Boxing Day 2004 magnitude 9 earthquake

This is the largest earthquake to have happened in the past 40 years and one causing great damage to areas around the Indian Ocean basin -- damage only just beginning to be known in these days subsequent to the event. The earthquake appears to have ruptured a few-hundred-kilometer-long underwater segment of the plate boundary between the Indian and the Eurasian plates between the northern end of Sumatra toward the Andaman Islands. Based on preliminary analyses of the data, the rupture lasted for almost three minutes due to the length of the fault.

The seismograms reflect this characteristic of the rupture. There are two shown from seismic observatories located on two islands in the Indian Ocean: Cocos Island and Diego Garcia. The P-wave arrives first at Cocos island (top), and later at Diego Garcia. Each seismogram begins with a very slowly growing P-wave arrival over the first few minutes of the event. Then the larger amplitude S-waves arrive, which dominate the rest of the traces.

These seismograms are extremely long: they represent an hour of elapsed time, yet are compressed into shorter stretches of audible time so that you can hear the low frequency energy emitted by this enormous earthquake. They start about 3 minutes after the earthquake. The frequencies are shifted upwards about 2560 times by the processing methods used here, and the seismograms shortened in time by a factor of 100.

The Cocos Island seismogram (top) plays in your left ear and the Diego Garcia seismogram (bottom) plays in your right. Diego Garcia is around 1000 km farther from the epicenter than Cocos Island, so you appear to hear a wave passing from left to right due to the stereo effect.

If you want to hear the low-frequency content of the signals better without distortion, there's an alternative audio track to listen to. Use headphones to listen to this one, which lets you hear the low frequency content of the seismograms better. The rumbling wanes to some higher frequency noises sounding like the whip of a fly-fishing rod being cast. These are high-frequency waves emitted downward from the earthquake that reflect off the far side of the Earth, arriving much later than the P- or S-waves travelling directly from the quake to the seismometers.
Boxing Day 2004 (magnitude 9) earthquake
Alternative audio