The ground beneath Mount Bulusan has not stopped shaking. Over a single week in May 2026, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) recorded 442 volcanic earthquakes beneath the 1,565-meter cone in Sorsogon province, southern Luzon. That works out to more than 60 quakes per day, all of them concentrated at shallow depths under the volcano’s northern flank, where superheated water meets rock baked by magma below. On May 20, PHIVOLCS raised Bulusan to Alert Level 1 and issued a pointed warning: a sudden steam-driven explosion is possible, and it could come with very little advance notice.
For the roughly 50,000 people living in the municipalities that ring Bulusan’s base, the alert is not abstract. This volcano has done this before.
What the seismic instruments are picking up
The 442 volcanic-tectonic earthquakes were logged between May 14 and May 20, 2026, according to the Bulusan entry maintained by the Smithsonian Institution’s Global Volcanism Program (GVP), which compiled the figure from the PHIVOLCS advisory issued that same day. The quakes clustered at depths of roughly 2.5 kilometers beneath the northern portion of the edifice, placing them squarely within Bulusan’s hydrothermal system rather than deeper in the magma reservoir.
That distinction matters. Earthquakes at this depth typically signal fluid movement and pressure buildup: groundwater being heated, expanding, and cracking through rock as it searches for an escape route. It is not the same as fresh magma pushing toward the surface, which would show up as deeper, longer-period seismic signals and measurable ground inflation. What PHIVOLCS is seeing instead is a pressurized plumbing system that could blow a valve.
In the Philippine volcano alert scale, Alert Level 1 indicates abnormal activity that has moved beyond baseline. It does not predict an imminent magmatic eruption, but it formally puts communities and local authorities on heightened watch for phreatic events: steam-driven blasts capable of hurling rocks and ash outward from the vent with almost no buildup.
Why phreatic eruptions are so dangerous
Phreatic explosions happen when trapped water flashes to steam faster than the surrounding rock can vent the pressure. The result is a violent burst of fragmented rock, ash, and steam that can travel several kilometers from the crater. Unlike large magmatic eruptions, which are often preceded by days or weeks of escalating warning signs, phreatic blasts can erupt from a seemingly quiet summit or flank vent in a matter of hours.
PHIVOLCS has emphasized that shallow hydrothermal activity of this kind can produce eruptions that occur with little warning. For villages on Bulusan’s slopes, even a brief event poses direct physical danger and can contaminate water supplies with volcanic sediment, disrupt farming, and ground local transportation under a layer of fine ash.
Bulusan’s own record underscores the point. Between 2016 and 2017, the volcano produced a series of phreatic eruptions that sent ash columns several kilometers into the sky, displaced farming communities across multiple barangays, and forced repeated evacuations of the permanent danger zone. Those events followed periods of elevated seismicity similar in character, though not identical in scale, to what instruments are recording now.
What the data does not yet show
The PHIVOLCS advisory confirms the earthquake count, the depth, and the alert level, but several monitoring indicators have not appeared in public bulletins so far. Ground-deformation measurements from tiltmeters and GPS stations on the flanks have not been reported. Inflation of the edifice would strengthen the case that pressure is building in a way that could escalate beyond a single phreatic blast. Similarly, sulfur dioxide emission rates, a standard marker of magmatic involvement, are absent from the public record. The lack of reported SO2 data is consistent with a hydrothermal rather than magmatic source, but it could also reflect a gap in reporting rather than a gap in emissions.
PHIVOLCS has also not released daily earthquake counts that would reveal whether the swarm is accelerating, holding steady, or tapering off within the seven-day window. A sustained or rising daily rate carries different implications than a burst that peaked early and then declined, and that trajectory will be critical to watch in the coming advisories.
Thermal imaging, whether from ground-based cameras or satellite passes, could detect subtle surface heating that often precedes steam venting. Reports from local residents of new fumaroles, stronger sulfur smells, or small ash puffs near the northern flank would also provide early clues that pressurized fluids are finding pathways to the surface.
What this means for people living near Bulusan
The practical message from PHIVOLCS is direct: the volcano is no longer behaving normally. Anyone entering the four-kilometer permanent danger zone around the crater, whether for farming, gathering forest products, or hiking, faces the highest risk. Local authorities typically respond to an Alert Level 1 declaration by restricting access to this inner zone, reviewing contingency plans, and coordinating with barangay leaders on communication protocols.
Even without formal evacuation orders, households on the volcano’s slopes are encouraged to identify safe shelters, prepare basic go-bags, and stay tuned to updates from municipal disaster offices. Short-lived ashfall can affect drinking water, respiratory health, and road visibility, so storing clean water and keeping dust masks accessible are simple steps that reduce exposure.
At the same time, the current alert level is deliberately calibrated to avoid unnecessary panic. There is no evidence in available reports of sustained deformation or elevated sulfur emissions that would point to a larger eruption building. Many periods of hydrothermal unrest at volcanoes end without a major explosive event, as fluids gradually escape and pressure dissipates on its own. The purpose of Alert Level 1 is to make sure that if Bulusan does produce a sudden blast, people are not caught off guard in the most hazardous areas.
Signals to watch in the weeks ahead
The next round of PHIVOLCS bulletins will be the most telling. If updated earthquake counts hold steady or decline, and no continuous tremor or surface gas emissions appear, the likelihood of a significant phreatic event will decrease, and the agency could eventually consider stepping the alert back down. If the daily quake rate climbs, if tremor shifts from discrete events to a sustained hum on seismometers, or if new steam vents open on the northern flank, authorities may need to tighten access restrictions or prepare for short-notice evacuations of specific high-risk barangays.
Bulusan is one of the most active volcanoes in the Philippines, with at least 16 recorded eruptions since 1852, most of them phreatic. Its pattern is to simmer, spike, and occasionally blow off steam in the most literal sense. The 442 earthquakes logged in a single week are a clear signal that the system is pressurized. Whether that pressure finds a quiet release or a violent one is the question PHIVOLCS is now watching around the clock.
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*This article was researched with the help of AI, with human editors creating the final content.
