Syrian regime stands - despite violent shaking by Hama uprising ... but it could be hit still harder if Israel launches attack into Lebanon

With explosive grudges hovering thickly over both sides of Israel's northeast border, there is a widespread feeling here now that even a modest spark could touch off a conflagration.

''For some time now, security circles have been convinced that an explosion in the north is just a matter of time,'' writes Zeev Schiff, Israel's foremost military analyst.

Since last July's cease-fire between Israel and the Palestine Liberation Organization (PLO), Israel has been watching with growing concern the buildup of PLO armaments in southern Lebanon. Israeli authorities say that the buildup includes 300 artillery pieces and rockets which could hit scores of Israeli settlements and towns.

In addition, the movement of Russian-made SAM antiaircraft missiles by Syria into southern Lebanon has been a source of irritation ever since Washington's special emissary, Philip C. Habib, talked Israel out of launching an air strike against them last summer.

At least three factors are tending to spur Israel toward exercising its military option in the north:

* The approach of Israel's final pullout from Sinai.

* Syria's current internal problems.

* Washington's apparently harder line toward Israel which reduces US leverage against such an option.

If Israel intended to strike against the PLO in southern Lebanon, and perhaps against the Syrian missiles there as well, it is clearly in its interest to do so in the ''days of grace'' before the Sinai pullout April 25. Egyptian neutrality is virtually guaranteed prior to the Israeli withdrawal.

Jerusalem indeed seemed on the verge of launching an attack across the border last month after a heavily armed Palestinian unit was intercepted in the Jordan Valley. Strong pressure against such action was exerted by Washington - and by residents of northern Israeli border settlements who feared resumption of a war of attrition.

The Begin government, however, has maintained its hair-trigger readiness. It has secretly dispatched the chief of military intelligence to Washington to point out the gravity of the border situation to US officials.

Washington, meanwhile, has been urging the Palestinians to avoid offering Israel any pretext for a strike. And the Syrians, isolated in the Arab world and struggling with a revolt at home, have been careful to avoid military reaction to Israel's annexation of the Golan Heights.

Nonetheless, even if Israel chooses a ''surgical'' strike against the Palestinians in southern Lebanon, observers here believe it will be difficult for Syria to stay out of the conflict. This, in turn, could bring in Iraq and Jordan - despite their current hostility to Syria, and despite Iraq's war with Iran.