Bulusan lies at the SE end of the Bicol volcanic arc in the Philippines. It is flanked by several other large intra-caldera lava domes and cones and its summit contains a 300-m-wide, 50-m-deep crater. Three small craters are located on the SE flank. Moderate explosive eruptions have been recorded since the mid-19th century. The previous eruption period, during June 2017, consisted of weak phreatic explosions and produced minor ashfall in nearby villages (BGVN 42:08). This reporting period covers a new eruption period during June 2022, characterized by similar phreatic explosions, based on information from the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

Summary of activity during January 2020-May 2022. Activity since the last eruption in June 2017 mainly consisted of occasional volcanic earthquakes and weak gas-and-steam emissions, sulfur dioxide emissions were below detectable levels since 2018, and the frequency of volcanic earthquakes had been at baseline levels of 0-2 earthquakes/day since May 2019. During 3-7 July 2020 a total of 72 volcanic earthquakes were detected, which included 43 low-frequency events. Diffuse white gas-and-steam plumes rose from the lower SE vent. Ground deformation data from continuous GPS measurements indicated short-term inflation of the edifice since late February 2020. On 6 July the Volcano Alert Level (VAL) was raised to 1, but activity remained relatively low, and on 26 October the VAL was lowered to 0 (normal conditions). Diffuse white gas-and-steam plumes continued to rise from the summit vents.

Unrest increased again during 7-10 May 2021, according to PHIVOLCS. There was a total of 186 volcanic earthquakes detected during 7-11 May. Inflation that was first recorded on the upper flanks on 6 March had continued. Sulfur dioxide emissions peaked at 48 tons/day on 20 May 2021, which was still within baseline levels. The VAL was raised to 1 on 11 May. During 12 June a total of 95 weak volcanic earthquakes were detected, in addition to diffuse white gas-and-steam plumes rising from the SW vent. Seismicity declined to the baseline level of 2-3 volcanic earthquakes per day during 1 July-17 August 2021, during which a total of 109 weak events were recorded. Short-term inflation had been detected since July 2020.

Activity during June 2022. During 4-5 June, PHIVOLCS reported 77 volcanic earthquakes. Ground deformation data from continuous GPS monitoring indicated short-term inflation, and electronic tilt data recorded an inflation event on the SE lower slopes beginning in April 2022. A phreatic eruption occurred at 1037 on 5 June 2022 and lasted approximately 17 minutes, based on seismic and infrasound monitoring by the Bulusan Volcano Network (BVN). Though cloud cover obscured most visual observations, a gray ash plume seen from Juban (Sorsogon Province) rose at least 1 km high and drifted W. Ashfall was reported to the NW in Puting Sapa, Añog, Guruyan, Catanusan, Buraburan, Bacolod, and Sangkayon in Juban, and Bolos in Irosin, Sorsogon Province. Residents of Añog, Guruyan, and Catanusan also reported rumbling sounds and a sulfur odor. As a result, the VAL was raised to 1. After this event, gas-and-steam emissions rose from the main crater, as well as the NW summit vent. A total of 27 weak volcanic earthquakes were recorded until 1600 by the BVN.

Increased seismicity began at 0500 on 9 June, which included a total of 45 volcanic earthquakes, including two low-frequency events. Most of these events were low magnitude and shallow. Another phreatic explosion was detected by the BVN at 0337 on 12 June, which lasted 18 minutes, based on seismic records. The eruption plume was not visible in webcams, but residents of Inlagadian (Casiguran municipality) briefly saw incandescence at the base of a plume. An explosion was felt by residents within 5 km, including in the barangays of Añog (Juban) and Inlagadian. Rumbling was heard in Sitio Bagong Barrio, and in the barangays of Santa Lourdes (Barcelona), Inlagadian and San Juan (Casiguran), Bentuco (Gubat), and Añog, Calateo, and Puting Sapa (Juban). Sporadic ash emissions were detected starting at 0430; ash emissions were visible rising from six vents: Blackbird Crater (the main crater), three explosion pits in the summit crater, and two vents on the NW and N sides of the summit. Gas-and-steam and ash plumes rose 400-750 m above the summit and drifted several kilometers NW. Ashfall was reported in areas as far as 50 km NW in Sorsogon City and Palanas, Pilar, Sorsogon Province, across Sorsogon Bay, and as far as Anislag (Daraga, Albay). Minor ashfall was detected in several barangays in the municipalities of Casiguran, Juban, and Magallanes. Sulfur dioxide emissions averaged 1,900 tons/day during 5-12 June. According to a Rappler news article, more than 11,000 people (at least 2,800 families) in Juban evacuated, and flights in and out of the Bicol International Airport were cancelled.

Unrest persisted through the rest of June. Gas-and-steam emissions rose 100-500 m above the summit and drifted NW, WSW, SW, and W. Daily sulfur dioxide emissions were 559-1,255 tons/day. Seismicity increased on 20 June; during 0500-1145 a total of 65 volcanic earthquakes were detected, including one low-frequency volcanic earthquake, though most were weak shallow events. During 24-29 June the BVN detected a total of 213 volcanic earthquakes, the strongest of which was an Mw 3.5 that occurred at 2327 on 24 June. Residents of Irosin and Bulusan in the Sorsogon Province felt the earthquake. A small-volume lahar began approximately at 1904 on 26 June during a thunderstorm, based on seismic and infrasound data. The event lasted 54 minutes and the Bulusan Volcano Observatory (BVO) confirmed thin, channel-confined lahar deposits along the Calang Creek on the SW flank, in the Cogon barangay.

Seismicity decreased to background levels during the third week of July. During 25 July-6 August sulfur dioxide emissions were 230 tons/day. On 21 August PHIVOLCS lowered the Alert Level to 0, reporting that unrest continued to decline to background levels.

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.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Rappler, Unit B, 3/F, North Wing Estancia Offices, Capitol Commons, Ortigas Center, Pasig City 1605, Bulusan Volcano erupts again, ash spreads to more areas, (URL: https://www.rappler.com/nation/phivolcs-bulusan-volcano-phreatic-eruption-advisory-june-12-2022-420am/).