Analyzing earthquakes is the job of peculiar minds that are comfortable with the abstractions of physics. But even trained specialists can get more from their data if it's presented in novel ways—like through their ears. And the rest of us can listen in, too.
Earthquakes Are Sounds
First of all, earthquakes are releases of energy, and the energy travels as various kinds of waves through the solid medium of rock. That means that they are sounds, just like the audible waves traveling through the gaseous medium of the air. In fact, earthquakes can be very noisy events.
I live in a place where small earthquakes are common. The little events are whumps and bangs, sounding a lot like a truck striking a steep driveway or a large slamming door. Neither we nor scientists get a lot of information from these except for the message that somewhere down there, rocks are breaking and grinding against each other.
People report that large earthquakes come and go with a loud roaring noise, like hundreds of jet planes taking off. That sound is the combination of ground motions and everything in the whole countryside that those ground motions disturb (trees waving in the wind, water moving, soil grinding), plus all the noise made by objects cracking and breaking. Add in thousands of car alarms, electrical systems exploding and people screaming, and it's clear that listening to the pure song of the underground earthquake itself is not possible for the unaided ear.
The basic physics of an earthquake rupture is fairly well described by simple equations. It stands to reason that at a certain level of abstraction and with clean data, the vibration patterns of an earthquake ought to make some sense to the ear. And the echoes of those vibrations off structures in the deep Earth ought to be informative, perhaps even in an intuitive way, just as snapping the fingers in a quiet room provides information about its size and shape and contents. If only we could hear them! But the frequencies of earthquake sounds are far too low for human hearing.
Recording and Playing Back Earthquakes
Sensitive research-grade seismometers are made to catch the best possible record of the earthquake's true sound. They're carefully isolated from surface noise by being placed in quiet locations, dug into the ground and surrounded by insulation. The promise is that these instruments will collect clean records of earthquakes, like a microphone in a well-built sound studio.
And for the most part they do just that. The patterns extracted from thousands of seismograms, by eye and by algorithm, have opened up the deep structure of the Earth to science. These days we can readily extract the basic focal mechanism of even tiny earthquakes, and larger ones provide signals reflected from deeper levels of the Earth's crust and mantle. But we have a stubborn urge to bring our five senses to bear. What do seismograms sound like? Can we, in effect, hear like the Earth hears? If we treat an earthquake like a finger-snap in a quiet room, can our ears relate to it?
These days we have a sophisticated control of sound, driven by advances in the music industry and the availability of massive computer power. A trickle of that talent has been diverted by seismologists to play with seismograms. The same earthquake recordings that are used to drive shaker tables at natural speed can be accelerated, with adjustments, into audio files that are sonically palatable. The most exciting of these sonifications (or audifications), for my money, is a recording of the Japan earthquake of March 2011 at sonifyer.org in which two days of seismograms are compressed into two minutes.
These accelerated seismograms, interpreted by the ear as sounds, have an oceanic quality. The Earth's background noise—its ambient seismicity—is reminiscent of experiencing an empty concert hall; you get a sense of space.
Then come the earthquakes. The first, fast P-waves translate to a crack or bang; the slower S-waves turn to a boom, and the really slow surface waves are rendered as deep rumbles. The Scripps Institution, in California, has a page where you can play an earthquake at different speeds. At the right speed, earthquakes sound a lot like thunderclaps and their rolling codas. Sped up more, they sound like the clanging of sheet metal being kicked. You even get hints of Caribbean steel-drum music. There's a reason for that.
The Earth's crust—more specifically, its lithosphere—acts like a rigid plate, and its behavior under stress can be modeled using the 19th-century equations that describe metal plates and beams. So small wonder that the physics of tectonic plates and the physics of steel plates tickle our ears the same way.
The signals that come from the earthquake source also have to travel through a body of rocks, and those affect the "sound" of a quake too. The U.S. Geological Survey has a set of pages called Listening to Earthquakes where you can hear quakes of different sizes, recorded in different settings. For more of that, you'll want to linger in the Earthquake Music page, by Zhigang Peng of the University of Georgia. Swiss researcher Florin Dombois hosts the auditory-seismology.org site with still more examples. And the mother lode of seismic data you can sonify yourself is at the Incorporated Research Institutions for Seismology (IRIS) site, in the SeisSound section.