The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported continuing unrest at Bulusan during 6-12 May. The seismic network recorded 18-124 daily volcanic earthquakes. Voluminous gas-and-steam emissions rose as high has 400 m above the summit and drifted NW, WNW, W, and WSW. Daily averages of sulfur dioxide emissions ranged from 466 to 1,117 tonnes per day; emissions were not reported on 11 May. A special advisory was issued on 12 May due to increasing seismicity. From 0000 on 8 May until 1330 on 12 May the seismic network recorded 309 volcanic earthquakes; of those 287 were volcano-tectonic earthquakes indicating rock fracturing at shallow depths (less than 4 km) beneath the SE flank and 22 were low-frequency earthquakes associated with fluid movement. The Alert Level remained at 1 (the second lowest level on a scale of 0-5). The public was reminded not to enter the 4-km-radius Permanent Danger Zone (PDZ) and to be vigilant within 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)