The ground beneath Mount Bulusan has not stopped shaking. Over a 10-day stretch ending May 20, 2026, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) recorded 442 volcanic earthquakes beneath the 1,565-meter cone in Sorsogon province, 55 of them at depths shallower than five kilometers. On the worst day alone, instruments logged 223 quakes in 24 hours, roughly half the entire swarm compressed into a single burst. PHIVOLCS warned that a steam-driven explosion could strike with little or no additional buildup, putting thousands of residents and regional aviation on notice.
What the seismic swarm tells us
Bulusan is not building toward a Hollywood-style lava eruption. The danger here is phreatic: superheated groundwater trapped inside fractured rock flashes to steam and blasts outward, hurling ash columns, rock fragments, and scalding vapor into the air. These explosions can go from quiet to violent in minutes, which is what makes the shallow quake cluster so alarming. Fifty-five events originating less than five kilometers below the summit mean pressurized fluids are interacting with rock close enough to the surface that the lid could blow before monitoring networks register a clear escalation.
Bulusan has done this before. The volcano produced a series of short-lived phreatic bursts in 2022 that dusted nearby barangays with ash but never progressed to sustained magmatic eruption. The current swarm fits that behavioral pattern, and PHIVOLCS has framed its warnings accordingly, focusing on sudden steam blasts rather than lava flows. Still, 442 quakes in 10 days is a rate that demands attention. The concentration of more than half those events into a single day suggests the system experienced a sharp pressure spike, not a gradual buildup.
Danger zones and flight warnings
PHIVOLCS enforced a Permanent Danger Zone around the crater and an Extended Danger Zone on the upper flanks, barring farming, tourism, and forest-product gathering in the areas most exposed to ballistic projectiles, ashfall, and fast-moving mudflows called lahars. Aviation authorities issued cautions for aircraft operating near the summit, a standard step when a volcano shows signs of possible ash ejection. Volcanic ash can sandblast cockpit windshields, clog jet engines, and reduce visibility along flight corridors across the Bicol region.
For the communities that ring Bulusan, the restrictions carry real economic weight. Many families farm plots that sit inside or at the edge of the exclusion zones, and every day the zones remain closed is a day without income from crops, livestock, or charcoal production. Local disaster officials are tasked with enforcing boundaries that cut directly into livelihoods while preparing contingency plans for rapid escalation: transport for vulnerable residents, temporary shelters, and communication systems capable of pushing warnings if conditions change overnight.
The alert level puzzle
The formal threat designation has shifted in a way that can confuse anyone watching from the outside. During the height of the swarm, PHIVOLCS raised Bulusan to Alert Level 1, indicating abnormal activity that could lead to eruption. Days later, the agency lowered the status back to Alert Level 0, but it kept the Permanent Danger Zone off-limits and cautioned that the alert could snap back up if monitored parameters climbed again.
That combination sends a mixed signal. A Level 0 designation technically means background-level activity, yet a closed danger zone says the volcano is still considered capable of producing a hazardous event. PHIVOLCS appears to be reading declining seismic energy while hedging against the possibility that the decline is temporary. For residents who have lived through prior ashfalls, the gap between the number on the alert scale and the restrictions on the ground can be disorienting: daily life looks normal, but official maps still mark nearby fields and trails as forbidden.
What the public record does not show
Several datasets that volcanologists normally use to distinguish a fading swarm from one that precedes eruption have not appeared in any public advisory tied to this episode. Ground-deformation readings from tiltmeters or GPS stations, which would reveal whether the volcanic edifice is inflating, are absent from both the Philippine Information Agency releases and the Smithsonian Institution’s Global Volcanism Program record. Sulfur dioxide flux measurements, a standard indicator of magma rising toward the surface, are likewise uncited. PHIVOLCS operates monitoring stations on Bulusan and almost certainly collected this data, but it has not been released in a form that outside analysts can evaluate.
That gap matters because the seismic numbers alone cannot answer the question residents most want answered: Is this winding down, or is it a pause before something bigger? Without deformation and gas data in the public domain, the honest answer is that no one outside PHIVOLCS can say with confidence.
What residents should watch for now
The Permanent Danger Zone remains closed regardless of the alert level, and PHIVOLCS has made clear that a return to Alert Level 1 or higher could happen quickly if seismicity rebounds. Phreatic eruptions at Bulusan have historically struck with minimal lead time, sometimes just hours between a new advisory and an actual explosion. Local governments have been advised to keep evacuation routes clear and staging areas ready.
For the tens of thousands of people living on Bulusan’s flanks, the calculus is familiar but no less stressful: carry on with daily routines while staying close to a radio or phone alert, keep a go-bag packed, and resist the pull to re-enter restricted farmland before officials give the all-clear. The mountain is quiet for now. Quiet, at Bulusan, has never been the same thing as safe.
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*This article was researched with the help of AI, with human editors creating the final content.
