This is a really neat bit of science, even though it is early days. It will require years of monitoring to maximize its value but obviously well worth doing. Most importantly, it provides an independent check on seismic data and its interpretation.
Any single method of data production is vulnerable to some form of interpretation error creep without a check variable that sees or does not see what one method senses.
One commenter suggests that this may explain the reactions of animals to pending seismic events. There I am more inclined to blame piezoelectric activity.
We now have a reason to collect all that taw data on a continuous basis. Several years is not very long if we will end up with steadily improving resolution. It is not as if we are dealing with turbulent activity.
Earth's 'hum' reveals hidden depths
Oct 5, 2009 3 comments
In Jules Verne's science-fiction classic of 1864, an intrepid professor embarks on a curiosity-driven Journey to the Centre of the Earth. While this type of exploration has remained firmly in the realms of fiction, scientists have since found an indirect means of "seeing" beneath the Earth's surface. Studying how earthquake-driven shock waves travel through the planet has provided geophysicists with a basic picture of the Earth's interior.
The limitation with this method of imaging, however, is that it is restricted to the zones where earthquakes occur, the vast majority of which are concentrated along plate boundaries. What is more, the technique can only be deployed when seismic activity is occurring – earthquakes are notoriously difficult to predict and they can strike in zones that have been seismically quiet for very long periods.
Now, Kiwamu Nishida at the
Mother Nature calls
Strange as it may seem, the Earth's atmosphere continuously rings out in a chorus of frequencies just below the reach of the human ear. This phenomenon is expressed at the Earth's surface as "infrasonic" waves – that is, waves with frequencies ranging 0.01–10 Hz – that are known to exist from acoustic recordings around the globe.
Until now, however, researchers have been unable to link variations in the Earth's hum with features in the mantle. This is largely because of the complicated processes involved the hum's generation and the presence of various other sources of ambient seismic noise.
Nishida and his team get around this problem by studying records of the Earth's ambient noise over a long time period – between 1986 and 2003. In this way, they were able to disentangle the hum from the different sources of ambient noise and observe how it varied over periods of 100–400 s. They could then link fluctuations and geographical variation in the hum with the physical and chemical composition of the mantle.
Mapping the mantle
The geophysicists measured the speed of sound in the mantle to depths of 340 km beneath Asia, North and South America and
Sebastian Rost, a geophysicist at the University of Leeds in the UK, is impressed by the new development and sees a lot of potential. "Understanding the structure of the upper mantle is important to better understand plate tectonics, especially the way subducted plates are recycled at subduction zones and new crust is generated at mid oceanic ridges."
Rost is slightly concerned, however, that the researches require a seven year timescale in order for the technique to work. "Seismic data are lost every day due to archive being closed to save money or decaying magnetic tapes or data not available for research in the first place."
This research appears in the latest edition of Science.
About the author
James Dacey is a reporter for physicsworld.com