For the first time, sound created by an undersea earthquake has been measured above the sea surface. First author of the article about this discovery, recently published in Geophysical Research letters, is Dr.
Läslo Evers, who works for the Royal Netherlands Meteorological Institute KNMI and TU Delft (CEG faculty).
The earthquake featuring in the article occurred southwest of New Zealand, on December 23 2004. Though very large, it received little attention, because it was followed about fifty-eight hours later by the devastating Sumatra earthquake and tsunami.
Evers and his colleagues found out that infrasound (sound with very low frequency) which was measured with land based microbarometers on Tasmania had to originate from this earthquake. This is surprising because sound created on the oceans floor can normally not be heard above the sea surface. The water-air interface functions as a kind of mirror, reflecting all sound. “But there is an exception”, says Evers. “If the wavelength of the sound is longer than the water column, then it can pass this interface.”
The sound registration was still puzzling though, because the wavelength of the earthquake acoustics was in the order of a kilometer whilst the earthquake occurred at a depth of more then four kilometers. By combining the data from the Tasmanian ground stations with data from under water microphones Evers and his colleagues were able to reconstruct the travel path of the acoustics and they conclude that the energy was diffracted by a seamount, the Macquarie Ridge, which at some points reaches up to less than a kilometer below sea surface.
“We thus found out that besides the hydro acoustic stations we can also use ground stations to measure what’s going on in the oceans”, says Evers.
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