An apparent Soviet violation this month of an agreement to limit the size of underground nuclear tests may lead the White House to demand that American electronic monitoring equipment be placed at the major Soviet test site, according to informed sources.

The Carter administration has been looking for a forceful response to the Soviet explosion of a test device "that probably was around 500 kilotons," a source said yesterday.

Since 1976, both the United States and the Soviet Union have said they would abide by the 150-kiloton limits of the still-unratified Threshold Test Ban Treaty. The treaty was signed in Moscow in the summer of 1974.

Establishment of a U.S. unmanned seismic station at the Soviet test site near Semipalatinsk in south-central Russia is only one of several proposals that will go before a White House committee this week.

Another plan calls for the United States to explode a 500-kiloton device of its own.

The Carter administration, sources said, appears determined to do something beyond the formal complaint that was lodged with Soviet Ambassador Anatoliy F. Dobrynin.

The monitoring plan would also permit the Soviet Union to set up one of its own devices at the U.S. nuclear testing site in Nevada and another at the site used by the British.

U.S. British and Soviet delegations have already studied the use of the unmanned seismic stations as part of the negotiations for a comprehensive test ban treaty.

The U.S. device would be able to measure seismic signals from any Soviet underground explosions at the Semipalatinsk site and then transmit them on to an American-based receiving station.

The United States has been developing the unmanned station at a West Virginia site for several years. In the summer of 1979, U.S. officials demonstrated it to a visiting Soviet scientific delegation.

This past summer in Geneva, the three negotiating countries agreed in principle on verification measures for a comprehensive test ban treaty that included use of several such unmanned stations in each country.

Sources said yesterday that British Prime Minister Margaret Thatcher has been contacted on the idea of putting in the stations and approved it.

During the comprehensive test ban treaty talks, the Soviets told negotiators that if the United States wanted to put one unmanned station inside the Soviet Union, it would have to send two so the Russians could open one up to assure themselves it did not contain intelligence collecting devices.

Sources within the administration and on Capitol Hill who are aware of the proposals now under consideration look on the monitoring plan as the one that is currently most likely to go to the president.

"We could not prepare for a 500-kiloton test before December," one official said. He added that it would not meet any specific problem need, although one could be devised.

U.S. analysts are studying recent Soviet defense moves trying to determine why the decision was made to conduct the largest nuclear device test since 1976, an explosion that so obviously exceeded the limits of the agreement.

"It had to have been on purpose, one scientist said after examining the data. He added that additional information may come in from fallout. There is some indication that the explosion vented radioactive material into the air. i

One group of analysts believes the explosion is part of a pattern snowing a hardening Soviet defense policies.

There is, however, another interpretation among administration specialists. In this view, the high-yield Soviet test may be an intentional signal by Moscow that Soviet patience is wearing thin with U.S. failure to ratify the Threshold Test Ban Treaty six years after it was signed to Moscow by the Soviet leader, Leonid Brezhnev, and then-president Nixon.

It could be just a "short across the bow," one official said, with the Soviets asking the United States, in effect, "why should we go on behaving as though this treaty was in effect?"

The Ford administration delayed sending the threshold treaty to the Senate for approval in 1975 and 1976 in the belief it could not be obtained.

Conservative senators were against it because the 150-kiloton limit was far below the size of the warheads they wanted built. Liberals opposed it on the ground it permitted some testing and would delay consideration of a comprehensive test ban.

While both sides have abided by the threshold levels -- overlooking some earlier Soviet shots that could have exceeded them -- both sides and the British have pushed ahead with talks on the comprehensive test ban agreement.

The Soviets have backed off somewhat from their initial position against all monitoring and on-site inspection on their soil.

There is, however, strong opposition to the treaty within the United States among the Joint Chiefs of Staff and the government's nuclear weapons laboratories if the Carter administration plan for no nuclear tests of any size is pressed.