WASHINGTON (AP) _ The Defense Department is working around the fact that one of its most vital Pacific bases has been shut down due to the threat of an eruption from a nearby volcano, a spokesman said Tuesday.

Some 15,000 Americans evacuated Clark Air Base on Monday after rumbling from the nearby Mount Pinatubo hinted that a major eruption might be on its way.

A huge explosion rocked the volcano Wednesday (Tuesday night Washington time) and the remaining 1,500 Americans and Filipino guards fled the base, according to a spokesman for the Philippine military.

''We don't know how long Clark Air Base will remain shut down. We can work around it for now,'' Pentagon spokesman Pete Williams told reporters earlier.

Most of the evacuees - primarily airmen and their families - were transferred to Subic Bay naval base, an installation 50 miles to the southwest.

Clark Air Base, located about 50 miles north of Manila, serves as a training facility for U.S. and allied flight crews.

It is an important transit point for U.S. forces from the western Pacific to the Indian Ocean and Persian Gulf.

Williams said 1,500 ''essential personnel'' remained at Clark, primarily for security reasons and to keep basic services operating.

In January, the U.S. military turned over a naval communications station near Clark to the Philippine military, but looters stripped the installation of virtually all its equipment. The site is no longer operational.

Clark's remaining personnel would have time to evacuate the base in the event of a volcanic eruption, and would do so by ground, Williams said.

All the aircraft from the base, including MC-130 transport planes, MH-53 Pave Low helicopters, C-12 King Air transport planes, were moved to Cubi Point Naval Air Station near Subic Bay over the weekend, the spokesman said.

Williams termed Clark a ''very important operating base'' because of its strategic location and the huge training area it provides U.S. forces from throughout the Pacific.

''We can work around those things short term, but we want to keep Clark open,'' he said, pointing out that the United States is negotiating with the Philippine government to retain Clark and Subic Bay.

But the talks have been stymied over how much the Americans will pay and how long the bases can remain in U.S. hands.

Asked about the long-term impact of the evacuation, Williams said it was impossible to tell.

''We can work around it in the short term, but I can't tell you what a long-term fix it would take if it were shut down for a long period of time,'' he said.

In Manila, the chief Philippine seismologist said that the volcano could rumble for years before a major eruption occurs.

Raymundo Punongbayan, chief of the Philippine Institute of Vulcanology and Seismology, said it could take years before a major eruption, during which time the volcano could remain active and register small, periodic eruptions.

Punongbayan said that a major eruption could also take place in a short period and that scientists simply had too little information on Pinatubo to be certain of their calculations. But he said a pattern of sporadic mini- eruptions over several years could make the area around the volcano virtually uninhabitable.