Report on Bulusan (Philippines) — 1 July-7 July 2020
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 1 July-7 July 2020
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2020. Report on Bulusan (Philippines). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 1 July-7 July 2020. Smithsonian Institution and US Geological Survey.
12.769°N, 124.056°E; summit elev. 1535 m
All times are local (unless otherwise noted)
PHIVOLCS reported that the seismic network at Bulusan recorded 53 volcanic earthquakes during 3-6 July, including 43 low-frequency events associated with weak shallow hydrothermal or magmatic gas movement. Despite visible degassing or steaming from the active vent in 2020, increased seismicity may still be followed by phreatic activity at the summit or from flank vents. Additionally, GPS data indicated short-term inflation since late February. On 6 July the Alert Level was raised to 1, indicating abnormal conditions, and the public was reminded of the 4-km-radius Permanent Danger Zone (PDZ). There were 19 volcanic earthquakes recorded during 6-7 July, and diffuse white steam plumes rose from the lower SE vent.
Geologic Background. 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.