy carried by ground shaking at each frequency) are often used to distinguish them.

Peak amplitude (peak ground acceleration, PGA) is physically intuitive. For instance, a horizontal PGA value of 0.6 g (= 0.6 times the acceleration due to gravity) suggests that the movement of the ground can cause a maximum horizontal force on a rigid structure equal to 60% of its weight. In a rigid structure, all points in it move with the ground by the same amount, and hence experience the same maximum acceleration of PGA. Horizontal PGA values greater than 1.0 g were recorded during the 1994 Northridge Earthquake in USA. Usually, strong grounds motions carry significant energy associated with shaking of frequencies in the range 0.03-30Hz (i.e., cycles per sec). Generally, the maximum amplitudes of horizontal motions in the two orthogonal directions are about the same. However, the maximum amplitude in the vertical directions is usually less than that in the horizontal direction. In design codes, the vertical design acceleration is taken as 1/2 to 2/3 of the horizontal design acceleration. In contrast, the maximum horizontal and vertical ground accelerations in the vicinity of the fault rupture do not seem to have such a correlation.

Authored by C. V. R. Murty for the Indian Institute of Technology Kanpur, Kanpur, India.

Electricity from mud
S.P. Gupta

Self-recharging bacterial batteries that clean up organic pollution as they generate electricity? Sounds more like science fiction than science. Microbiologists are coming closer to making microbial fuel cells a reality. They harnessed bacteria to generate electricity from underwater sediments. The microbes make excess elections that they stick directly to graphite wires, which in turn send current to a second wire much like a car battery. For fuel, the bacteria use organic material in the sea floor. These bacterial batteries will probably never power a car, but they should be adequate to run underwater sensors. Derek Lovely, a microbiologist at the University of Massachusetts, Amherst, with Daniel Bond led the work on these non-conventional energy sources.

Because organic sediments are so abundant, there could be an inexhaustible source of fuel. And because many pollutants are organic, these portable generators might also help get rid of hazardous materials. The whole field is very exciting, because the work has broad potential for both helping pollution cleanup and providing a cheap power supply. This research has come closer to developing accessible marine batteries as a way to meet our electricity needs.

This was not the first time to notice that microbes could steal electrons from oxygen-deficient mud and somehow transfer them to electron-accepting rods placed into the oxygen-containing sediments over-head. But Lovely and his colleagues, take a concept that has been known for a while and make good on it.

They used lab fish tanks to recreate the ocean’s saltwater environment. Collaborator Leonard Tender, positioned graphite wires which act as electron-accepting anodes into oxygen containing water to deceive electrons. In three different experiments, they measured the number of electrons transferred to the anode and then to the cathode. Even in these crude experiments, the current was enough to power a small calculator.

After several weeks, the researchers identified the microbes that were growing on the mud-implanted electrodes. To their surprise, Lovely and his colleagues found that one type of microbe — Desulfuromanas acetoxidans, from a family called geo-bacteraceae —had all but taken over the battery electrode, ousting the others. These geo-microbes are famous for their ability to detoxify toluene and other organic solvents, notes microbiologist Caroline Harwood of the University of Iowa.

Earlier, microbiologists had shown that different microbes could move electrons from oxygen-deficient to oxygen-rich substances through intermediate substances that they produced. The microbes were involved, but not directly with the electrode, Lovely explains. But geobacters, as the family is commonly called, need no such gobetweens. They can convert the mud’s organic matter directly, and that might prove quite useful in pollution control.

Before using organic pollutants to fuel electricity production leaves the realm of science fiction, Lovely and his colleagues warn, the work needs to be replicated in field conditions. Harwood points out that the bacteria might quickly exhaust local organic fuels and have to be moved to a different spot. The efficiency of the transfer also needs improving, something that Lovely and others are fervently working on, otherwise, it would take fields of electrodes to get enough energy to power many undersea devices.

NEW PRODUCTS & DISCOVERIES
The invisible keyboard

Soon, drumming your fingers on the conference room table may no longer be a sign of boredom. Instead, it may be a way to take notes.

How? Korea-based Samsung’s Scurry and Switzerland-based Senseboard Technologies’ Senseboard (see picture) are virtual keyboards that use sensors on the back of your hands to track the movement of your fingers.

An onboard processor maps the location of each virtual key tap to a keyboard layout, then transmits the corresponding character wirelessly to a PDA, cellphone, or other mobile device.

Both virtual keyboards will be available later this year. Prices not set. Popular Science

Automatic docking guidance

Scientists have developed an automatic, video-based docking guidance system that replaces the groundcrew flagman for the last few metres and centimetres in the docking process.

To allow passengers to move into the terminal building as quickly as possible and without being exposed to the weather, telescoping bridges or jetways are becoming more and more common, replacing rolling gangways and airport buses.

