PHIVOLCS reported that during 12-16 October the seismic network at Bulusan recorded 2-6 volcanic earthquakes per day. During 12-13 October steam plumes rose as high as 500 m above the active vents and drifted SE and SSE. The network recorded 24 volcanic earthquakes from 16 to 17 October. At 0736 on 17 October a 24-minute-long phreatic explosion at the SE vent generated an ash plume that rose 1 km.

PHIVOLCS noted that while the SE vents are within the 4-km-radius Permanent Danger Zone (PDZ) they are part of a fissure that extends 2 km down the upper S flank. Eruptions from this area pose a greater risk to the populated barangays of Mapaso and Patag in Irosin, and San Roque in Bulusan. In response the Extended Danger Zone (EDZ) was increased and as of 18 October stretched an additional 2 km past the PDZ. The Alert Level remained at 1, indicating abnormal conditions and a 4-km-radius Permanent Danger Zone (PDZ).

Geological Summary. Luzon's southernmost volcano, Bulusan, was constructed along the rim of the 11-km-diameter dacitic-to-rhyolitic Irosin caldera, which was formed about 36,000 years ago. It lies at the SE end of the Bicol volcanic arc occupying the peninsula of the same name that forms the elongated SE tip of Luzon. A broad, flat moat is located below the topographically prominent SW rim of Irosin caldera; the NE rim is buried by the andesitic complex. Bulusan is flanked by several other large intracaldera lava domes and cones, including the prominent Mount Jormajan lava dome on the SW flank and Sharp Peak to the NE. The summit is unvegetated and contains a 300-m-wide, 50-m-deep crater. Three small craters are located on the SE flank. Many moderate explosive eruptions have been recorded since the mid-19th century.

Source: Philippine Institute of Volcanology and Seismology (PHIVOLCS)