Sorsogon, Philippines. Hundreds of volcanic earthquakes have been rattling the ground beneath Bulusan volcano since mid-March 2026, prompting the Philippine Institute of Volcanology and Seismology (PHIVOLCS) to raise the alert level and warn nearby residents that sudden, violent steam explosions could strike with almost no advance notice.
The 1,565-meter stratovolcano at the southeastern tip of Luzon has a long record of exactly this kind of eruption: fast, steam-driven blasts that hurl rock and ash into the sky before anyone can react. The last significant sequence, between 2016 and 2017, sent ash columns several kilometers high and forced evacuations across multiple towns in Sorsogon province. Now the mountain is restless again, and the pattern looks familiar.
What PHIVOLCS has recorded
PHIVOLCS raised Bulusan to Alert Level 1 after its seismic network logged 475 volcanic earthquakes beginning March 15, 2026. Alert Level 1 signals abnormal activity that could lead to an eruption and triggers formal hazard advisories for communities surrounding the summit.
The agency imposed a four-kilometer permanent danger zone around the crater and added a two-kilometer extended danger zone on the southeastern flank, where past eruptions have channeled pyroclastic density currents, ballistic rock fragments, and ashfall. Aviation authorities were also warned about potential airborne hazards.
A follow-up advisory sharpened the concern. In a 24-hour observation window ending at midnight on May 1, 2026, PHIVOLCS recorded 223 volcanic earthquakes centered roughly 2.5 kilometers beneath the northern part of the edifice. That depth places the seismic source squarely in the hydrothermal system, the zone where superheated water and gas can flash to steam and punch explosive blasts through the surface without the magma movement that typically precedes larger eruptions.
A sulfur dioxide measurement taken in early 2026 showed emissions averaging about 75 tons per day, a level consistent with active degassing but not necessarily a large magma body pushing toward the surface. PHIVOLCS explicitly warned of possible steam-driven blasts and flagged lahar risks along drainage channels, especially during heavy rain, because loose volcanic debris on Bulusan’s slopes can be swept into fast-moving mudflows with little warning.
Why Bulusan’s history makes it one of the country’s deadliest cones
Bulusan is one of the most frequently active volcanoes in the Philippines, and its pattern of sudden phreatic blasts has made it one of the archipelago’s deadliest. Because steam-driven explosions can occur with virtually no precursory warning, people caught in the open near the summit face extreme danger. The Smithsonian’s Global Volcanism Program catalogs dozens of eruptions dating back centuries, with phreatic events dominating the modern record. In 2006, a series of blasts deposited ash on surrounding towns and displaced thousands. Between 2016 and 2017, repeated phreatic explosions sent ash columns as high as three kilometers and triggered evacuations across Sorsogon. The combination of frequent eruptions, minimal warning time, and densely populated surroundings, with roughly 50,000 people living in municipalities that border the volcano’s slopes, places Bulusan among the most hazardous volcanic systems in the country.
Those episodes share a common thread with the current unrest: shallow earthquake swarms concentrated in the hydrothermal system, followed by sudden surface explosions. But the pattern is not perfectly predictive. Some swarms at Bulusan have ended without any eruption at all, while others have escalated rapidly. That unpredictability is precisely what makes the volcano dangerous and why PHIVOLCS treats even moderate seismic upticks seriously.
What scientists still do not know
Publicly available advisories leave several gaps. PHIVOLCS has not released detailed ground-deformation or tiltmeter data for the period of unrest. Inflation of the volcanic edifice would be one of the clearest signals that pressurized fluids or magma are building at shallow depths. Without that information, outside scientists cannot independently judge whether the swarm reflects a stable hydrothermal disturbance or a system building toward something larger.
The sulfur dioxide figure comes from a single measurement date. Gas flux at active volcanoes can shift dramatically over hours, so one snapshot tells little about the trend. Whether emissions have risen, fallen, or held steady since that reading is not addressed in any advisory reviewed for this report. A sustained increase in SO2 would strengthen the case for magmatic involvement; a decline would support the interpretation that the unrest is primarily hydrothermal.
The exact magnitudes and precise locations of individual earthquakes within the swarm have not been published beyond summary totals and a general depth estimate. Volcanologists tracking unrest typically watch for migration patterns, clusters of quakes shifting upward or outward over time, as a sign that fluid pathways are opening. The aggregated count of 475 events confirms elevated activity but does not reveal whether the swarm is intensifying, stabilizing, or winding down.
No direct statements from PHIVOLCS scientists, local officials, or residents near the volcano have appeared in the advisories reviewed for this report. The information presented here is drawn entirely from official PHIVOLCS advisories distributed through Philippine Information Agency news releases and from the Smithsonian’s Global Volcanism Program database. Readers should note that PIA articles summarize PHIVOLCS advisories rather than reproducing the original monitoring bulletins in full.
What the alert level means for people on the ground
Alert Level 1 does not mean an eruption is imminent. It means abnormal processes are underway beneath the volcano and that sudden explosions are plausible. For communities near Bulusan, the distinction matters: phreatic blasts do not build slowly like magmatic eruptions with advancing lava. They are short, violent bursts of ash, gas, and rock that can kill or injure anyone caught in the open near the summit.
That reality is why PHIVOLCS treats the four-kilometer exclusion zone as non-negotiable during periods of unrest. Pyroclastic density currents and ballistic projectiles are most lethal closest to the crater. The extended danger zone on the southeastern flank reflects the agency’s knowledge of past flow paths and the topography that funnels hot gases and debris downslope.
Lahar hazards add a second layer of risk that can persist long after the shaking stops. Heavy rainfall can transform loose ash and rock into fast-moving slurries that follow river channels and gullies for kilometers. Communities downstream from Bulusan’s slopes, some of which cannot see the volcano itself, can still be hit by mudflows capable of burying roads, damaging bridges, and flooding low-lying neighborhoods. PHIVOLCS’ lahar warnings are aimed at those residents as much as at people living near the crater.
How Bulusan’s alert could change in the weeks ahead
The clearest signal of escalation would be a move from Alert Level 1 to Alert Level 2 or higher, which would indicate that monitoring data, whether seismic, gas, or deformation, had shifted toward a greater likelihood of eruption. A return to Alert Level 0 (Normal) would mean seismicity and emissions had dropped back to background levels.
For now, Bulusan sits in a state of heightened vigilance: abnormal, but not yet showing a clear pre-eruptive trajectory. Residents in and around Sorsogon province are being told to stay out of the exclusion zones, prepare evacuation routes, secure essential supplies, and monitor updates from PHIVOLCS and local disaster offices. Given Bulusan’s track record of erupting with little warning, those are not abstract precautions. They are the most practical defense available when a volcano’s next move remains genuinely uncertain.
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*This article was researched with the help of AI, with human editors creating the final content.
