Scientists studying the Nevado del Ruiz volcano that exploded last week, burying 25,000 people in a massive mud flow, said today the mountain could erupt again soon.

"Many volcanoes that go off this way have more than one big bang," said John Tomblin, of the U.N. Disaster Relief Coordinator and member of a group of geologists who rushed to Colombia after the explosions.

Colombian authorities have sought to calm fears of another eruption, issuing daily communiques suggesting that activity inside the volcano has subsided. But Colombian and foreign specialists who have established a monitoring site in this city, 35 miles west of the mountain, said the government's assurances may be overly optimistic.

"The situation looks very much the way it did before Nov. 13," when the last eruption occurred, Tomblin told a group of U.S. geologists today within earshot of several reporters. "This is cause for anxiety."

Colombian scientists here said that five weeks before the Nov. 13 explosion they had warned government officials and rescue authorities that Navado del Ruiz might blow and had predicted that mud flows would sweep through the river valleys below the volcano, swallowing the town of Armero. This is, in fact, what happened.

Pablo Medina, executive director of the Volcano Studies Committee and spokesman for the international group assembled here today showed reporters a copy of a "volcanic risk map" prepared by a team of Colombian experts and picturing Armero engulfed in mud.

A preliminary version of the map, Medina said, was completed Oct. 7 and submitted to the Ministry of Mines, governmental authorities in the region and rescue organizations. He added that a final version, which differed little from the preliminary one, was presented to the government on the day the volcano erupted.

"It's our feeling that accuracy of prediction [of the map] was quite high," Medina remarked.

In recent days, teams of U.S., Swiss, Spanish, Japanese, French and Colombian engineers and geologists have been scrambling up the volcano to install new sensing devices to keep close watch on seismic activity. Three of five sensors that had been on the mountain were destroyed in last week's explosion, said Medina.

These seismographs differ in some cases from the previous instruments used because they have the capacity to transmit readings to a central control station. The readings of the old seismographs had to be retrieved in person, resulting in delays of 24 hours or more.

Two of the more modern sensors are already operating on the volcano, one to the west of the 17,000-foot-high peak, the other to the north of it. In the past few days, they have detected a number of minor eruptions, the geologists said.

These included what one foreign geologist described as "a swarm of small quakes" lasting 20 minutes which started at 8 a.m. yesterday. A two-minute explosion that vented steam from the crater was recorded at 2 this morning.

The volcano, said Medina, is "in an excited state."

Experts here are reluctant to share observations about the volcano in view of the political sensitivity the issue has taken on. They said they have been instructed by the office of President Belisario Betancur not to make public predictions about the likelihood of another explosion.

The monitoring station in Manizales has a direct phone line to Victor Ricardo, general secretary of the presidency. "We feed him information," said Medina. "It's up to him to transmit it within the boundaries of prudence, taking into account national politics."

For the Colombian government, the decision of what to say publicly about the volcano's condition is a difficult one.

On the one hand, regional and national authorities have been criticized for failing to prepare adequately for an eruption of Nevado del Ruiz.

On the other hand, riverside communities at the base of the volcano are already jittery. Any alarming word about a potential new explosion could trigger a panic.

In the experts' view, Nevado del Ruiz's future is hard to predict.

"It's a very bad-tempered volcano," said Medina. A member of the U.S. Geological Survey team here likened the Colombian volcano, which had been dormant for 140 years, to Mount St. Helens in Washington state. It, too, had been dormant for years before erupting five years ago. "The less often they erupt the more dangerous they become," said the American volcanologist.

Scientists are still not in agreement on what occurred during last week's explosion. The prevailing explanation is that the volcano sent massive amounts of ash and incandescent pumice stone soaring into the air, which landed on the glacier at the volcano's summit and melted some of the ice. This water cascaded down the mountain, picking up surface debris and mud, then surged into riverbeds, causing a giant mud flow.

Another theory, which several U.S. geologists here said is being investigated, is that the mud flow was caused by the rupture of a large pond that may have been near the volcano's top.

[The Associated Press reported from Bogota that President Belisario Betancur's Cabinet authorized $8 million to rebuild bridges, roads and oil pipelines in the stricken area. The Congress was expected to allocate $12 million to aid the 50,000 people left homeless.]