Mapping Earth's Unseen Region

Article excerpt

The first global map of the Earth's core-mantle boundary, a 200-mile-wide swath of geology 2,000 miles beneath the planet's upper crust, has been rendered by Michael E. Wysession, assistant professor of earth and planetary sciences, Washington University in St. Louis. The core-mantle boundary, a blazing (6,000 [degrees] F) chemical cauldron, is the rocky interface between the Earth's hard mantle and its liquid iron core. Of all the unseen regions of the planet, the core-mantle boundary most resembles the Earth's surface in terms of chemical, fluid, thermal, and liquid-solid interactions. The new map will help geologists understand how the Earth is evolving and how, from its dynamic inception 4,500,000,000 years ago, it is cooling off in space.

Geologists literally can not see what is beneath the Earth's surface. Instead, they are limited to remote sensing through seismic-wave analysis of the world's deep earthquakes. Combined with rapid and sophisticated computation, they can get images of structure and activity. To produce his map, Wysession analyzed eight years of earthquake data, focusing on digital diffracted P waves, which, of all seismic waves, spend most of their time and energy traveling around the core-mantle boundary. Seismic P waves travel through enormous slabs of rock in a domino effect, the way each metal link pushes the next in an extended, shaken Slinky. Diffraction works on the same principle and lets people hear sound around the corner of a building--the waves actually bend around a structure. Thus, they arrive at their destination muffled, though intact.

Wysession solved the mapping difficulty by overlaying the seismograms of 543 diffracted P waves atop synthetically generated seismograms from a computer program. He then determined the difference in travel time between the diffracted P waves and accompanying PKP waves from the same earthquakes. Unlike diffracted P waves, which bounce from an earthquake's hypocenter down 2,000 miles along the boundary, PKP waves go directly through the inner core of the Earth. By measuring the time differential between these two waves and using the synthetic model to eliminate discrepancies, Wysession determined if the P waves traveled slow or fast around the core-mantle boundary. …