PHIVOLCS lowered the Alert Level for Bulusan to 0 (on a scale of 0-5) on 21 August, noting that unrest had further declined to background levels. The frequency of volcanic earthquakes declined to baseline levels during the third week of July. Deformation data showed short-term inflation at the SE flank, though long-term data showed no deformation associated with the volcano. Sulfur dioxide emissions averaged 1,900 tonnes/day during 5-12 June and declined to about 230 tonnes/day during 25 July-6 August. Steam-laden emissions from the active vents declined to low-to-moderate levels. PHIVOLCS reminded the public not to enter the 4-km-radius Permanent Danger Zone (PDZ) nor the 2 km Extended Danger Zone (EDZ) on the SE flank.

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)