LATE on the evening of May 28, 1993, something shattered the calm of the Australian outback and radiated shock waves outward across hundreds of miles of scrub and desert. Around the same time, truck drivers crossing the region and gold prospectors camping nearby saw the dark sky illuminated by bright flashes, and they and other people heard the distant rumble of loud explosions.
The mysterious event might have been lost to history except for the interest of Government investigators in Australia and the United States who eventually came to wonder if the upheaval was the work of the Japanese doomsday cult accused of the poison-gas attack on Tokyo subways in 1995 that killed 12 people and hurt thousands.
The fear was that the terrorists had acquired nuclear arms or other weapons of mass destruction and had been testing them that night in the Australian wilds.
The hope was that the upheaval was an earthquake, a mining explosion or even a meteorite strike from space, any natural event.
The evidence was ominous. Investigators discovered that the cult, Aum Shinrikyo, had tried to buy Russian nuclear warheads and had set up an advanced laboratory on a 500,000-acre ranch in Australia near the puzzling upheaval. At the ranch, investigators found that the sect had been mining uranium, a main material for making atomic bombs.Continue reading the main story
The clues were judged worrisome enough to set in motion a wide scientific investigation that is still going on today.
''Many experts had dismissed the possibility of nukes'' in the hands of terrorists before the emergence of Aum Shinrikyo, Dr. Gregory van der Vink, head of the science investigation, said in an interview. ''But the group was into biological and chemical weapons and was attempting to acquire nuclear ones. I'm still amazed.''
At the request of Senate investigators, the science inquiry was led by the Incorporated Research Institutions for Seismology, or IRIS, where Dr. van der Vink is director of planning. The IRIS consortium is based in Arlington, Va., and financed by the National Science Foundation, a Federal agency. It has more than 80 member institutions at universities in the United States. It also has more than 100 seismometers on all continents, the largest global network of these devices, which pick up faint vibrations traveling through the earth's rocky interior.
One aim of scientists using the IRIS network is to try to distinguish natural disturbances from nuclear explosions, a goal that recently took on new significance as the United Nations called for an end to all explosive testing of nuclear weapons around the globe. Much research, including that by IRIS members, now aims at monitoring such a ban.
Pinning down the nature of the outback incident was seen as an important test of the emerging skills.
The doomsday cult that caused the worry first caught world attention after the Tokyo subway attack of March 20, 1995, which involved the deadly nerve gas sarin, and was the world's first large chemical strike by terrorists. The cult was quickly implicated and is now charged in Japan with planning a virtual civil war meant to be carried out with some of the world's deadliest weapons.
After the Tokyo attack, government investigators around the world raced to learn more about the shadowy group. Aum Shinrikyo, or Supreme Truth, turned out to have accumulated some $1 billion and to have won more than 50,000 converts in at least six countries.
In the United States, the permanent subcommittee on investigations of the Senate Governmental Affairs Committee, at the urging of Senator Sam Nunn, mounted a major investigation of the cult.
Senate investigators found that the group had thousands of disciples in Russia and had bought guns, a military helicopter and other weapons there. The group's construction minister, Kiyohide Hayakawa, the reputed mastermind of the cult's efforts to arm itself, went to Russia 21 times from 1992 to 1995, visiting there a total of 180 days.
Senate investigators say the cult recruited at least two nuclear scientists in Russia.
Notebooks later seized from Mr. Hayakawa show he wanted to buy the ultimate munition there. In one entry, he asked, ''How much is a nuclear warhead?'' and listed several prices.
In Australia, the activities were just as troubling. Cult members arrived in April 1993, a little more than month before the mystery blast. Mr. Hayakawa, apparently fresh from visits to Russia, was among the initial party. After visiting several remote sites, the group bought a 500,000-acre sheep farm in Banjawam, Australia, about 400 miles northeast of Perth. The site has a known uranium deposit.
The cult eventually brought in chemicals, gas masks and respirators, and picks, shovels, mining equipment and a mechanical ditch digger. It also set up a laboratory stocked with computers, glass tubing, glass evaporators, beakers, Bunsen burners, mixing bowls and a rock-crushing machine.
Documents seized from Mr. Hayakawa include some 10 pages written during his visit to Australia in April and May 1993 that refer to the whereabouts of Australian properties rich in uranium, including one reference praising the high quality of the ore.
The disturbance shook the earth on May 28, 1993, at 11:03 P.M. local time, but it was not until after the Tokyo attack of March 1995, that an Australian geologist, Harry Mason, brought the seismic upset to the attention of Australian Federal Police and Senate investigators. He was prompted in part by public disclosures in June 1995 of uranium mining at the cult's ranch.
Seismic observatories in Australia tracked the event to a location 28.47 degrees south latitude, 121.73 degrees east longitude, a remote area near the cult's ranch. But uncertainty in the readings was such that the actual site of the disturbance could have been anywhere in 485 square miles of Australian hinterland.
