PHIVOLCS raises Alert Level for Bulusan volcano

Increased activity under Bulusan volcano, Philippines prompted PHIVOLCS to raise the Alert Level for the volcano from 0 to 1 on October 13, 2022. The last eruption at this volcano took place in June 2022.

The Bulusan Volcano Network (BVN) has recorded a total of 126 weak and shallow (0 – 5 km / 3.1 miles deep) volcanic earthquakes from 05:00 LT on October 11 to 15:00 LT on October 12.¹

Most of these events occurred in the northwestern sector and the summit area of Bulusan and are attributed to rock-fracturing processes within the edifice.

In addition to these earthquakes, several monitored parameters indicate increased hydrothermal activity and overall unrest as follows:

• Ground Deformation data from continuous GPS measurements and radial tilt indicate short-term inflation of the southern flanks of Bulusan since April 2022. Moreover, a pronounced and ongoing inflationary trend that began in late September 2022 was detected by most of the electronic tilt stations on the southwestern and southern slopes. This current inflation cycle signifies the ongoing pressurization of the shallow hydrothermal system.
• Volcanic Carbon Dioxide (CO2) concentrations measured monthly in springs in the southeastern sector of Bulusan have been increasing since July 2022, in conjunction with increasing spring temperatures since April 2022 in monitored hot springs in the southwestern sector.
• Sulfurous Odor was also reported by residents of Sitio Talistison, Brgy. Mapaso, Irosin and Brgy. San Roque, Bulusan on October 10 and 11, 2022. In contrast, only very weak to moderate emission of steam-laden plumes is transpiring at the summit crater and northwest vents.

In view of the above, DOST-PHIVOLCS has raised the alert status for Bulusan from Alert Level 0 to Alert Level 1, which means that the volcano is currently in a state of low-level unrest.

Local government units and the public are reminded that entry into the 4 km (2.5 miles) radius Permanent Danger Zone (PDZ) must be strictly prohibited and that vigilance in the 2 km (1.2 miles) Extended Danger Zone (EDZ) on the southeast sector must be exercised due to the increased possibilities of sudden and hazardous phreatic eruptions.

People living within valleys and along river/stream channels especially on the southeast, southwest and northwest sector of the edifice should be vigilant against sediment-laden stream flows and lahars in the event of heavy and prolonged rainfall should a phreatic eruption occur.

A powerful phreatic eruption took place at the volcano at 02:37 UTC (10:37 LT) on June 5, 2022:

Powerful phreatic eruption at Bulusan volcano, Philippines

Another powerful eruption took place on June 12, covering nearby communities in heavy ash:

New eruption at Bulusan volcano covers nearby communities in heavy ash, Philippines

Geological summary

Luzon’s southernmost volcano, Bulusan, was constructed along the rim of the 11-km-diameter (6.8 miles) dacitic-to-rhyolitic Irosin caldera, which was formed about 36 000 years ago.

Bulusan 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 Bulusan 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 of 1 565-m-high (5 134  feet) Bulusan volcano is unvegetated and contains a 300-m-wide (984 feet), 50-m-deep (164 feet) crater. Three small craters are located on the SE flank. Many moderate explosive eruptions have been recorded at Bulusan since the mid-19^th century.²

References:

¹ BULUSAN VOLCANO UPDATE – October 12, 2022 – PHIVOLCS

² Bulusan – Geological summary – GVP

Featured image credit: PDERG