Sound track of the earth

TO PROBE THE innards of the earth, scientists no longer need to bore holes into the crust. According to Pierre Poli and his colleagues at the Institute of Earth Science in France, seismic noise can unravel the mysteries of the earth.

Seismic or ambient noise is the collective sound that arises from ocean waves, storms, human activity, earthquakes, volcanoes and other sources and gives rise to seismic waves. It depends on physical properties of the earth’s interior like rock composition, temperature and pressure. Conventionally, scientists have tried to map the earth’s interior with the aid of seismic waves produced by earthquakes. But the method has its drawbacks. “Main limitation with seismology is that we do not control the source of energy that we use for the imaging,” says Poli. “Big earthquakes cannot be predicted and take place in a limited number of locations. Finding ways to image deep earth that are not dependent on earthquakes is, therefore, of great importance.”

The scientists processed year-long data from 42 seismic recording stations in northern Finland. After eliminating seismic waves from earthquakes, they compared the data from different stations. The findings were then compared to the existing model of the earth. Although most methods for imaging the earth make use of waves propagating along the earth’s surface, the scientists used “body waves” instead. “Our results are the first to show that so-called body waves propagating deep in the earth can be recovered through analysis of seismic noise,” says Poli.

The body waves that are generated in the ocean basins travel downwards into the deep earth. According to the scientists, signals from the body waves generated in the ocean basins are recorded at a seismic station. After these waves travel into the earth, they are reflected back to the surface by the interfaces they encounter. This is registered at a second seismic station. The scientists used this data to image the thickness and structure of the transition zone between the lower and the upper mantle of the earth’s crust. The results were published on November 23 in Science.

In a related perspective published in the same issue of Science, German Prieto, assistant professor of Physics at the University of Andes in Columbia, says, “There have been multiple seismic deployments across continents in last 10-20 years; analysis of that data using this technique should reveal further evidence for body waves from the deep earth. The combined results may help resolve the topography of the mantle transition zone.”

Planets; Science; … Science & Technology