PHIVOLCS reported that increased seismicity at Bulusan that began on 14 October continued. The seismic network recorded a total of 121 volcanic earthquakes from 0500 on 14 October to 1500 on 25 October. Out of those, 37 were volcano-tectonic events associated with rock fracturing and were located at depths of 1-9 km beneath the NW and SE flanks of the volcano. Ground deformation data from electronic tiltmeter stations continued to record inflation at the S flank, first detected in February, and at the NE flank since September. The Alert Level was raised to 1 (the second level on a scale of 0-5) and PHIVOLCS reminded the public 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)