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A CAT scan for earth.

Using seismic waves from earthquakes and underground explosions in much the same way doctors utilize X-rays to look inside the human body, geologists are painting the most detailed pictures ever of the Earth's great rifts. Working in the Kenyan portion of the African Rift, they have employed new techniques to chart, in three dimensions, the subterranean features of the longest continuous crack on the planet's face.

"This was like a CAT scan for the Earth," notes University of Wisconsin-Madison seismologist Robert P. Meyer, describing a set of experiments that concentrated scores of portable seismic recorders along the rift in an effort to get a snapshot of its hidden structures. Rifts are important because scientists believe they are a manifestation of the same processes that spawn oceans and cause continents to collide.

The African Rift extends from the Red Sea through the Gulf of Aden and across half the length of Africa. Unlike many active rifts, which occur under oceans, the African Rift extends onto the continent, making its study by researchers relatively convenient. Until recently, rifts were regarded simply as the surface manifestations of splits in the Earth's crust. These splits, many geologists believed, were caused by the diverging motions of the great plates that compose the Earth's surface.

Rift valleys were thought to denote abortive splittings of the crust, geologic events akin to the ancient event that split North America from Europe and Africa and created the north Atlantic Ocean, Although the African Rift and others have been studied extensively and their surface structures are fairly well-known, their underlying features are little understood. As scientists sift through the results of major field experiments, an intriguing picture of the deep structures of the African Rift is emerging.

The Kenya Rift International Seismic Project (KRISP) - a major international effort involving scientists from Kenya, Germany, Great Britain, Denmark, Italy, and the U.S. - was begun in 1985 and has involved some 50 researchers, technicians, and support staff. A two part experiment, with an active phase wherein seismic waves were created by underground explosions, and a passive phase during which the shock waves of numerous distant earthquakes were recorded, the goal was to sounded the rift to depths of 120 miles of more. Using 54 portable seismographs, placed at 15- to 20-mile intervals in lines roughly perpendicular to the rift, the KRISP team was able to make the most detailed study ever of the underground features of the African Rift.

Working much like a giant CAT scanner, the seismic arrays were used to sense, from all directions,the waves generated by numerous distant earthquakes and underground blasts. With the aid of computers, these underground "snapshots" were combined to give scientists their first three-dimensional image of the rift structures that lie far below the Earth's surface. "The goal," Meyer explains, "was to find out not only what happens in the crust, but also below the crust."

The picture that is emerging from the KRISP experiment shows an abrupt thinning of the Earth' crust near the center of the rift valley and slightly thicker to the west than to the east. These variations in thickness - as much as nine miles in some places - were unexpected and suggest an extension of the Earth's crust forced by a huge subterranean dome of partially molten rock.

The most logical explanation for this underground dome, Meyer suggests, is a hot spot at the boundary of the Earth's core and mantle Pushed upward by the energy from the hot spot, the dome, in turn, exerts force on the crust and causes it to extend and rift.
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Title Annotation:seismic charting
Publication:USA Today (Magazine)
Date:Jun 1, 1993
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