Mount Bulusan, a volcano in Sorsogon province at the southern tip of Luzon, produced more than 200 volcanic earthquakes in a single 24-hour period, a seismic burst that far exceeds anything recorded during the volcano’s recent unrest episodes. The Philippine Institute of Volcanology and Seismology (PHIVOLCS) has warned that the shallow swarm could precede a sudden steam-driven explosion, and the agency’s 4-kilometer permanent danger zone around the summit remains strictly enforced.
The spike has put farming communities on Bulusan’s flanks on edge. Tens of thousands of people live in municipalities ringing the volcano, and even a modest phreatic blast can shower nearby areas with rock fragments, hot debris, and ash.
What the seismic data shows
According to the Philippine Information Agency, PHIVOLCS recorded 223 volcanic earthquakes during one observation window, with the events clustered at depths of roughly 2.5 kilometers beneath the volcano’s northern flank. Some reports have placed the single-day count as high as 252, though PHIVOLCS has not published a timestamped seismic catalog that would pin down the exact peak. Either figure represents a dramatic jump from earlier bouts of elevated activity, when daily counts were notably lower, based on Philippine News Agency reporting on past unrest.
The shallow depth of the quakes points toward hydrothermal processes rather than deep magma movement. When superheated groundwater trapped beneath a volcanic edifice flashes to steam, it can violently shatter rock near the surface, producing what volcanologists call a phreatic explosion. Bulusan has a well-documented pattern of exactly this behavior. During its most recent notable sequence in 2016 and 2017, the volcano fired off several phreatic blasts that sent ash columns thousands of meters into the air and dusted surrounding towns, according to records maintained by the Smithsonian Global Volcanism Program.
PHIVOLCS has also moved to tamp down a recurring public concern: whether recent strong tectonic earthquakes elsewhere in the Philippines might be triggering the Bulusan swarm. In a general statement reported by the Philippine News Agency, the agency said that strong tectonic quakes are not directly linked to volcanic eruptions, noting that regional tectonic stress and localized volcanic seismicity operate through fundamentally different mechanisms. Several significant tectonic earthquakes have struck the Visayas and Mindanao in recent months, fueling speculation that the events are connected, but PHIVOLCS treats them as independent phenomena in its monitoring protocols.
Key gaps in the picture
The most consequential unknown is Bulusan’s current formal alert level. PHIVOLCS uses a five-tier system, from Alert Level 0 (quiet) to Alert Level 5 (hazardous eruption in progress). None of the available public reporting specifies whether the agency has upgraded the alert in response to the swarm. That distinction matters directly: an alert-level raise would trigger mandatory evacuations within expanded radii and activate local disaster-response protocols affecting thousands of households.
Also absent from public reporting are sulfur dioxide flux measurements and ground-deformation data from the current episode. These two parameters are critical for distinguishing a pressurized groundwater system from intruding magma. Rising sulfur dioxide output would suggest fresh magma degassing at shallow depth, while stable or low emissions during a seismic swarm lean toward a steam-driven scenario. Without those numbers, scientists cannot estimate with precision whether Bulusan is building toward a phreatic blast, a larger magmatic eruption, or a gradual return to quiet.
The duration of the swarm is another open question. Short-lived bursts that decay within days often reflect transient pressure shifts in the hydrothermal plumbing. Swarms that intensify or persist for weeks can signal a more sustained energy source, such as magma slowly shouldering its way into shallow rock. The current data window is still narrow, and PHIVOLCS has not yet released a trend analysis showing whether the daily earthquake count is climbing, falling, or holding steady.
What Bulusan’s history suggests
Bulusan is sometimes overshadowed by the Philippines’ more famous volcanoes, particularly nearby Mayon, but it is one of the country’s most frequently active. The Smithsonian database records multiple episodes over the past two decades in which seismic swarms produced phreatic blasts but did not escalate to full magmatic eruptions with lava flows or pyroclastic currents. That track record offers some reassurance, but it is not a guarantee. A sustained count above 200 volcanic earthquakes per day for more than 48 consecutive hours, especially if paired with rising gas emissions, would represent a departure from the historical pattern and raise the probability of a more serious escalation.
Seismic instruments are highly sensitive to rock fracturing and fluid movement underground, but they provide only indirect clues about the exact state of magma or groundwater. At comparable volcanoes worldwide, shallow, tightly clustered quakes with no clear magmatic gas signal have most often preceded steam-driven blasts rather than lava-producing eruptions. Exceptions exist, which is why volcanologists rely on multiple data streams rather than seismicity alone.
What communities around Bulusan are doing now
The 4-km permanent danger zone exists for exactly this kind of scenario, and PHIVOLCS advisories carry legal weight under Philippine disaster-management law. Local governments around Bulusan have well-rehearsed contingency plans drawn from past eruptions, including pre-identified evacuation centers, protocols for suspending school classes, and procedures for clearing ash from roads and rooftops. These measures are designed to scale with the alert level: if PHIVOLCS raises the status, authorities can widen restricted areas and relocate at-risk families on short notice.
For now, officials are focused on information campaigns, making sure farmers, tour operators, and transport providers understand the current restrictions and why they are in place. Even a small phreatic explosion can be lethal to anyone near the vent, but its destructive reach drops off sharply beyond the established perimeter. Staying outside the danger zone, preparing for possible ashfall, and tracking official PHIVOLCS updates remain the most effective steps residents can take while the science catches up to the seismicity.
Why the next 72 hours of Bulusan monitoring data matter most
Bulusan’s sudden spike is a reminder that long quiet stretches near active volcanoes do not mean the underlying system has gone dormant. The plumbing beneath the summit remains dynamic, and episodes of unrest can surface with little warning. As of late May 2026, PHIVOLCS continues round-the-clock monitoring, and the next 48 to 72 hours of seismic data will go a long way toward clarifying whether this swarm is winding down or just getting started.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
