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Australia Gets Rocked By Seismic Mystery

September 1997

It's a tale from Down Under, set against a backdrop of international terrorism. On a dark night in May 1993, somewhere in the empty miles of the dry-as-dust Australian outback, aborigines prospecting for gold saw a streak blaze through the sky, and felt the ground shake.

It happened near a ranch owned by the Japanese cult Aum Shinrikyo, the group accused of the poison-gas attack on Tokyo subways in 1995. Investigators in Australia and the United States raised concerns that the seismic event might be the result of cult activities. Cult followers had recently acquired land in the outback, and were known to be mining uranium and carrying out weapons tests there.

The U.S. Senate's Permanent Subcommittee on Investigations requested that scientists affiliated with the NSF-supported Incorporated Research Institutions for Seismology (IRIS) in Washington, D.C. look into the incident.

After sleuthing through the data, the investigators concluded that the event was not a nuclear detonation. They offered an alternative hypothesis, which they presented at the American Geophysical Union meeting in May.

"If the eyewitness accounts are credible, the seismic signal was most likely created by the impact of an iron meteorite about two meters in diameter," says IRIS seismologist Gregory van der Vink. Such a meteorite could survive passage through the atmosphere and impact Earth with sufficient energy to create the seismic signal picked up by one of the stations in the Global Seismographic Network.

But, as there is no previously known digital seismic signal from a meteorite impact, "we have nothing to compare this record to," adds Christel Hennet, van der Vink's colleague at IRIS. The IRIS scientists--along with researchers Danny Harvey of the University of Colorado, Chris Chyba of the University of Arizona, and Vipin Gupta of Sandia National Laboratory--estimate that this kind of meteorite impact is a once-in-a-decade event. What's more, they say, it should have created a crater.

The researchers are looking for a hole the size of a football field, difficult to overlook in populous regions of the world, but perhaps hard to find in the wide Australian outback. "We now have a good determination of the location from analyzing the seismic records," says van der Vink. "We're closing in on an answer to this mystery."

If a crater is never found, he says, and there is no credible alternative explanation for the eyewitness accounts, "We may never know what happened that night, in that remote corner of Australia."

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