Behind the turgid drama of the SALT II debate, the Soviet Union has been engaged in intensive underground weapons testing with indications of repeated violations of two test-ban treaties, on limiting the size of weapons tested, the other prohibiting "venting" of radioactive fallout into the atmosphere.

This hyperactive Soviet testing, the most intensive in years, has been overlooked in the vigorous Carter administration effort to ratify the new strategic arms limitation treaty. But there are growing indications that the Kremlin may be systematically cheating in the rush to complete a vase "data base" for building new and better nuclear explosives before U.S.-Soviet agreement on the comprehensive test-ban treaty now being negotiated.

A key issue here is U.S. trust of its Russian negotiating partner, a trust that President Carter's administration had seemed particularly inclined to accept.

Lack of healthy suspicion was the crux of the tragic failure of the United States to talk privately to Ludmilla Vlasova, the Soviet ballerina whose husband defected and who might done so herself if she had had the chance. Alerting Soviet diplomats here in advance to the face that the United States wanted that private talk made it impossible. She was spirited off to an Aeroflot IL-62 by Soviet secret police.

The Soviets' record of actual and attempted cheating is unambiguous, beyond the ability of Soviet, apologists to disprove. Yet such "gentlemanly" conduct as Deputy Secretary of State Warren Christopher's telephone call alerting the Soviet embassy here to U.S. interest in the Vlasova affair persists. In the infinitely more important case of Soviet underground weapons testing, trust in Soviet declarations or intentions could lead to graver results.

During 1978, at least three underground tests (among the estimated 27 exploded in the Soviet Union) are believed to have violated the 1974 "threshold" test-ban treaty with its 150-kiloton ceiling. In addition, at least 13 of the 1978 tests almost certainly produced illegal "venting"-- the escape of poisonous radioactive materials such as krypton and other fission products into the atmosphere.

The three probable violations occurred in August, November and December. Seismic instruments-- not on Soviet territory because Moscow rejects on-site inspection-- indicated that each of the three tests were well above the 150-kiloton limit.

This year, as the Russians continued their most active underground test program in 15 years, a June explosion is believed to have had the force of 225 kilotons and a July shot possibly as high as 400 kilotons. The United States has never announced the July test.

Soviet refusal to permit on-site inspection by seismic sensor -- and the imprecision of distant seismographs -- make it virtually impossible to prove violations. This is true despite sensitive Swedish seismic instruments that pick up and record most Soviet tests. But Soviet violations of the 1963 treaty can be proved by the empirical evidence of dust particles collected in the atmosphere over Japan, Korea and other areas.

"It is intolerable that the United States should permit the Soviets to cheat at the margin without even raising a protest," one specialist told us privately. Yet, no violation has yet been charged under either the 1974 treaty (which sets the ceiling on explosive power) or the 1963 Limited Test-Ban Treaty (which prohibits venting).

The farthest the Carter administration has ventured is to "raise a question" with Moscow about one of the tests this summer and three of the 1978 tests. The predictable Soviet response: nyet , and you can't prove we did. Furthermore, Moscow is armed with a strange concession it got from the United States: two tests a year may "slightly" exceed the limit.

What makes this produce sleepless nights for some U.S. officials is its implication in the nightmarish world of atomic weaponry. The data base now being accumulated by the Soviets, with a test program perhaps three times that of the United States and rising, will probably be used to perfect warheads for the new crop of post-SALT II long-range missiles.

Thus, if the comprehensive test-ban-- a near-total ban on all tests for a period of perhaps five years-- is agreed on anytime soon, the Soviets could leapfrog U.S. technology by translating its test data into new types of higher-yield, lower-weight weapons with the United States powerless to catch up.

The Soviets have no pyramided their testing program to today's awesome level just for fun. The testing of warheads that violate treaties has its grim purpose: to surpass U.S. weapons technology. Refusal of the United States to challenge Moscow out loud on its presumed violations thus becomes dangerously counterproductive.

Worse, it ensures the very gains in Soviet weaponry that the president insist will be curbed by SALT II-- thus undermining waht he regards as his most important mission.