What if the whole fleet sank?

The chain of explosions that crippled the Soviet Union's Northern Fleet could help both superpowers move toward a new perspective on security needs. The blasts that shook the munitions depots at Severomorsk in May, if seen from a certain viewpoint, could help break the US-USSR impasse on arms control.

Last summer the Soviet merchant marine became stuck in Arctic Ocean ice; now the Northern Fleet has been disabled by explosions destroying two-thirds of its surface-to-air and ship-to-ship missiles.

Will the West now intensify its pressures or even attack the USSR? The answer is ''no.'' But what if the entire Soviet Navy were crippled? Would the West Europeans then try to stretch out payments for Siberian gas? Would the United States raise the price of wheat or build even more missiles? Would NATO do more to support Solidarity or the Mujahideen? Would Washington demand a two-party system for the Supreme Soviet? Would US Navy Secretary John F. Lehman Jr. carry out his designs for US ships to bombard the Soviet coast?

The Politburo knows, one hopes, that the answer is ''nyet.'' Western democracies have no plans to coerce the USSR if Soviet defenses falter. While Axis armies fought Russia in World War II, no Western democracy has sent troops againt the Soviet regime since 1921. None plans to do so today, although the Pentagon seeks arms to preempt any Soviet first strike.

The Kremlin could cut its forces by a huge margin and still have plenty left to maintain order within the USSR and its border empire. Even if NATO forces kept to present levels, the USSR could reduce conventional forces and still repel any Western attack. Moscow could pare its nuclear arsenal by half and still deter atomic blackmail from any quarter.

Does a similar logic hold for Western security needs? What if the US Sixth or Seventh Fleet - or both - were disabled? Would this raise the odds of Soviet attack or attempted extortion? Not by much. Outsiders cannot know the Kremlin's intentions, but the USSR has never attacked a major Western power. Soviet troops have seized real estate on the USSR's periphery and have sought to keep friendly regimes in power, from Prague to Kabul. For a decade they have been active in the third world, but Soviet actions in Angola, or even in Afghanistan, have been low risk compared with the dangers Moscow would face if it threatened America's European or Asian allies. (The most belligerent Soviet moves in Europe, as in 1948 and 1958-61, were aimed more at stabilizing East Germany than at dominating West Germany.)

Soviet sea power has grown because neither alliance willingly permits its rival to enjoy a decisive advantage in any realm of the competition. NATO navies have long had a strong lead over the Warsaw Pact, which the USSR has slowly redressed. If Moscow now kept its Northern and Baltic Fleets at home, it would probably gain, because Norway and other Scandinavian governments might become less militant. Except for its submarines, the Soviet Navy serves no urgent purpose: Neither alliance believes that a limited war in Europe can remain limited; in an all-out nuclear exchange most surface ships would be superfluous.

The security of each rival is assured by a tremendous redundancy. As Congress considers the defense budget a similar understanding emerges: US security will not suffer if Stealth, Trident, B-1, or MX - or all - are delayed or shelved. Either superpower could lose one or more fleets, or armies, or air wings; the other would probably do little to exploit the ensuing vulnerability.

Russia's temporary weakness gives the West an opportunity - at little cost - to show by deed that it is ready to ''jaw'' instead of confront. Rather than harass the Soviets by pushing into their northern waters, NATO could cut back on its forward deployments, keeping more ships in port and circumscribing naval maneuvers. Such restraint could help both sides discuss how they might cut today's bloated arsenals and prevent deployment of new weapons so as to provide for the minimum security requirements of each alliance.