Belfast — This week's bombings in central London by the Provisional wing of the illegal Irish Republican Army (IRA) have raised several complex theories about the motives behind the terrorist acts.
However, security sources in Northern Ireland, with long experience of such outrages, regard the London explosions as simply another example of classic guerrilla tactics by a terrorist organization. One security source said: ''It was not a new phenomenon, but it was a classic.''
Some observers have suggested that the bombs were a reprisal for the jailing of Gerard Tuite, who was sentenced to 10 years for possessing explosives. Others say the London attacks were timed to coincide with the visit to America of James Prior, the Northern Ireland secretary of state.
On the other hand, the atrocities gave Mr. Prior's visit maximum publicity in the United States. He used the opportunity to condemn the IRA and to appeal to Americans not to give money to support their cause.
Security sources in Northern Ireland point to three possible motives for the bombings:
* There was undoubtedly a mood of national pride in Britain in the wake of the Falklands victory. Thus the Provisionals' statement: ''Now it is our turn to properly invoke Article 51 of the UN statute and properly quote all Thatcher's fine phrases on the right to self-determination of a people,'' might be taken at face value. Article 51 was a keynote of the British prime minister's argument that the Falkland Islanders had a right to determine their own future.
* It is known that bombs in London create much more publicity than explosions in Belfast, and this has proved to be the case again.
* Security sources say the Provisionals have taken a beating in the long war of attrition since the early '70s. The figures bear out this argument. In 1972, 467 people died and there were 1,495 explosions. Last year the figures were 101 and 398, respectively. This year, so far, 38 people have died and there have been 153 explosions.
The Provisionals themselves have created counterpropaganda. Several weeks ago many residents of west Belfast had to leave their homes while the Army tried to defuse a massive van bomb left in the area. It exploded and created widespread damage. The security forces have also made numerous arrests. So far this year some 400 people are facing serious charges, including murder and the possession of weapons.
Given such a background, the Provisionals' need for a spectacular coup was self-evident.
Despite attempts by London police to find bombmaking equipment and terrorist information throughout last winter, the IRA has again been able to slip through the London security net. Many thousands of people move from Britain to Ireland and back regularly. Short of a total clampdown, fool-proof security is impossible.
An IRA cell in England can exist for weeks with impunity. It can be well organized, with minimum security leads. Money is no problem. The IRA still receives funds from the United States,but security sources claim this is no longer crucial. The IRA funds itself from extortion, rackets, and robberies. This year some $1.5 million has been stolen in Northern Ireland alone, and much of this has found its way into IRA coffers.
In moral terms, the Provisionals' action was indefensible. Technically, it was a considerable military coup in the enemy's heartland. Some people have suggested the bombs were partly an attempt to affect the British plan to hold elections to a new Ulster assembly in October. But the political evidence so far suggests that Ulster politicians will fail to agree anyway, irrespective of IRA bombs.
Police now say that even more care is needed. ''The more we succeed on the long haul, the more they must try to strike back,'' said a spokesman. ''The IRA must always try to restore their credibility and they are particularly dangerous when they are trying to do that. It behooves everyone to have the utmost vigilance.''
Clearly the long war of attrition, like the IRA threat itself, is far from over.