Mr. Mason's 19-page report summarized interviews he had conducted with people who were in the remote area on that clear moonless night. They saw the sky blaze, heard loud explosions and felt the ground shake, in one case knocking beer cans off a table.
Mr. Mason noted that earthquakes were very rare in the region and that mining explosions were illegal at night. ''I currently believe that a nuke is a very real possibility but a meteorite and an earthquake cannot be ruled out either,'' he wrote Senate investigators in October 1995.
With that information in hand, the investigators contacted IRIS, which was happy to help and had a number of seismometers in Australia. Though the most sensitive unit was inoperative at the time of the blast, another in the town of Narrogin, near Perth and some 400 miles from the disturbance site, was recording data that evening.
Its tracing showed a smooth line that erupted into a fit of wiggles, suggestive but inconclusive. Joining Dr. van der Vink in the tricky analysis were Dr. Christel B. Hennet of IRIS, Dr. Danny Harvey of the University of Colorado and Dr. Christopher Chyba of Princeton University.
Eventually, the IRIS team calculated that the event was 170 times larger than the largest mining explosion ever recorded in the Australian region, to helping rule out that possibility. The disturbance was calculated as having the force of a small nuclear explosion, perhaps equal to up to 2,000 tons of high explosives. In contrast, an atom bomb with a power of about 15,000 tons of high explosives leveled Hiroshima, Japan. But the signature of the disturbance seemed to be more that of an earthquake or a meteorite strike than a nuclear explosion.
Typically, the shock waves from nuclear blasts begin with a very distinct wave or spike as earth and rock are violently compressed. The signal then tends to become more fuzzy as surface rumblings and shudders and after shocks create a seismologic din.
With earthquakes, it is usually the opposite, with gentle jostling suddenly becoming much bigger and violent.
The main problem for analysis of the mystery event is that such clear distinctions tend to break down when the size of the disturbance under study is relatively small, as was the case in the Australian outback.
Nevertheless, the IRIS experts judged that the violent episode was probably natural in character rather than being a manmade blast, prompting official Washington to breathe a sigh of relief.
No mention of the seismic riddle was made in well-publicized hearings that Senator Nunn ran in late October and early November 1995 on Aum Shinrikyo as a case study of what fanatics with large financial resources can do to acquire weapons of mass destruction.
On Dec. 15, 1995, Mr. Nunn wrote the head of IRIS, David Simpson, thanking him for analytic aid. The scientific work helped Senate investigators ''rule out certain terrorist activities,'' he said.
Still, one remote but still worrisome possibility remained. Aum Shinrikyo had apparently studied seismic warfare and artificially triggering earthquakes, a terrifying prospect for quake-prone Japan.
The cult apparently sent a party of its members to the former Yugoslavia to study the work of Nikola Tesla, the discoverer of alternating current who toyed with the theory of seismic weapons before he died in 1943. At the Tesla Museum in Belgrade, the members seem to have reviewed Tesla's thesis and other research papers concerning such weapons.
Many geophysicists view seismic warfare as a fantasy that is highly unlikely to ever materialize. Even so, the subject has been quietly studied for decades by governments worldwide, including the Soviet Union and the United States during the cold war.
So IRIS investigators kept pressing ahead to solve the Australian riddle, trying to determine if the event was an earthquake or a meteor strike. The team worked with computers and lengthy calculations, eventually showing that an iron meteorite striking the earth at an oblique angle could have created the seismic upset. A meteorite five or six feet wide would have dug out a crater some 300 feet in diameter, the team calculated.
Despite preliminary searches, no impact crater has been found in the Australian outback.
Even so, Senate investigators are increasingly confident that the episode was natural in origin. ''Eventually, we got information that led us to believe the group was out of the country at the time of the blast,'' said John F. Sopko, senior counsel to the subcommittee. ''That pretty much eliminated the possibility of a weapons test.''
The fall 1996 issue of the IRIS newsletter presents the analysis of the mystery event -- the first public disclosure -- and concludes that ''the meteorite impact scenario is consistent with the eyewitness observations and with the energy levels derived from seismic records.''
The team also estimates that such meteorite strikes on land occur at the rate of about once every six years, complicating the prospects for monitoring a comprehensive nuclear test ban.
In an interview, Dr. Hennet noted that IRIS had recently received readings from other seismometers in Australia that should help determine if the meteorite thesis is correct.
She also noted that in 1993 during the mystery event, IRIS had 63 stations in its Global Seismographic Network. Now it has more than 100, sharply increasing the group's ability to conduct its scientific sleuthing.
If the peacefulness of the Australian outback is suddenly shattered again, Dr. Hennet said, ''we could probably identify the nature of the event with a high degree of confidence.''Continue reading the main story