President Carter's decision to build the MX blockbuster missile will put into effect only part of the Pentagon's master plan for modernizing the nation's strategic arsenal, defense officials said yesterday.

The president chose the MX mobile missile, they said, as one way to make the U.S. nuclear offense less vulnerable to increasingly accurate Soviet warheads.

Still facing Carter are other tough decisions, including how may cruise missiles to deploy and whether to change the character of the U.S. missile submarine force by arming it heavily enough to knock out Soviet ICBMs - a capability called counterforce.

The president made his MX decision, Defense Department officials said at a background briefing at the Pentagon yesterday, in the context of how to make all three legs of the strategic "triad" - land-based of ICBMs, submarine missiles and "air breathing" bombers and cruise missiles - less vulnerable in the 1980s and 1990s.

Carter was not pressed to make decisions right away on anything but the land-based ICBMs, partly because Pentagon specialists believe the Soviets do not yet threaten the other two legs of the traid to the same degree.

Defense Secretary Harold Brown has predicted that Soviet Warheads will be able to knock out "most Minuteman" ICBM silos "in the early 1980s." The current U.S. land-based strategic missile offense consists of 54 old Titan ICBMs and 1,000 Minuteman missiles. All are in underground silos, making them easy targets for Soviet gunners.

The White House, in confirming that Carter has decided to build the MX, yesterday said that this harder-to-hit mobile missile would "strengthen the stability of the strategic balance."

White House Spokesman Rex Granum said further that Carter "doesn't believe we can have serious discussions with the Russians if they have any advantage in the arms race."

Secretary Brown has argued that since the Soviets already enjoy the advantage of threatening existing U.S. land ICBMs with near total destruction in a surprise strike, the United States must respond in the same way - meaning a counterforce capability on both sides.

Defense officials conceded yesterday that, given the action-reaction pattern of the arms race, the deployment of the MX may drive the Soviets to making their own land missiles mobile and thus harder to hit.

Granum said yesterday that Carter is "prepared to discuss" the implications of both sides deploying mobile missiles when he meets with Soviet Premier Leonid I. Brezhnev in Vienna June 15-18 for the signing of the strategic arms limitation treaty (SALT II).

Each MX is designed to carry in its nose 10 H-bombs, each with an explosive power of 335 kilotons. The Carter administration intends to deploy 200 MX missiles, meaning 2,000 nuclear warheads, and to dismantle some older Minuteman missile to stay within the limits established by SALT II.

To confront Soviet gunners with more than just a moving target, the Pentagon plan calls for shuttling each MX missile between underground stations spread along 20 miles of a special railway 10 feet below the surface of the earth.

The Soviets, defense officials assert, would not know which of the 8,000 stations planned for the network held MX missiles. The Soviets would have to aim at least one warhead at each of the 8,000 stations to be sure of destroying the MX missiles in a surprise strike, a discouraging proposition, according to the Pentagon.

Some arms control specialists counter that such deployment schemes will only result in a warhead race as each side deploys enough to cover all the missile hiding places the other constructs.

Although the Pentagon's deployment plan calls for digging 200 trenches, each 20 miles long and holding one missile, on government land in Arizona, New Mexico, Nevada and Utah, it could be accomplished in stages. The Pentagon estimates it will cost $30 billion and become operational in 1986.

During the last several weeks or arguing within the administration over the best way to reduce the vulnerability of the U.S. nuclear offense, Adm. Stansfield Turner, director of the Central Intelligence Agency, was said to be at the forefront of those who contended sending more strategic missiles to sea in submarine was preferable to trying to give land missiles more protection.

Now that Carter has killed the B1 bomber and decided to build the MX, he must soon make other tough choices, including whether to keep developing the Trident II missile for the new Trident submarine fleet now under construction. The Trident II missile, like the MX, would have enough accuracy and punch to destroy Soviet ICBMs.

But historically, the United States has advertised its missile submarine fleet as strictly a second-strike, retaliatory force which does not threaten Soviet ICBMs.Deploying Trident II would mark a significant change.

Another hard decision is how many cruise missiles to deploy and whether to build an airplane to carry them rather than settle for the B52 bomber as the launching pad.

Although Carter has pledged to work toward eliminating nuclear weapons, Pentagon weapon plans now before the president could result in nearly doubling the nation's current arsenal from 9,200 warheads to 17,000. CAPTION: Illustration, The MX Blockbuster Missile, The Washington Post