But docking at the extendable jetways requires a high level of precision. The pilot must stop the aircraft at the exact right parking position. Because the view from the cockpit is very limited, this process is difficult without outside help by marshals.

The new system developed at the Fraunhofer Institute for Information and Data Processing IITB in Karlsruhe by Volker Gengenbach does away with the requirement of a marshal, a report in Fraunhofer Gesellschaft said.

In the new system the groundcrew first enter the craft type and expected arrival time into the computer. A video camera at the outside wall of the building then acquires the aircraft as it taxis in.

Using the available model data, the docking approach path and optimal stopping position are calculated. "The approach line is measured 10 to 20 times per second," explains Gengenbach. PTI

Viruses to build microprocessors

As it is getting tougher and more expensive to shrink conventional microchip technology, scientists have succeeded in turning plant viruses into the building blocks of microprocessors thereby bringing a breakthrough in miniaturisation.

The viruses are just 30 nanometres across, far smaller than the 130-nanometre wide components in today’s microchips.They provide the perfect scaffold for tiny electronic systems because they can be made to arrange themselves into crystal-like arrays.

This raises the tantalising possibility of self-organising circuits, which need little or no intervention to help them build useful three-dimensional structures that can be populated with circuit components.

Until now, nanotechnologists have only constructed flat nanocircuits, using components like carbon nanotubes as transistors, but what they wanted to achieve was to find a way for these molecular circuits to build themselves, a report in New Scientist said.

For their work, chemist MG Finn and virologist Jack Johnson, both of the Scripps Research Institute in La Jolla, California, selected cowpea mosaic virus, a common pathogen which stunts the growth of the black-eyed pea plant.

This virus is encased in a protective protein coat that has 20 faces and 12 corners, or vertices. The researchers inserted DNA segments into the virus’s genome that cause the pathogen to produce cysteine amino acids on the vertices of its viral shell.

The resulting cysteine complex at each vertex sports sulphur-containing thiol groups, which bind readily to gold.

So when the team added ultrafine gold particles to the cysteine-loaded viruses, they ended up with viruses studded with a pattern of gold electrodes. PTI

Plastic surgical material

A plastic material that can be made to tie itself into a knot shows promise for use as a surgical thread and for some medical implants, associated Press reports.

The material, made of thermoplastic polymers that can be absorbed by the body, can be engineered to retain a memory of a specific shape and to then transform itself into that shape when warmed to body temperature, said Robert Langer, the co-author of the study published in the electronic version of the journal Science.

The other author is Andress Lendlein, a former researcher at the Massachusetts Institute of Technology, now teaches at the University of Technology in Germany, Y Tevtththehe Techissbhe.

Langer, a professor of chemistry at MIT, said the plastic could be used to make implants or bone screws that are not much bigger than a piece of string when they are inserted into the body. Once they warm up, the devices change to form the appropriate implant.

"In a test on mice, we showed we can make these sutures (surgical stitches) actually tie themselves," said Langer.

He said that since the material has a memory, it could be threaded into an incision as a loose knot. When it warms to the body’s temperature, the material "remembers" its designed shape and size and shrinks to tighten the wound. Later after the wound is healed, the material dissolves and is harmlessly absorbed by the body.

"It is like a smart suture," said Langer. "That could be very important in closing and incision in a place that is hard to reach by surgeon. PTI

SCIENCE & TECHNOLOGY CROSSWORD

Clues :

Across

1. An ion that has a positive and a negative charge on same group of atoms.

7. Connecting an electric conductor to the ground.

8. A specialised Indian body advising on matters relating to energy.(abbr.)

9. Longer is this, heavier is the beam.

12. Bangalore based research institute named after C.V.Raman.

13. A colourless crystalline solid acid used in beverages.

15. A rain gauge is called so.

18. Tendency of solvent when partitioned from a more concentrated solution, to diffuse through into that solution.

20. A substance having nuclear carbon skeleton of the sterol or a similar structure.

22. One of the best engineering firms.

23. A mineral found in crystals separable into thin transparent plates.

24. An alloy of Iron, Carbon and other metals in small proportions.

Down

1. A current produced in a semiconductor when subjected to strong electric field.

2. A small piece of insulation.

3. A transition metal having Ir as its symbol and 77 atomic number.

4. A device used to maintain the temperature in an appliance within a range.

5. A disease usually occurring after 6 to 12 minutes strenuous activity.

6. An inert gas used in lights and signs.

10. …..head is the area surrounding a coal mine.

11. Acid prepared by oxidation of ethyl alcohol with acidified potassium permanganate.

14. Equivalent to degree in engineering.

16. Most important system for PCs.

17. A system to find the direction and distance of an object by analysing the reflection of microwaves.

18. An electric circuit through which no current is passing.

19. One of 4 quantum numbers assigned to an electron and having only 2 values.

21.A mixture of naturally occurring hydrocarbons.

Solution to last week’s crossword.