PHIVOLCS reported that a phreatic explosion occurred at 0458 on 19 October from the vents on Bulusan's upper SE flank. The seismic network recorded the event as an explosion-type earthquake that lasted nine minutes. Dense weather clouds obscured views although limited observations indicated that the plume rose 1 km. A 20-minute-long phreatic explosion occurred from the summit crater at 1234 on 21 October. A thin layer of ash was reported in Casiguran and Gubat, and trace amounts of ash fell in barangays in Barcelona, Casiguran, and Gubat. At 1531 on 23 October a 15-minute-long phreatic explosion from the summit vent produced an ash plume that rose 2.5 km and drifted WSW. Small pyroclastic flows traveled about 2 km down the flank. Trace ashfall was reported in multiple barangays in Irosin Town, ashfall 0.5 mm thick was reported in the municipality of Juban, and the most ash, 1 mm thick deposits, were found in barangay Puting Sapa, Juban. A second and much smaller explosion was recorded at 1539 from the SE vent and generated an ash plume that rose 500 m. Rumbling and a sulfur odor was noted in several nearby areas. 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)