A Better DIY Seismometer Can Detect Faraway Earthquakes

Arthur T Knackerbracket has processed the following story:

This basement seismometer is relatively compact yet still sensitive enough to detect the low-frequency vibrations from distant earthquakes.

In September of 2023, I wrote in these pages about using a Raspberry Pi-based seismometer-a Raspberry Shake-to record earthquakes. But as time went by, I found the results disappointing. In retrospect, I realize that my creation was struggling to overcome a fundamental hurdle.

I live on the tectonically stable U.S. East Coast, so the only earthquakes I could hope to detect would be ones taking place far away. Unfortunately, the signals from distant quakes have relatively low vibrational frequencies, and the compact geophone sensor in a Raspberry Shake is meant for higher frequencies.

I had initially considered other sorts of DIY seismometers, and I was put off by how large and ungainly they were. But my disappointment with the Raspberry Shake drove me to construct a seismometer that represents a good compromise: It's not so large (about 60 centimeters across), and its resonant frequency (about 0.2 Hertz) is low enough to make it better at sensing distant earthquakes.

My new design is for a horizontal-pendulum seismometer, which contains a pendulum that swings horizontally-or almost so, being inclined just a smidge. Think of a fence gate with its two hinges not quite aligned vertically. It has a stable position in the middle, but when it's nudged, the restoring force is very weak, so the gate makes slow oscillations back and forth.

[...] Most DIY seismometers use a magnet and coil to sense motion as the moving magnet induces a current in the fixed coil. That's a tricky proposition in a long-period seismometer, because the relative motion of the magnet is so slow that only very faint electrical signals are induced in the coil. One of the more sophisticated designs I saw online called for an LVDT (linear variable differential transformer), but such devices seem hard to come by. Instead, I adopted a strategy I hadn't seen used in any other homebrewed seismometer: employing a Hall-effect magnetometer to sense position. All I needed was a small neodymium magnet attached to the boom and an inexpensive Hall-effect sensor board positioned beneath it. It worked just great.

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