Six underground blasts took place in the Soviet Union's western Kazakh Republic yesterday, according to the U.S. Geological Survey.
The blasts were recorded by the survey at five-minute intervals beginning at 1 a.m. EDT in the area north of the Caspian Sea, said spokesman Russell Needham, a geophysicist at the survey's earthquake information center. The blasts ranged from 5.3 to 5.6 on the Richter scale.
He said the survey was unable to say whether the disturbances were nuclear tests, but said that "nothing in nature occurs exactly like that in five-minute intervals."
In the past, Needham said, the survey has monitored seismic disturbances in similar intervals in the area near the Caspian Sea that have later been determined to be nuclear tests.
He said he did not know the explosive power of the blasts, which were also recorded in Sweden, Switzerland, Italy and India. Reuter reported that the Swiss National Seismological Service said the blasts were equivalent to between 50 kilotons and 70 kilotons of TNT--about three times more powerful than the bomb dropped on Hiroshima in 1945.
U.S. officials said the occurrence of the blasts would not be unusual if they were conducted within the Soviet Union's normal test areas. The Soviet Union generally conducts its underground weapons tests in the eastern Kazakhstan area of Semipalatinsk or on the island of Novaya Zemlya in the Arctic Ocean, they said. The disturbances that were recorded yesterday took place more than 1,300 miles west of Semipalatinsk.
"If they were outside the normal test area, it would be unusual, and cause for further analysis," said Joseph Lehman, spokesman for the U.S. Arms Control and Disarmament Agency.
Lehman also said the sequence of six explosions at regular intervals was "unusual," but added that it was "unclear what such a series of tests might indicate."
Both the Soviet Union and the United States are parties to the limited test ban treaty of 1963, which bans testing of nuclear weapons in the atmosphere. Each side is also observing the limits of the threshold test ban of 1974, which limits the explosive power of underground tests to the equivalent of 150 kilotons of TNT.
A spokesman for the Department of Energy, which monitors and conducts the underground tests for the United States, said he had not been notified of any underground nuclear tests in the Soviet Union yesterday. The spokesman, James Cannon, said, "It takes some time to verify these things, especially over the weekend."
In Stockholm, scientists at the Seismological Institute at Uppsala University said this was the third time they have recorded similar blasts from the area of the Caspian Sea near the Ural River, United Press International reported.
Indian and Swedish scientists speculated that the blasts might be connected with the construction of a new waterway, the news agency reported.