EARTHQUAKES HAVE LONG HELD a special fascination for science buffs. In the past, this column has described seismographs no fewer than eight times, so you might think there would be little more to say. But something exciting has just happened that makes this topic well worth revisiting. Using a breakthrough technology, amateurs can now, for about $100, easily build seismographs that are robust and that approach professional quality.

The breakthrough is a remarkable micromachined accelerometer on a silicon chip. Made by Analog Devices in Norwood, Mass. (telephone number: 800-262-5643, ext. 3), the ADXLOS chip costs about $20 and can detect fantastically small accelerations–less than five thousandths of 1 g (g is the acceleration caused by the earth's gravity and equals a change in velocity of 9.8 meters per second every second). That sensitivity is sufficient to detect extremely tiny tremors. The largest acceleration it can detect is 5 g, which is just about the limit of the largest earthquakes recorded.

The chip requires several resistors and capacitors to function as a seismograph. The circuit diagram on page 102 shows the arrangement. This design ignores the constant acceleration of gravity, so you do not have to level your seismograph precisely. The device is, however, quite efficient at detecting shaking rates from 0.1 hertz (one shake every 10 seconds) up to 100 hertz, which is the frequency range in which an earthquake releases most of its fury.

Unfortunately, the ADXLOS chip can be destroyed by the stray, static electrical charges that build up on our bodies. We often have up to 4,000 volts sitting on our fingertips, just waiting to be discharged to any convenient ground. We usually do not even feel these static discharges, because the amount of current that flows is relatively low. But those tiny pulses of energy can fry these delicate chips. Never handle them without keeping yourself discharged. Tying a wire between your wrist and something grounded, such as a metal pipe under the sink, is a good way to prevent static disasters. Also, dropping the chip on a hard surface may break the inner mechanism, so lay a soft towel over your workbench when building the circuit.

My seismograph uses three ADXLOS chips; two measure horizontal accelerations, and one measures vertical accelerations. Mount these chips on ordinary circuit boards. The tab on the chip points along its sensitive axis. Mount two chips such that these tabs face away from each other at 90 degrees–one forward, the other facing right (that is, so they span the x-y plane). Affix a separate piece of circuit board at a right angle to the first, making it stand upright [see Figure 1], then mount the third accelerometer with its tab facing straight up to record vertical accelerations.

House the completed unit in a weatherproof box and orient your seismograph so that the y axis points to true (not magnetic) north and the x axis points due east. (Hiking supply stores sell local topographic maps that indicate the difference between true and magnetic north.) Ideally, the seismograph should be bolted directly onto a concrete foundation that descends at least one meter into the ground. Failing that, bolt it to a weighted base and rest it on a surface–preferably concrete–away from foot and automobile traffic.

The chip's output is 2.5 volts at rest. A 1-g acceleration changes it by 0.4 volt. In general, the acceleration (a) along the chip's sensitive axis is given by a = (V_out - 2.5)/0.4, where V_out is the voltage output and a is in units of g.

These days your most important scientific tool is usually a home computer. Research often entails converting measurements (in this case, an acceleration) into voltage, which can then be digitized and fed directly into your computer. Vernier Software in Portland, Ore. (telephone: 503-297-5317), makes a versatile interface that will directly link your computer to this seismograph. It is their three-channel "Multipurpose Lab" card ($310), available for both IBM-compatible and Macintosh systems. BSOFT Software in Colurnbus, Ohio (telephone: 614-491-0832), offers several hardware solutions for IBM compatibles for under $100, as does Prairie Digital in Prairie Du Sac, Wis. (telephone: 608-643-8599), which also sells a serial device driver for Macintosh aficionados.

Finally, there is LabView, put out by National Instruments in Austin, Tex. (telephone: 800-433-3488). This truly amazing program will read data from any interface and enable your computer to mimic almost any scientific instrument imaginable. You can buy the student edition for just $65 from Prentice Hall (telephone: 800-947-7700).

You will still need a bit of programming wizardry to avoid filling up your hard drive with useless data. The software accompanying your computer interface should be able to analyze data and select what to record on the fly, so you will need to write a program that makes those decisions [see sidebar]. Working with this modification to the commercial packages mentioned above. So be sure to pick the software most suitable for your level of programming skill.

If you do not have a computer, you will need a chart recorder [see "The Amateur Scientist, SCIENTIFIC AMERICAN, November 1955, July 1966, May 1970 or March 1972]. There is a bit of a problem with chart recorders, though. The voltage shift from each chip can be either positive or negative depending on the way the device lurches. If the different voltages are simply added together, then a negative horizontal "zig" would cancel a positive vertical "zag" and erroneously reduce your estimate of the actual acceleration.

To get around this problem, you will need to combine the signals into a single voltage that is a good measure of the total strength of the shaking. The summation circuit shown below does the trick. It takes the absolute value of the voltage shifts and adds them together.

The Society for Amateur Scientists is organizing an international network of amateur seismological observation stations. To get involved or to learn more about amateur seismology, send $5 to the Society for Amateur Scientists, 4951 D Clairemont Square, Suite 179, San Diego, CA 92117. You may download the information for free from its Web site at http://www.SAS.com/

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.