The 1,560-meter Mt. Bulusan caused no damage to surrounding areas when it began smoking and spewing ash Wednesday night, but light ashfall was reported five kilometers to the north and traces of ash were found in Sorsogon City, 15 kilometers farther north, Phivolcs said.
The Office of Civil Defense said it distributed 1,000 facemasks in the area and called in fire trucks to hose down falling ash along roads.
In an interview with The STAR, Jaime Sincioco, officer-in-charge of Phivolcs’ volcano monitoring and eruption prediction division, said Bulusan’s recent behavior indicates "the possibility of life-threatening volcanic flow."
There are 45 barangays around the volcano with some 45,000 residents.
Sincioco said Phivolcs raised the second of its five-stage alert level at about 11:30 p.m. Wednesday when it issued a warning that the volcano "may produce hazardous magmatic eruption" and reiterated its warning for people to refrain from going within four kilometers of the summit because of possible sudden explosions that could send superheated volcanic debris down its slopes.
"Areas beyond the permanent danger zone may be included, depending on the nature of the anticipated (volcanic) activity," Phivolcs said.
Sincioco cited areas nearest the volcano as Juban in the northwest, Irosin in the southwest, Gubat in the northeast, Bulusan in the northeast, Barcelona in the northwest and Casiguran in the north.
Sincioco said that at about 8:17 p.m. last Wednesday, Bulusan exploded with ash some two kilometers into the air. Another such explosion was recorded by four seismographs around the volcano at 8:30 p.m. The explosions, he said, were phreatic, or resulting from the contact between heated rocks underground and subterranean water.
Another phreatic explosion occurred at about 2:26 a.m. yesterday, but the height of the ash column it produced in the air was not determined due to lack of visibility, according to the Phivolcs monitoring station in Irosin town, some six kilometers southwest of the volcano.
"We expect an increase in the frequency of phreatic explosions," Sincioco said.
Scientists also warned that rains over the volcano could trigger lahar flow that could affect nearby towns.
Sincioco stressed, however, that such lahar flow would be much smaller than similar flows following Mount Pinatubo’s eruption in 1991, which buried entire villages in the following years.
On Bulusan’s slopes towards Casiguran town, ashfall deposits were measured only at four millimeters, he said.
At Mt. Pinatubo, volcanic debris accumulation on the slopes was as thick as 20 meters.
Phivolcs volcanologist Ed Laguerta said ashfall deposits ranged from 0.5 to four millimeters in Bulusan, Juban, Irosin, Casiguran and Sorsogon City after Wednesday night’s explosion.
Casiguran town was the most affected by the ashfall, prompting Mayor Edwin Jamor to suspend classes at all levels and request the provincial government for fire trucks to hose down deposits that had blanketed major roads.
"Rains could push ash materials through the natural drainage channels on the slopes. Even old materials could be washed down," Sincioco said.
He noted that during typhoon "Caloy" last month, some lahar flow was already noted on the slopes towards Irosin where the volcanic debris accumulated at a spillway in Barangay Cogon.
In its alert notice yesterday, Phivolcs said the explosions at Bulusan "appear to be increasing in frequency and reflect intrusion and ascent of magma. Ash analyzed has been found to contain glass particles that indicate magma involvement." — Cet Dematera, Celso Amo, Sheila Crisostomo, Ding Cervantes, AP, AFP