The people of Colima get rightly scared when the earth shakes. In 2003 there was a magnitude 7.8 that flattened buildings all across the town and killed a number of people. Considering the amplitude of each grade on the Richter scale is 10x larger than the previous and that this earthquake was therefore ~1000x more powerful, it must have been pretty scary.
On Friday I discovered how to convert the traces from the seismograms into sound files so that we can 'listen' to the earthquakes. Volcano seismologists have been doing this sort of thing as part of their analysis of the seismic signals for ages. They use it to detect changes within the plumbing of volcanoes that may one day be a method of predicting eruptions. I did it because the results sound cool. The sounds produced are not real - they are vibrations of the earth that have been sped up 50x so that we can hear them with our ear. But you can clearly tell the difference between various types.
The trace above is from an earthquake. It sounds like this. It is like a cross between someone slamming a door and the rumble of distant thunder. This is the noise of big slabs of rock grinding past each other in a sudden jerk.
This trace is from an explosion at the volcano. It sounds like this. This is the sound of pressurised gases explosively bursting free from the crater, carrying ash and rock fragments with them. It is easy to hear the energy tail off as the pressure is released.A third kind signal comes from harmonic tremor. It sounds like this. It is a bit like whale-loving or a broken trumpet. This type of signal is a bit special, as it is thought to be caused by seismic waves travelling within a gas- or fluid-rich conduit beneath the volcano. Different frequencies are amplified or cancelled-out like sound waves in a music instrument. Volcano seismologists use the changing frequencies of the signals to estimate things like pressure beneath the ground.
