PHIVOLCS reported that the Alert Level for Bulusan was lowered to 0 (on a scale of 0-5) on 24 April following a decline in activity after a phreatic eruption on 13 May 2011. The frequency of earthquakes decreased to baseline levels of 0-2 per day, measurements indicated deflation since late November 2011, and steaming activity from the crater and known thermal vents had been frequently weak compared to more moderate steam emissions during periods of unrest. Entry into the permanent danger zone, defined by a 4-km radius around the volcano, remained prohibited.

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)