Mapping the Bottom of the World

On any given day, our planet experiences about one thousand observable earthquakes. The majority of these events are small, and few result in anything substantial. For centuries, most of the world didn’t even know they were happening. Even today, the best means we have for observing them is pretty crude—and our ability to predict them is even more so.

Just ask the residents of Plainfield, Connecticut. The town hadn’t seen any seismic activity since 1975, and even then it was so intermittent and minor it barely warranted mentioning in local geological annals. However, in the first six months of 2015, Plainfield experienced more than three hundred quakes. Experts still can’t say why. Nor can they say why it took us until August of this year to discover a major fault in Virginia, the same state that, in 2011, witnessed the largest earthquake east of the Mississippi since 1897.

In fact, if seismologists are certain of one thing, it’s that earthquakes remain the least understood of all our natural disasters. This past summer, I visited one of the United States Geological Survey (USGS) offices responsible for creating our nation’s seismic hazard maps. The seismologists there told me it’s all still a fair amount of guesswork—and that’s especially true in places like the Eastern United States, where scientists have precious little data.

Why is this?

Geophysicist and sound artist JT Bullitt thinks one reason may be that we tend to ignore the auditory identity of the world. “Humans perceive the world through a tiny keyhole. That’s particularly true when it comes to sound,” says Bullitt. “The physical world is talking to us all the time, but most of the sounds are largely inaudible to our ears. There’s this huge wilderness of sound inside the Earth and on the surface of the Earth, and we’re just not paying attention to it. How can we possibly know what’s really going on?”

The short answer is that we can’t yet—not with any real certainty. For nearly two thousand years, our best understanding of seismic activity has been gleaned from instruments known as seismographs. The first one was invented by Chinese mathematician Chang Heng in 132 CE. The size and shape of a large urn, Heng’s “earthquake weathercock” was embossed with eight dragons evenly spaced around the exterior of the vessel. Inside the mouth of each dragon rested a bronze ball. Any seismic activity would knock one or more of these balls out of the mouth of the dragon and into the upturned mouth of a matching bronze toad set on the floor below. The sound of the ball hitting the toad would alert a watchman, who could make some basic conclusions about the strength and location of an earthquake based on which balls had moved and how loudly they had clanked into the toad’s mouth.

The technology has advanced some since Heng’s time, but not all that much. Today, seismographs include a weight that swings freely within a secure compartment and a base rooted on—or in—the Earth. During an earthquake, seismic waves shake the base and the compartment, but the weight remains unaffected. The distance between it and the shaking compartment registers as a type of line graph known as a seismogram: The faster the shaking, the shorter the lines. The harder the shaking, the longer the lines.

A modern seismogram will show when a seismic wave arrived, how fast it was moving, and how strong it was. But it does so the same way that sheet music tells you about the key signature and rhythm of a Bach concerto. Sound, after all, is nothing more than waves created by vibration. An originating object creates a disturbance—a finger plucks a guitar string; a metal tine is struck against the side of a table. That disturbance is then transported across space one particle at a time in an ever-widening band. An earthquake works the same way: moving faults create a disturbance, the...