PHIVOLCS lowered the Alert Level for Bulusan to 0 (on a scale of 0-5) on 25 January, noting that activity at the volcano was at baseline levels. Specifically, sulfur dioxide flux had been below detectable levels since 2018, the frequency of volcanic earthquakes had been at baseline levels (0-2 earthquakes/day) since May 2019, and overall ground deformation data indicated that there was no pressurization from subsurface magma. Weak gas emissions from hydrothermal activity continued. PHIVOLCS reminded the public of the 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)