Your own seismograph

EARTHQUAKES are devastating, but studying them is very fascinating. Specialists say that a major earthquake is often preceded by small signal tremors that mostly go undetected. By taking adequate precautions, damages due to violent rolling and tumbling of the earth can be minimised, according to experts. That is where a new device developed by the Analog Devices Inc of Norwood, Massachusetts, us comes to the forefront. With this break- through, even amateurs can build seismographs that cost as low as us $ 100 but are at par with the professionally advanced ones.

The essential feature of the new seismograph is a micromachined accelerator on a silicon chip. The ADLX.05 chip costs about us $20 and can detect amazingly small accelerations - less than five thousandths of g (g is tJW acceleration due to earth's gravity). It can mea- sure upto 5 g, which translates to roughly 8.0 on the Richter scale, but at that intensity one does not need a seismograph to understand that something is wrong. The device can detect shake rates from 0.1 hertz ( that is, one vibration every 10 seconds) to 100 hertz, the vibration frequency of very strong earthquakes.

The seismograpb uses three chips, two for measuring horizontal acceleration and the third for measuring vertical acceleration. These are mounted on ordinary printed circuit boards (PCBS) and a tab on each chip, gi@es its sensitive axis. Two chips for measuring horizontal accelerations are mounted at right angles to each other and the chip for the vertical acceleration is then fixed on another PCB perpendicular to the first one. The device is then put into a heavy wooden box (to give it stability) and lowered into a hole in the ground at a depth of about a metre.

The tremors are detected by a change in the output voltage of the chips and calculations accompanying the three-chips set specify that a change of 0.4 volts signifies a change in the acceleration by one g. This very simple, 'do it yourself model, however, is very sensitive to stray static electric charges. Users should never directly handle the chip without first discharging themselves, (we often have up to 4,000 volts of static electric charge right on our fingertips).

The new model has taken the amateur scientific community in the us by storm and three software companies - Vernier Software in Portland, Oregon; BSOFT software in Columbus, Ohio and National Instruments in Austin, Texas, have developed interface programmes that can digitise the voltage readings and feed them directly into a home computer. The software costs about us $65 and these user-friendly programmes are all set to push high-tech seismography right into your backyards (Scientific American, April 1996).

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