The Philippine Institute of Volcanology and Seismology (Phivolcs) detected an increase in Bulusan Volcano’s seismic activity since Thursday, June 9, which could lead to another phreatic or steam-driven eruption “within the next few hours.”

In an advisory issued late Thursday, Phivolcs said 45 volcanic earthquakes, including two low-frequency quakes, have been recorded by the Bulusan Volcano monitoring network since 5 a.m.

“Most of these were weak and shallow events, but may indicate that a phreatic eruption could possibly occur within the next few hours,” it warned.

Phivolcs reminded the public that Bulusan remains under Alert Level 1, which means that it is currently in an abnormal condition.

“Local government units and the public are reminded that entry into the four-kilometer radius permanent danger zone must be strictly prohibited and that vigilance in the two-kilometer extended danger zone on the southeast sector must be exercised due to the increased possibilities of sudden and hazardous phreatic eruptions,” Phivolcs said.

Likewise, Phivolcs asked civil aviation authorities to advise pilots to avoid flying close to the volcano’s summit as ash from any sudden phreatic eruption can be hazardous to aircraft.

“People living within valleys and along river/stream channels especially on the southeast, southwest and northwest sector of the edifice should be vigilant against sediment-laden stream flows and lahars in the event of heavy and prolonged rainfall should phreatic eruption occur,” it said.

Mt. Bulusan had a phreatic eruption on Sunday, June 5, which produced an ash plume more than a kilometer high and left parts of Sorsogon blanketed in ash.