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Title: Earthquake Detector Finds Solid Earth Core
Source:
Discovery Channel
URL Source: http://dsc.discovery.com/news/briefs/20050418/earthcore.html
Published: Apr 20, 2005
Author: Larry O'Hanlon, Discovery News
Post Date: 2005-04-24 17:36:40 by robin
Keywords: Earthquake, Detector, Finds
Views: 15
April 20, 2005 — For the first time, the solid inner core of the Earth has been directly detected and its existence confirmed, seismologists have reported. New evidence of a solid iron inner core to the planet comes from a digital broadband seismic array in Germany that is located in a lucky enough position to have captured a faint, but telltale, seismic signal. The signal was sent through the Earth from a particularly clear sort of earthquake deep in the crust on the other side of the planet. The seismic discovery was announced in the April 15 issue of the journal Science. "The earthquake they used was at the right depth and magnitude," said seismologist Adam Dziewonski of Harvard University, referring to the discovery team from University of California-Berkeley and the University of Tokyo. "It provided a lot of energy in just the right phase." That energy traveled from the earthquake rupture zone, deep in the crust under the South Pacific, downward into the Earth's mantle in the form of pressure waves, called P-waves, which are a lot like sound waves. When those P-waves pass into the liquid outer core, they are changed into shear waves (S-waves) and deflected slightly, similar to how light is shifted when it enters water and makes a straight straw look bent. The S-waves then pass into the solid inner core, deflecting a bit more and becoming P-waves again. Then same series is done in reverse as the seismic waves pass out of the inner core, through the outer core and mantle and up to the Gräfenberg Seismic Array in Germany, about 140 degrees away from Tonga on the great circle of the Earth. If the Earth were homogenously solid or liquid, the seismic waves would have passed straight through to 180 degrees without any deflections or changes. The special, inner-core-detecting series of wave changes is called PKJKP, with each letter denoting a change as it passes from source to seismograph. The J is the part of the wave that passes through the solid inner core. Further confirmation of the PKJKP will come from additional work on historic seismograms, said co-discoverer Aimin Cao of University of California, Berkeley. For instance, there could be more PKJKPs to discover in Japanese seismic data, he said. The Gräfenberg Seismic Array was a good place to start looking, Cao said, because it is the oldest array of its type and has the most data search through for hidden PKJKPs. Other seismologists have been trying to detect PKJKPs since the 1970s, said Dziewonski, but failed. "This is the first one that I really believe is convincing," he said. Besides requiring the serendipitous alignment of the right earthquakes with the right seismic array, Dziewonski said, you just have to be lucky to catch seismic waves passing through the relatively small inner core. "It's very small," Dziewonski said, "less than one percent of Earth's volume."
Cutaway of Planet Earth
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