TWO: HOW DO WAVES PROPAGATE IN THE EARTH?

IIa. Spatialization of sound
Now we zoom out, and listen to earthquakes at a global scale. First, to communicate the propagation of waves with sound, we return to the hand claps. If you are using earphones, you will hear the movement of the clap sounds from left to right and back, to give a feel for wave propagation around the globe. In the sound ring, the claps will move around you.


For the rest of the sounds in the website or exhibit, this spatialization represents the distribution of stations around the globe, as shown here:



Here are sounds from the exact same earthquake in Japan that we just listened to on a local scale, near the source, as the waves propagate across Japan. Now we are listening to the waves propagate around the Earth, much farther away from the source. The waves have much much smaller amplitudes, none that you could really feel, and are recorded with much more sensitive seismometers.
Niigata Earthquake, Global sounds (GSN stations), (5 hours of data), 20 sps

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IIb. DIFFERENT KINDS OF WAVES: Surface and Body
Surface waves feel the surface, moving relatively slowly with larger amplitudes.
Body waves move through the inside, more quickly and smaller amplitude, higher frequency...
To illustrate the differences, we compiled a small collection of others' movies:

1. Surface wave propagation:
Charles AMMON's movies of the data from the transportable array (TA) of seismometers of the Earthscope project show beautifully the wave propagation across the surface of the Earth:
http://eqseis.geosc.psu.edu/~cammon/QA/12Sep2007-Sumatra-3c-divx.mov




2. Body wave propagation:
Paul Tackley's page of wave movies:
http://www.gfd.geophys.ethz.ch/~pjt/wavemovies/movies.html
see movie 2a.

3. Body wave propagation:
A 3-D (spectral element) simulation of wave propagation in the globe, from Jeroen Tromp's group, Princeton (formerly CalTech).
From Jeroen Tromp's group:
http://www.gps.caltech.edu/~jtromp/research/global.html
and

http://www.gps.caltech.edu/~jtromp/research/movies/bolivia1.avi


IIc. EARTHQUAKE SOURCE: Shallow vs Deep
Here we compare two seismic sounds from the Andes, to illustrate the importance of the depth of an earthquake in terms of how its waves propagate, and also how we know the depth of an earthquake.


Andes, Mw 5.7, off coast of Peru, 2004/12/18, 17:57 GMT
shallow source: 24 km.
12 hrs of data, 20 sps

This earthquake is shallow, near the trench of the subduction zone. Much of its energy is converted to surface waves, which dominate the signal around the globe.

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Argentina, Mw 6.8, 2005/03/21 12:23 GMT
Depth: 572 km (!)
12 hrs of data, 20 sps


The source of this earthquake is very deep, almost 600 km below the surface, inside a cold, rigid subducting slab. Because the source is so far from the surface, it has no surface wave energy... only body waves. For shallower earthquakes, the surface waves dominate the waveform and the sound. Here, you can hear the pinging of the body waves as they hit different stations around the globe.
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(Now watch the deep bolivia wave propagation movie again!)

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