IN 1998, Japanese seismologists noticed deep rumbles in the ground that could not be linked to any earthquakes. They suggested that variations in atmospheric pressure might be causing a drumming on the Earth鈥檚 surface, leading to the low-frequency hum.
But Barbara Romanowicz of the University of California at Berkeley was not convinced. 鈥淔rom the beginning, I had a hunch that the oceans might be involved,鈥� she says. 鈥淏ut then I had to prove it somehow.鈥� So she and her colleague Junkee Rhie collected data from networks of seismometers in California and Japan, and worked out the direction the hum signal was travelling and its source on each of 60 earthquake-free days in one year. They conclude that the hum is strongly linked to stormy seas.
Between January and March, the hum originated mainly from the north Pacific. Then the source shifted to the Southern Ocean before moving back to the northern hemisphere in October (Nature, vol 431, p 552). The source of the hum seemed to be tied to winter in each hemisphere, when ocean storms are at their worst.
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Romanowicz suggests that the Earth鈥檚 crust shakes as storms set the oceans sloshing, causing the hum, which has a frequency range of between 2 and 7 millihertz.