On 12 October PHIVOLCS raised the Alert Level for Bulusan to 1 (on a scale of 0-5) due to increased seismicity, inflation, and gas emissions. From 0500 on 11 October to 1500 on 12 October the seismic network detected 126 weak earthquakes located at depths of 0-5 km. Most of the events were concentrated beneath the NW flank and summit area and were indicative of rock-fracturing processes. Other data suggested increased hydrothermal activity and unrest including increased temperatures and carbon dioxide emissions at area hotsprings beginning earlier in the year, pronounced inflation of the SW and S flanks that began in September, and a sulfur odor reported by residents of Sitio Talistison in Barangay Mapaso (Irosin) and Barangay San Roque (Bulusan) during 10-11 October. Daily earthquakes totaled 164 during 13-14 October and 79 during 14-15 October; daily counts were 26-32 during 15-18 October. Sulfur dioxide emissions were 302-386 tonnes per day during 15-16 October. 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)