MOUNT MERAPI, Indonesia – Deafening explosions of hot gas rattled evacuees miles (kilometers) from an Indonesian volcano Monday, the latest eruption in a deadly week. The country reported increased rumblings at 21 other active mountains, raising questions about what’s causing the uptick along some of the world’s most prolific fault lines.

No new casualties were reported in Mount Merapi’s new blast, which came as Indonesia also struggles to respond to an earthquake-generated tsunami that devastated a remote chain of islands. The two disasters unfolding on opposite ends of country have killed nearly 500 people and strained the government’s emergency response network. In both events, the military has been called in to help.

Merapi has killed 38 people since it started erupting a week ago. Monitoring officials have also raised alert levels at some of the 129 active volcanoes in Indonesia, with two under watch for possible eruption within two weeks and 19 showing increased activity — more than double the usual number on the watch list, an official said.

Indonesia, a vast archipelago of 235 million people, is prone to earthquakes and volcanos because it sits along the Pacific “Ring of Fire,” a horseshoe-shaped string of faults that lines the western and eastern Pacific. Scientists could not say for certain what was causing the increased volcanic activity, though two theorized the earth’s tectonic plates could be realigning and one noted growing evidence that volcanos can affect one other.

About 69,000 villagers have been evacuated from the area around Merapi’s once-fertile slopes — now blanketed by gray ash — in central Java, 250 miles (400 kilometers) east of Jakarta, the capital.

Booming explosions sounded during Monday’s eruption, which shot massive clouds from the glowing cauldron and sent ash cascading nearly four miles (six kilometers) down the southeastern slopes, said Subrandrio, an official in charge of monitoring Merapi’s activity.

Even in the crowded government camps, miles (kilometers) away from the mountain, the sound of the explosions sent evacuees scurrying for shelter.

More than 800 miles (1,300 kilometers) to the west, meanwhile, a C-130 transport plane, six helicopters and four motorized boats were ferrying aid to the most distant corners of the Mentawai Islands, where last week’s tsunami destroyed hundreds of homes, schools, churches and mosques. The tsunami death toll stood at 431 Monday, the National Disaster Management Agency said on its website.

President Susilo Bambang Yudhoyono said relief efforts must be sped up, expressing dismay that it took days for aid to reach the isolated islands, though he acknowledged that violent storms were largely to blame.

Last week’s killer wave was triggered a 7.7-magnitude earthquake along the same fault that caused the 2004 temblor and tsunami that killed 230,000 people in a dozen countries. The fault line, which runs the length of the west coast of Sumatra island, is the meeting point of the two of the Earth’s dozen major plates, which have been pushing against and under each other for millions of years, causing huge stresses to build up.

Both earthquakes and volcanos can result from the release of these stresses. As plates slide against or under each other, molten rock can break the surface via a volcano or the energy can be released in an earthquake.

The government has raised alert levels of 21 other volcanos to the second- and third- highest levels in the last two months because they have shown an increase in activity, said Syamsul Rizal, a state volcanologist, said Monday. Many of those are already rumbling and belching out heavy black ash.

Indonesia has several volcanos smoldering at any given time, but another government volcanologist, Gede Swantika, said that normally only five to 10 would be at the third-highest alert level — which indicates an increase in seismic activity and visible changes in the crater. It is rare for any to be at second-highest — which signifies an eruption is possible within two weeks.

He said monitors noticed more volcanos were exhibiting seismic activity starting Sept. 2.

Geophysicist Pall Einarsson of the University of Iceland said that such an increase could be an indication that some of the volcanos — if any are very close — could be affecting one another. He said this idea is a new one for volcanologists, but they are increasingly seeing evidence of interplay between neighboring mountains.

Geologist Brent McInnes said as he hadn’t seen the raw data but would find such a rash of volcanic activity significant.

“If it’s true that there are over 20 volcanos demonstrating increased levels of seismic activity, then that is something we should pay attention to,” said McInnes, a professor at Australia’s Curtin University who has done extensive volcanic research in Indonesia.

He said such an increase could indicate “a major plate restructuring” — a major shift in the plates’ position, rather than simply the usual jostling. “That would be significant.”

But seismologists also caution that while eruption patterns can be studied, neither earthquakes nor volcanos can be predicted with any precision.

“My theory is that it is just a normal, random fluctuation of volcanic activity,” said John Ebel, professor of geophysics at Boston University.

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Associated Press writers Thomas Wagner in London, Achmad Ibrahim in the Mentawai islands and Kay Johnson, Niniek Karmini, Irwan Firdaus, Ali Kotarumalos and Kristen Gelineau in Jakarta contributed to this report.