Listening to Earthquakes

To start with, here is a famous recording to illustrate the kinds of sounds we actually hear during an earthquake, from the 1976 Friuli earthquake in Italy:

A boy was transferring his Pink Floyd record to cassette by holding a microphone up to the speaker connected to the turntable. When the first wave arrived, the P-wave (or compressional wave), the needle jumped. Then the second, larger set of waves arrived, the S-waves (or shear waves). These caused the needle to scratch across the record and then bounce off… and then people realize what is happening and start to panic. But the sounds that you hear are secondary sounds– windows and lights rattling, pieces of concrete, rock and wood rumbling, etc. The actual seismic waves are much lower frequency.

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On the screen, seismic waves look just like recorded sound waves. To hear them, we speed them up by simply changing the “sampling rate”, the time between each recorded data point, and then make a sound file. The range of frequencies we can hear is much narrower than the range of frequencies in seismic data, so we can shift different parts of the data into our range of hearing by speeding it up by different amounts. In the next sequence of movies, we will be listening to increasingly low frequencies.

Starting with data recorded near the earthquake location, here is a wave propagating across Japan, from the 2007 Niigata earthquake.

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Here is a movie (with no sound) to show you how those waves look when they travel through the Earth’s interior, and how the data recorded at two seismometers look very different, because they are sampling that wave field at different spots on the surface:

And here’s a zoom in to how we listen to earthquakes, and how two different earthquakes can sound very different, when only one aspect is changing, in this case, the depth.

This movie (part of our SeismoDome show at the Hayden Planetarium) couples sonified data from 8 seismometers around the globe to a 3D simulation of seismic waves propagating through and around the planet (body and surface waves respectively), after the Tohoku Magnitude 9.0 earthquake in Japan, in 2011:

And here is a simulation of the even longer wavelength, lower frequency waves that occur days after a very large earthquake, called the free oscillations. There are many different modes of these kinds of waves at different frequencies, and you can see how they can add to each other to make it look like there are moving waves (called surface waves, as you saw in the previous movie). Eventually they stop moving, and the Earth just rings like a bell: Free Oscillations: