Report on Bulusan (Philippines) — February 2008
Bulletin of the Global Volcanism Network, vol. 33, no. 2 (February 2008)
Managing Editor: Richard Wunderman.
Bulusan (Philippines) Ash ejections continued to at least late 2007
Please cite this report as:
Global Volcanism Program, 2008. Report on Bulusan (Philippines). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 33:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200802-273010.
12.769°N, 124.056°E; summit elev. 1535 m
All times are local (unless otherwise noted)
Our last report on Bulusan described explosive eruptions and ashfall during 10 October 2006 to 12 May 2007 (BGVN 32:04). This current report will cover the events from late May 2007 to January 2008. There were ash-bearing eruptions on 31 July and 4 October 2007. Hazard concerns also included steam-driven explosions, lahars, and related flooding.
The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported on 20 May 2007 that seismicity remained high following an explosion on 12 May (BGVN 32:04). The seismic network detected 673 volcanic earthquakes during five days. The epicenters were located along a NW-SE trend. Ground deformation measurements conducted on 17 May on the NE flank revealed 4 mm of inflation since 7 April, measurements in a series which have shown continued inflation since June 2006. Sulfur dioxide flux measurements were 165-315 tons per day (t/d), below a baseline level of 500 t/d. The Alert Level was raised in mid-May from 1 to 2 (out of 5) due to the increased seismicity and inflation. On 22 May, heavy rain triggered lahars, but they were confined and did not affect populated areas. On 25 May 2007 sulfur emission reached 500 t/d.
During mid-2007, scientists from PHIVOLCS conducting an aerial investigation discovered lahar deposits and three steaming fissures. Scientists also observed steam plumes that rose to altitudes of 1.6-1.7 km and drifted NW and NE. The S flank had inflated by 3 mm. Residents near the base of the volcano noted the odor of sulfur dioxide.
No significant activity was reported during June 2007. Steaming from the active vents and fissures generally consisted of weak to moderate emissions of steam. On 13 July 2007, PHIVOLCS lowered the Alert Level to 1 due to a decline in activity including decreased seismicity, and lower than baseline sulfur dioxide emissions. On 19-21 June the NE and SE flanks were deflated when compared to previous surveys. Sulfur dioxide emission rates were 50-400 t/d.
On the morning of 31 July 2007 an explosion produced an ash plume that rose to an altitude of 6.6 km and drifted WSW and WNW. Initial field reports indicated that light ashfalls were experienced in Cogon, Gulang-gulang, Puting Sapa, Bolos, Monbon and Gabao in Irosin, and Sangkayon and Buraburan in Juban. Small to moderate sized earthquakes and ash explosions continued. On 2 August, white steam plumes rose from active craters and fissures.
On 28 September 2007 the number of volcanic earthquakes increased and PHIVOLCS noted a possible eruption. Explosions at 0134 and 0139 on 4 October 2007 caused a blanket of thick ashfall in sixteen villages that resulted in minor injuries and damage. Instruments recorded 40 volcanic earthquakes and eight short harmonic tremors during a 24 hour interval ending at 0526 that day. Moderate steaming from fissures were found on the SW flank.
According to the news source Southen Luzon Bureau, on 15 October 2007 PHIVOLCS found an additional six points of emission around the volcano, three each on the NW and SE slopes. Several other emission points had stopped on the N, SSW, and SW slopes. Overall, nine emission points were active. News reports also mentioned that residents in the village of San Rogue noted bulging of the ground. A deformation survey was allegedly conducted, but results were not available in PHIVOLCS reports.
In the 24 hours from 0800 on 6 January 2008, at least seven minor earthquakes were recorded, but no steaming was noted. Although the Alert Level remained at 1, authorities began to enforce a no-entry policy in a 4-km radius.
Geologic Background. 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.
Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph); Southern Luzon Bureau, Philippine Daily Inquirer, PO Box 2353, Makati Central Post Office, 1263 Makati City, Philippines (URL: http://newsinfo.inquirer.net/).