I have only once felt an earthquake—in 1985, when a magnitude-4 temblor occurred just north of New York City. It wasn't until I heard the news reports later that I realized the vibration that had awakened me at 6 a.m. was, in fact, a small earthquake.
Many earthquakes have since vibrated the ground beneath my feet. It's just that those vibrations, having traveled long distances through the Earth, have (thankfully) been too small to feel. If I had a suitably sensitive seismometer, though, I'd be able to measure them.
I recently decided that I needed to give this a try. Searching the interwebs, I found no shortage of leads about how to build a DIY seismometer. Fundamentally, these consist of a magnet attached to a mass, with a nearby pickup coil. The mass is suspended so that it remains largely motionless when the ground shakes. The shaking does vibrate the coil, however, inducing a voltage in it due to its relative motion through the magnet's magnetic field. The problem is that the DIY seismometer designs I was seeing were large and ungainly contraptions. I wondered whether I could build a more compact one using a geophone.
Geophones are commonly used in the oil and gas industry for seismic surveying, where the seismic waves are artificially generated to probe the ground below. On land, special trucks—called "thumpers"—do the job. The seismic waves they produce reflect back up from layers of rock and are sensed using geophones.
From IEEE Spectrum
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