Mount Bulusan, one of the most frequently active volcanoes in the Philippines, has produced 442 volcanic earthquakes in roughly ten days, and the country’s monitoring agency says a sudden steam-driven eruption remains possible. The seismic swarm began accelerating on the evening of May 18, 2026, beneath the 1,565-meter stratovolcano in Sorsogon province at the southern tip of Luzon, about 390 kilometers southeast of Manila. PHIVOLCS is holding Bulusan at Alert Level 1, its lowest elevated status, but the intensity and shallow depth of the quakes point to mounting pressure in a volcano that has a well-documented habit of erupting with little warning.
A surge of shallow quakes
Between 4:00 p.m. on May 18 and the close of the most recent monitoring window on May 28, PHIVOLCS instruments recorded 442 volcanic-tectonic earthquakes beneath Bulusan. Fifty-five of those events struck at depths shallower than five kilometers, according to the Smithsonian Global Volcanism Program, which cross-references Philippine monitoring data in its weekly reports. PHIVOLCS placed the average source depth at roughly 2.5 kilometers.
That shallow clustering is significant. At 2.5 kilometers, the earthquakes are occurring near the hydrothermal system, the zone of superheated groundwater and fractured rock that sits above any deeper magma source. When pressure builds in that zone, groundwater can flash to steam and blast through the surface in what volcanologists call a phreatic eruption. These events can hurl rocks, generate ash clouds, and trigger mudflows on steep slopes, all with very little lead time.
“The increased seismicity raises the chances of phreatic eruptions,” PHIVOLCS stated in its advisory for the monitoring period covering May 14 through May 20, warning that communities on Bulusan’s flanks should treat any rapid escalation in activity as a signal to move.
Inflation on two flanks
Seismicity is not the only signal. GPS instruments have tracked ground inflation on Bulusan’s north flank since June 2025 and on its upper west flank since January 2026, according to PHIVOLCS advisories distributed through the Philippine Information Agency. Inflation on two separate flanks over different timescales suggests a broad pressurization zone rather than a single, tightly focused intrusion.
The north-flank signal, now nearly a year old, overlaps spatially with the area where many of the 55 shallow earthquakes have been detected. That alignment raises the possibility that a localized pressure source is driving both the swelling and the seismicity, though PHIVOLCS has not confirmed a direct link.
Meanwhile, degassing at the summit has remained weak to moderate, and the most recent sulfur dioxide measurement came back low. In many volcanic systems, low SO2 paired with intense shallow seismicity can mean that heat is building inside a sealed hydrothermal cap rather than venting harmlessly. If that cap fractures, the release can be explosive. But a low reading can also simply mean that no significant new magma is pushing toward the surface. Repeat measurements over the coming weeks will help PHIVOLCS distinguish between those scenarios.
Bulusan’s track record
What makes the current swarm especially worth watching is Bulusan’s history. The volcano produced at least 17 phreatic eruptions between 2006 and 2017, according to the Smithsonian’s eruption catalog. Several of those events sent ash columns more than a kilometer above the summit and deposited fine debris on farming communities in the municipalities of Irosin, Juban, and Casiguran, which ring the volcano’s lower slopes. In June 2016, a phreatic blast forced the evacuation of more than 9,000 residents from villages within the permanent danger zone.
Bulusan’s pattern is consistent: periods of elevated seismicity and mild inflation, followed by short, sharp steam explosions that can catch nearby communities off guard. The current swarm fits that template, which is part of why PHIVOLCS has explicitly warned that phreatic eruptions “cannot be ruled out” even at Alert Level 1.
What Alert Level 1 means on the ground
Alert Level 1 does not mean “no threat.” It means the volcano is above its normal background activity but is not yet showing signs of an imminent magmatic eruption. Under this status, entry into the four-kilometer permanent danger zone around the summit is prohibited. Broader evacuation orders have not been issued, but PHIVOLCS has directed local governments to review contingency plans and reminded residents that even a modest steam blast can be deadly at close range.
One gap in the public record is population exposure. PHIVOLCS bulletins and the Smithsonian digest do not include figures for how many people live inside or immediately outside the exclusion boundary. Bulusan’s lower slopes are home to rice paddies, coconut farms, and several barangays, but precise counts have not appeared in official reporting for the current unrest period. That makes it difficult to estimate the scale of any evacuation a higher alert level would trigger.
No statements from municipal disaster offices or barangay officials have surfaced to confirm whether residents felt any of the 442 quakes, or whether springs and fumaroles have changed in temperature or flow. Felt reports and fumarole observations often serve as early, ground-level confirmation that subsurface changes are reaching the surface. Their absence does not mean nothing is happening; it means the monitoring picture right now is limited to instrumental data and official bulletins.
Rainy season adds another layer of risk
Sorsogon province is entering its wet season, and heavy rainfall on a volcano already showing signs of unrest raises the risk of lahars, fast-moving flows of water, ash, and debris that can travel far beyond the permanent danger zone along river channels. Even without an eruption, rain saturating loose volcanic deposits on Bulusan’s steep flanks can mobilize material from previous blasts. If a phreatic eruption were to coincide with a typhoon or sustained downpour, lahar hazards would escalate quickly.
PHIVOLCS routinely warns communities downstream of Bulusan’s major drainages to watch for rising water levels and unusual sediment loads during heavy rain, guidance that carries extra weight during a period of elevated seismicity.
Why the next PHIVOLCS bulletin matters more than usual
The situation at Bulusan as of late May 2026 is best understood as a period of heightened unrest with a specific, well-characterized hazard: sudden steam-driven explosions from the summit or flank vents. The 442-earthquake swarm, the shallow source depths, and the months-long inflation trend all point to a pressurized system. Low gas emissions leave open the question of whether that pressure is building toward a release or slowly dissipating.
If PHIVOLCS raises the alert level, it will signal that new data, perhaps stronger deformation, higher gas output, or a more intense seismic swarm, have shifted the risk assessment. Until then, the agency’s own language is clear: a phreatic eruption remains on the table, and communities on Bulusan’s flanks should treat any rapid change in the volcano’s behavior as a signal to move.
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*This article was researched with the help of AI, with human editors creating the final content.
