Harder Than It Seemed

The international Conference of Experts that met in Geneva last summer concluded that a worldwide system of 180 inspection stations could detect nearly all underground nuclear tests. Any explosion of even the modest energy of 5 kilotons, they figured, could be distinguished from the noise made by small earthquakes and other natural causes. Only about 100 seismic "events" a year would be borderline cases. These could be followed up and checked by other means.

Two weeks ago the U.S. Government announced that new information has cast doubt on the effectiveness of the 180-station system. Last week the U.S. Department of Defense released the data that led to this conclusion. To measure the seismic effects of underground tests, 16 special seismographs were set up in a line extending from the Nevada atomic proving ground to Maine, 2.500 miles away.

The stations listened attentively during October while the AEC made underground tests in faraway Nevada. The rock waves came through all right, but not quite as strongly as had been anticipated. At distances above 700 miles, only explosions of more than 20 kilotons could be identified clearly as manmade. To sum up, said the panel, the 180-station detection system might be confronted by 1,500, not 100, natural seismic shocks a year that could not be distinguished from an underground test explosion. This number would presumably overburden the checking system as presently outlined.

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