Jaffna, the volatile epicenter of the Tamil insurgency, is keeping Sri Lanka on the knife's edge between a return of this island nation to a semblance of normality and the ignition of another wave of communal violence throughout the country.

Outwardly, the palm-shaded fishing town on the northern peninsula of the pear-shaped Indian Ocean island is calm--like most of Sri Lanka--a week after the ambush slaying of 13 Sinhalese Sri Lankan Army soldiers touched off the worst ethnic violence since the country's independence in 1948.

But that is because government troops are enforcing stringent emergency reulations under a tight curfew and the Tamil Tigers guerrilla organization is lying low, at least for the duration of an intensive manhunt by the security forces.

Beneath the appearance of calm and orderliness, however, is a pervasive fear that one terrorist attack in Jaffna by the Tigers to show their supporters that they have not been irrevocably scattered by the Army would trigger new violence throughout Sri Lanka--formerly Ceylon--that would make last week's wave of killing and arson pale by comparison.

"If one Sinhalese soldier is killed in Jaffna, I don't even want to contemplate the consequences. If the Tigers leave it alone, Sri Lanka can be normal by Monday," said T.D.S.A. Dissanayaka, spokesman for the Foreign Ministry.

The official death toll since rioting began July 25 is 267, but the actual toll is believed to be much higher. Most of the victims have been Tamils killed by majority Sinhalese, who originally migrated here from West Bengal in India but are ethnically different from Tamils, who migrated from southern India.

Security officers here said the ambush that left 13 Sinhalese soldiers dead last week was carried out with military precision not previously shown by the Tigers.

The Army patrol was caught in cross fire when it slowed to detour around a road excavation in the well-to-do Thinnavela neighborhood and a land mine was detonated, blowing up the lead vehicle. Although immediately afterward there were reports that soldiers went on a rampage, looting and burning houses, there was no evidence of damage in the area this week when this correspondent and a colleague were taken under heavy guard to the site and through much of downtown Jaffna.

The city was eerily quiet even before the start of a 3 p.m. curfew, as three vehicles filled with soldiers with submachine guns at the ready toured Jaffna streets for about two hours.

Sinhalese soldiers, almost none of whom speak Tamil, appeared on edge as Jaffna residents scurried off the streets at the sight of the approaching patrol. A few minutes after the beginning of the curfew, the convoy stopped and searched a city bus that was completing its route behind schedule.

A radioed report that a "petrol bomb" had been found sent the patrol looking behind a stone wall, but the nervous-appearing lieutenant who poked at the suspicious object with a stick concluded with some embarrassment that the "bomb" was a discarded bottle.

The lieutenant suggested that the bottle had been left to demonstrate that the local Tamils have no good will for the Sinhalese security forces. The troops on the patrol were from the Rajarat Rifles Brigade, which is well known here for a mutiny two months ago in central Sri Lanka during which troops criticized the government for being too conciliatory toward the Tigers and attacked a number of Tamils.

Almost nowhere was there a sign of life or movement, except for the occasional appearance of a frightened face at a darkened window, or a cow ambling nonchalantly in the street.

Army Col. Michael Silva, acting commander of the security forces here, said the government's plans for the time being were to continue seeking intelligence about the Tigers, cordoning off neighborhoods and conducting house-to-house searches and interrogating suspects.

"I personally believe that we can break the movement because the people of Jaffna are tired of violence and we can get the information," Silva said. He said the security forces are continuing to identify suspected terrorists, adding, "after some time they break down under interrogation."

The problem, Silva said, is that the Tigers guerrillas can easily blend into the densely populated, completely Tamil population of about 120,000, or, if necessary, slip under cover of darkness by small boat across the 18-mile-wide strait to southern India for a temporary sanctuary.

"Our overall strategy is to isolate the Tigers from the population, but it is very difficult. They have no difficulty in disappearing among people who will harbor them either in sympathy or fear," Silva said.

The Tiger guerrillas, the radical arm of the Tamil separatist movement that seeks to establish in the northern provinces a separate nation for the 3.5 million Tamils in Sri Lanka's 15 million population, were organized in 1975.

Estimates of their strength range from under 200 to more than 1,000, although judging by the frequency and pattern of attacks in recent years they probably do not number more than 500. The Army has recorded about 60 incidents so far this year, ranging from political assassinations and ambushes of Army patrols to sabotage and bomb threats.

Army Maj. Sarath Manasinghe, an intelligence officer in headquarters here, said the guerrillas are split into several factions, including the Tamil Eelam Liberation Organization, the People's Liberation Organization of Tamil Eelam and the Tamil Eelam Liberation Army. Eelam is the name of the independent state the Tamils seek.

Manasinghe said the Tigers, many of whom come from peninsula villages that traditionally produce tough, weather-hardened fishermen, are usually high school graduates between 18 and 28 years old. He said 70 suspected guerrillas were arrested last year, 16 of whom were "confirmed" as Tigers.

Silva said many of the Tigers are armed with shotguns and .30-caliber hunting rifles, but that some have automatic rifles and submachine guns taken from soldiers here or occasionally obtained in southern India.

Most of the financial support of the Tigers comes from Tamils working abroad, according to Silva, although he said five Tigers captured in 1979 claimed to have been trained by the Palestine Liberation Organization in Lebanon and said a small cadre of PLO-trained guerrillas is still operating here.

Silva said he was skeptical about reports that Tigers were being trained in camps in southern India, although he said small-scale individual training by retired Indian Army instructors probably was taking place there. He said that occasionally wounded Tigers guerrillas are taken by outboard motorboat to India for treatment.

Attempts to contact the 17 members of Parliament who represented the Tamil United Liberation Front party during a visit to Jaffna were unsuccessful. The Parliament, controlled by President Junius R. Jayewardene's party, is expected to put the party under severe restriction.

Tamil sources in Colombo said the 17 were in hiding because they feared being jailed. Fifty-two Tamil prisoners have been killed in prisons since the current wave of ethnic violence began.

The front, the last remaining moderate voice and buffer between the government and the Tigers, boycotted Parliament Thursday for the vote that would, in effect, strip the party of a reason for existence.