SORSOGON CITY -- After being quiet for 11 years, Mount Bulusan in Sorsogon province, southeast of Manila, erupted late Tuesday night, spewing ash clouds as high as 1.5 kilometers into the sky and prompting government warnings to residents not to go near the volcano.

The Philippine Institute for Volcanology and Seismology (Phivolcs) reported a steam-driven explosion from the summit of the 1,559-meter volcano at 10:58 p.m. on Tuesday. It lasted 20 minutes and was accompanied by an earthquake.

Resident volcanologist Bella Tubianosa said that because of the westward direction of winds, only the villages of Cogon, Tinampo, Gulang and Bolos in Irosin town and the villages of Bacolod, Sapa, Sangkayon and Biriran in Juban town were covered by the ash fall.

More explosions

"Phivolcs expects more explosions to occur in view of the volcano's recent reactivation," the agency said in its bulletin issued at 8 a.m. yesterday.

accompanied the explosion at 10:58 p.m., but three more earthquakes followed at 11:30 p.m., 11:32 p.m. and 11:37 p.m.

"Technically, it's an eruption, although it's only an ash explosion since the materials spewed were mostly ash and not pyroclastic ones," Ed Laguerta, Mount Mayon resident volcanologist who is based in Legazpi City in Albay, said late yesterday afternoon.Laguerta, who went to Bulusan to assist in monitoring activities, said there was not much hazard or threat to life at present but only inconvenience to local residents, except for those near the crater.

He said no succeeding activity had yet taken place after Tuesday's ash explosion.

Bulusan usually exhibits a "one-shot deal" -- meaning, once it explodes, it will take days before another activity occurs, he said.

Laguerta noted the reactivation of certain fissures down the slopes, as manifested by smoke emissions.

Tubianosa said the ash ejection was preceded by at least 63 volcanic quakes and a loud grumbling.

Phivolcs warned people to stay away from areas within 4 km of the crater that has been designated as a permanent danger zone (PDZ). The zone, which covers portions of Juban, Barcelona, Bulusan and Irosin towns, is off-limits to the public because this will be affected by sudden explosions.

Nearby communities near the zone have been warned against more ash explosions that could generate life-threatening volcanic flows.

At 6 a.m. yesterday, the Phivolcs' seismic network around Bulusan recorded 18 low-frequency volcanic earthquakes, 41 high-frequency ones, and four explosion-type earthquakes.

High-frequency earthquakes are caused by the breaking of rocks. Low-frequency earthquakes are those caused by the movement of fluid or gas.

24-hour vigil

"What we are warning against are the more dangerous kinds of explosion," Phivolcs Director Renato Solidum said.

Hazardous eruption is characterized by tall eruption columns, pyroclastic flows moving down the slope, and lava flows.

Tubianosa said she could not yet tell if the volcano's situation would still worsen but her office was keeping a 24-hour vigil.

Aerial survey

Arnel Capili, regional director of the Office of Civil Defense, said an aerial survey of the volcano would be conducted today.

On Sunday, the Phivolcs raised Alert Level 1 in Bulusan after recording 73 high-frequency volcanic earthquakes, much higher than the usual five volcanic quakes in 24 hours. The next day, its seismic network recorded 58 volcanic earthquakes, compelling the agency to issue a warning on a possible explosion, which occurred Tuesday night.

"These are phreatic explosions, which are a precursor to an eruption ... These indicate volcanic unrest," Phivolcs volcanology officer-in-charge Jaime Sincioco said in a phone interview.

Sincioco said his office was treating the explosion as phreatic. This type of explosion occurs when water impounded in the crater flows into the cracks and meets the hot rocks, converting it to steam.

Disaster council

Yesterday, Gov. Raul Lee of Sorsogon immediately called the Provincial Disaster Coordinating Council to a meeting to assess the situation and prepare for any eventualities.

Lee asked the Department of Education to make available its classrooms in the affected areas and even in towns that might be hit by ash fall for possible evacuation.

He mobilized the Philippine Army, Philippine National Police and the Bureau of Fire Protection to clean up the national highway.

Bulusan, 43 kilometers south of the capital city Sorsogon, is one of the country's 22 active volcanoes. The Phivolcs defines "active" as volcanoes that have erupted in the past 500 years.

Before Tuesday's ash explosion, Bulusan's last activities were recorded from Nov. 27, 1994 to January 1995. It only caused ash fall in villages in Bulusan, Barcelona, Casiguran, Juban, Irosin and Gubat towns and was accompanied by at least 14 minutes of weak tremor.

The Philippines lies in an area in the Pacific basin that is vulnerable to earthquakes and volcanic eruptions.

Mount Pinatubo in Central Luzon, north of Manila, erupted in 1991 in the century's biggest blast, burying dozens of villages under tons of mud after lying dormant for 600 years.

More than 800 people died in the wake of Pinatubo's eruption, mostly from diseases in overcrowded evacuation camps.