Explosions that might not shock

Press, 14 October 1985, Page 12

Explosions that might not shock

The problem in policing a test ban treaty is whether you can be sure of detecting a secret nuclear test. DAVID FAIRHALL, of the “Guardian,” examines the troubles of a treaty.

The underground shock waves of the Mexican earthquake reverberated round the earth for hours afterwards. In a world where all nuclear bomb testing was supposed to be banned by treaty, it would have been an ideal moment for a nation intent on cheating to try out a secretly developed weapon.

The relatively short sharp bang of the illicit underground test would be lost in the. great rolling noise of the earthquake, masking its record on the sensitive seismic recorders operated by potential enemies.

But big earthquakes happen unpredictably, perhaps once a year. How easy would it be to cheat for the rest of the time?

Disarmament campaigners have recently refocused their attention on the prospect of outlawing all nuclear tests, even small underground ones that do not release dangerous radioactive fallout. A Comprehensive Test Ban Treaty of the kind the Americans, Russians and British tried to negotiate some years ago would have a direct impact on the arms race. It would also — incidentally — help to persuade the non-nuclear weapon states that have renounced nuclear arms under the Non-Pro-

liferation Treaty that it is worthwhile keeping their bargain. The Soviet leader, Mr Gorbachev has caught the mood — with what sincerity remains to be seen — by annnotmcing a five-month unilateral moratorium on nuclear testing and inviting President Reagan to follow suit. This United States Administration says it will not reopen negotiations for a comprehensive test ban treaty until it has completed tests for new weapons like the Midgetman missile and not even then until it’s sure the Russians cannot cheat.

The scientific question of whether small underground disturbances (that is earthquake tremors, nuclear- bomb tests or chemical explosions can reliably be detected, located and identified by modern seismic techniques has become crucially important. A number of American scientists, notably the seismologist Dr Jack Evernden of the United States Geological Survey, have already gone on record to the effect that there is no longer any technical obstacle to the comprehensive test ban. Dr Evernden puts his faith in a new technique of high freqency seismic analysis whose advocates claim will be able to detect small nuclear explosions

down to the equivalent of one kiloton (a thousand tons) of TNT. Dr Evernden and others latched on to the new technique some time ago when one particularly well placed sub-array in the Norwegian seismic network detected high frequency vibrations from a series of smallish Soviet tests thousands of miles away in the Caspian Sea area.

The results were so clear, it was possible to extrapolate that a distinctive trace would have shown up from a much smaller test, with a yield of no more than 30 tons. A new Norwegian recording station to exploit this possibility NORESS (Norwegian Experimental Seismic Station) was formally opened near Oslo on June 3. It’s now busy “listening” to whatever it can hear from local quarry blasting and other disturbances that may equate to a distant Soviet underground test. The trouble, of course, is that a highly sensitive seismograph is capable of picking up all kinds of extraneous noise from railway trains to waves beating on a rocky shore.

If the telltale traces can be filtered out of the NORESS recordings, the next job is to find at least two other sites, as widely spaced as possible, so as to use the different timing to calculate distance from the explosion and enable its position to be plotted. That is not going to be at all easy, according to the cautious Ministry of Defence scientists, since unless you can find an

exceptionally good geological path — such as evidently exists in one part of Norway - high frequency tremors will only propagate for a few hundreds miles. Even an optimist like Dr Evernden seems to assume that full verification will involve a large network of stations scattered round the Soviet Union — and of course the United States. Until now, the main way of distinguishing small earthquakes from man-made explosions has been to compare the waves which come right through the core of the earth with those that run along the surface. The ratio between the two sorts of waves tends to be consistently different, so that by plotting it for each recorded disturbance, you get two reasonably clear groups — earthquakes and explosions — with a small proportion sitting doubtfully on the borderline. . It is this approach, supported by other established techniques, that underlies the British Foreign Office position — that an illicit test could be carried out without appearing above the seismic detection threshold. A test of up to about 50 kilotons, might be deliberately “decoupled” that is set off in a large cavern — so as to muffle the vibrations. Without access to the U.S.S.R., and using current techniques, the Defence Ministry team reckon they can reliably detect decoupled tests down to about 100 kilotons.

This does not mean that a Comprehensive Test Ban Treaty is

unnegotiable. The ban President Carter was trying to achieve would only have lasted for three years anyyray before being reviewed in ; the light of actual experience; and in that time the disarmament community believes it might have led to other things. Without i the necessary political will and a‘certain measure of trust on all sides, no such agreement would work. Some suspicious American‘ sicentists now believe the Russians are exceeding the limits of the Threshold Test Ban Treaty, but they cannot be sure since they have not been able to calibrate ‘their seismic recorders against a J test of known, independently checked yield in a geological structure! that has been properly mapped. This is where the United States call for on-site inspection comes ini In the context of a complete test ban, on-site verification means being able to send in a team to inspect the site of a suspiciously bomb-like disturbance. The Soviet delegation to the negotiations on a comprehensive test ban treaty accepted this in principle, and also agreed to abandon its; “peaceful nuclear explosions” programme, that is explosions used to shift earth or divert rivers. ■

Both; these concessions were seen at. the time as evidence that the Kremlin really wanted a treaty. The talks were abandoned at the point where the three delegations were arguing about the procedure for on-site inspections.