TOKYO -- Mount Unzen shot up huge new flows of red-hot rocks and superheated gases yesterday, including an evening outburst that scientists said was bigger than the one that killed 37 people in southern Japan Monday.
Police reported no casualties by late last night but expanded the evacuation zone at the foot of the mountain, urging an additional 1,300 residents of Shimabara, a city of 45,000, to leave their homes. The new evacuations brought to more than 8,500 the number of people who have been forced to move into school gymnasiums and other temporary shelters.
The newest eruptions set houses and forests afire along the Mizunashi River, which has repeatedly carried much of the fiery outflow down the hill. Thousands of small volcanic rocks rained down on Shimabara, and fallout from the giant plume thickened the gritty coating of ash and dust that has built up in the area since the volcano became active in November after nearly two centuries of dormancy.
Japanese television showed lightning bolts piercing the thick plume of hot gray ash and gases, with towers of flame bursting through seams that extended along the mountain's ridgetop and down its eastern slope.
Farmers herded cattle and other livestock onto covered trucks as the evacuation area expanded.
Electricity went out in Shimabara for about 15 minutes during last night's biggest eruption, but there was no immediate explanation for the blackout.
Last night's eruptions also spread volcanic debris toward the nearby town of Fukae, which had not been hit Monday. News reports said that forest fires were visible from the Fukae town hall.
Scientists monitoring the eruptions said they hoped that today they would be able to get close enough by helicopter to see whether yesterday's eruptions were triggered by the collapse of a lava dome that had been building up and clinging to the hilltop's edge more and more precariously since midweek.
It was the growth of the lava dome that prompted evacuation of more than 7,000 people late last week.
By yesterday afternoon, before the new eruptions, scientists estimated that 900,000 cubic meters of the lava dome were perched on the mountaintop, growing by about 100,000 cubic meters a day. Estimates were that 700,000 cubic meters of volcanic matter had poured down the hillside since September, including Monday's deadly flow but not including yesterday's.
The mountain has sent tons of ash and volcanic gas as high as 300 feet into the air for most of the week.
Scientists warned yesterday that there was still the potential for an eruption that could dwarf anything seen so far. They said existing conditions could lead to the collapse of the entire eastern face of the 4,455-foot mountain.
That could send a plume of ash and superheated gas a mile or more into the air, unleash a windstorm of air and gases several hundred degrees hot through Shimabara and greatly expand the area covered by the pyroclastic flow of red-hot rock and other volcanic matter.
Such an outburst, they said, would be preceded by bigger earth tremors, by splits in the earth's surface and by big changes in underground water levels, any of which should be taken as the cue for a general evacuation of Shimabara and nearby towns.
Yesterday's biggest eruption, at 7:51 p.m., was accompanied by about 25 minutes of earth tremors. The Mount Unzen meteorological observatory said it was the biggest since the mountain became active in November.
Mount Unzen's last eruption, in 1792, was Japan's worst recorded volcanic disaster. It touched off landslides and a tidal wave, killing an estimated 15,000 people.