At 2300 on 12 June PHIVOLCS stated that unrest at Bulusan had again increased, with a total of 95 weak volcanic earthquakes occurring during the previous 17 hours. Diffuse white steam plumes rose from the SW vents. Inflation of the upper flanks first recorded on 6 March in tilt data was sustained. GPS data indicated short-term inflation starting in May, though the long-term pattern since May 2019 showed overall deflation. The seismic unrest was likely due to shallow hydrothermal activity; the Alert Level remained at 1 and the public was reminded not to enter 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)