Sorsogon City, Philippines - Despite a relatively calm Mt. Bulusan, lahar started to flow down its slopes into a river in Irosin town on Sunday, hours after the rain that began Saturday night.

Close to 100,000 residents in slopes around the volcano, particularly in Irosin and Juban, feared flash floods in case torrential rains followed.

But Crispulo Diolita Jr., volcanologist at the Bulusan Observatory Center based in Cabid-an, Sorsogon City, told the Inquirer on the phone on Sunday that the lahar flows were still confined to the Rangas River channel.

Philippine Institute of Volcanology and Seismology (Phivolcs) and local officials rushed to Rangas River in Barangay Cogon, Irosin, on Sunday morning due to reports of heavy lahar flow triggered by the rain.

Lahar and other volcanic sediments started flowing from the slopes of Mt. Bulusan at 10 a.m. on Sunday, causing widespread fear of lahar overflowing to residential areas close to the river system.

"We were alarmed because what we see were mostly lava, sand and big rocks cascading down the slopes of the volcano at a velocity of about 70 kilometers per hour. There were minimal water component compared to the previous lava flows in November and December," said Roniel Frayna, a resident of the village of Cogon located at the foot of the volcano.

Government volcanologists earlier warned of lahar flows in the event of a downpour as tons of ashes and other volcanic debris were deposited along the slopes of the Cogon gully.

Mt. Bulusan dumped some 700,000 cubic meters of debris in an ash explosion on Feb. 21, adding to the stockpile from previous explosions since Nov. 6. 2010.

Irosin Councilors Roque Dorotan Jr. and Christian Lim, who went to Rangas River with Cogon Barangay Chair Eduardo Endaya, assured they would be ready to evacuate residents if there were any sign the lahar flow would endanger residents. The rain stopped at noon Sunday but the lahar flow continued.

A Phivolcs team went to Cogon as well as Monbon and Patag areas in Irosin to get more samples of volcanic debris and to also assess the volume of lahar flows, the town officials said.

Ed Laguerta, chief volcanologist for Sorsogon, however, said that an intermittent rain could be helpful as it could "wash away ash and other volcanic fragments little by little."

The Phivolcs said in its bulletin Sunday that its Telemetered Seismic Network recorded five volcanic earthquakes during the past 24 hours. Steam activity was not observed due to clouds obscuring the vents and summit crater.

Mt. Bulusan remained at alert level No. 1. This means that the source of activity is hydrothermal and shallow.

Entry to the 4-km radius Permanent Danger Zone is strictly prohibited, since the area is at risk to sudden steam and ash explosions.

Phivolcs also reiterated its warning against aviation activities near the crater and pilots need to be very watchful of sudden ash explosions which could prejudice their flights and the lives of people on board.