Shishaldin volcano, a 9,373-foot stratovolcano on Unimak Island at the eastern end of Alaska’s Aleutian chain, has been producing persistent seismic tremor, frequent infrasound signals, and elevated sulfur dioxide emissions for weeks, according to the Alaska Volcano Observatory. The sustained unrest has kept the volcano at a YELLOW/ADVISORY alert level since mid-April 2026, a designation that signals activity above normal background but stops short of indicating an imminent eruption. For the roughly 4,500 residents of Unalaska and Dutch Harbor about 60 miles to the east, and for airlines flying heavily trafficked great-circle routes between North America and Asia, the activity warrants close attention.
What monitoring instruments are detecting
The clearest picture of Shishaldin’s current behavior comes from the U.S. Geological Survey’s Alaska Volcano Observatory, which publishes both weekly and daily bulletins through its Hazard Notification System. A weekly update published May 1 stated that Shishaldin “continues to exhibit volcanic unrest,” with instruments recording numerous small earthquakes, frequent infrasound signals, and persistent volcanic tremor throughout the preceding week. Satellite spectrometers detected elevated sulfur dioxide on most days during that period, a sign that magma at depth is releasing substantial amounts of volatile gas.
An earlier daily bulletin from mid-April described a “strong sulfur dioxide plume” visible in satellite data, paired with elevated seismic and infrasound activity. The most recent daily bulletin, issued May 2, reported that unrest persists, though cloud cover prevented visual confirmation of any surface changes at the summit. Seismic and infrasound sensors continued to register elevated activity even when webcams were blinded by weather, confirming that the unrest is sustained rather than a short-lived spike.
Three instrument types are doing the heavy lifting. Seismometers record earthquakes and continuous tremor caused by fluid and gas movement underground. Infrasound arrays pick up low-frequency pressure waves generated by explosions or vigorous gas releases at the vent. And satellite-based spectrometers, including the European Space Agency’s TROPOMI instrument aboard Sentinel-5P, measure atmospheric sulfur dioxide concentrations from orbit, allowing AVO to track gas output even when local weather blocks ground-based views. USGS research has shown that co-locating seismic and infrasound sensors improves detection and characterization of volcanic activity, making the combination a standard part of the monitoring toolkit across the Aleutians.
Why Shishaldin’s history matters here
Shishaldin is one of the most frequently active volcanoes in North America. Its most recent significant eruption, in July and August 2023, sent ash columns above 40,000 feet, triggered a RED/WARNING alert, and forced the Anchorage Volcanic Ash Advisory Center to issue multiple advisories affecting trans-Pacific air traffic. That eruption was preceded by weeks of escalating seismicity and gas emissions not unlike the pattern being observed now.
Earlier research reinforces the connection between the current signals and real volcanic processes. A study by Caplan-Auerbach and McNutt covering 2003 to 2004 recorded seismo-acoustic signals at Shishaldin directly associated with degassing explosions and low-frequency seismicity. The current pattern of persistent tremor, frequent infrasound detections, and strong SO₂ plumes is consistent with the gas-driven activity described in that work. The parallel does not guarantee the same outcome, but it gives the current signals a concrete scientific precedent and supports AVO’s decision to maintain a heightened alert level.
Critically, Shishaldin’s record also includes extended periods of elevated unrest that eventually subsided without producing significant ashfall. The volcano does not always follow the same script, which is precisely why AVO has held at YELLOW/ADVISORY rather than escalating to ORANGE/WATCH, the next tier, which would indicate a heightened potential for eruption.
What remains uncertain
Several gaps in the available data limit how far any assessment can go. AVO bulletins describe sulfur dioxide emissions as “elevated” or note a “strong” plume, but none of the published updates include quantitative emission rates or plume altitude measurements. Without specific numbers, it is difficult to compare the current episode to the 2023 eruption or to benchmark it against other active volcanoes globally. The qualitative language tells scientists the volcano is degassing above normal levels, but not by how much or whether the trend is climbing.
Cloud cover has repeatedly blocked visual observations of the summit, meaning there is no confirmed evidence of new vents, lava flows, or changes to the crater from webcam or ground-based imagery. The AVO bulletins in the current series have not referenced satellite thermal anomalies either. The absence of visual confirmation does not mean nothing is happening at the surface; it means the monitoring network is currently relying almost entirely on geophysical and atmospheric chemistry instruments.
Perhaps the most consequential unknown involves the depth and volume of magma feeding the unrest. Seismic tremor and infrasound show that gas, and possibly magma, are moving. But without complementary data such as ground deformation measurements from GPS stations or detailed gas-ratio analyses, scientists cannot determine whether the system is steadily pressurizing toward an eruption or simply venting built-up gas in a way that may eventually relieve internal pressure. No AVO scientist has publicly offered a timeline or probability estimate for an eruption during this episode.
What the alert level means in practice
AVO uses a four-tier alert system. GREEN/NORMAL indicates typical background activity. YELLOW/ADVISORY, where Shishaldin sits now, means the volcano is exhibiting signs of elevated unrest. ORANGE/WATCH signals heightened or escalating unrest with increased potential for eruption. RED/WARNING means an eruption is imminent or underway. Each step up triggers progressively more urgent coordination with the Federal Aviation Administration and the Anchorage Volcanic Ash Advisory Center, which is responsible for issuing SIGMETs, the formal warnings that reroute commercial flights away from ash hazards over the North Pacific.
The YELLOW/ADVISORY designation is a status indicator, not a prediction. It tells the public and aviation authorities that the volcano requires close monitoring and that conditions could change rapidly, but it deliberately avoids implying that an eruption is expected on any particular timescale. If AVO escalates to ORANGE or RED, the practical consequences for air traffic and nearby communities would intensify quickly.
What to watch for next
For communities downwind and airlines operating across the North Pacific, the practical takeaway as of early May 2026 is cautious awareness. The monitoring network has detected sustained, above-background activity that has persisted long enough to rule out a brief anomaly. At the same time, the lack of visual confirmation of eruptive activity, the absence of quantitative gas data, and Shishaldin’s mixed historical record all argue against assuming a particular outcome.
The signals that would mark a meaningful escalation include a sharp increase in earthquake frequency or magnitude, the appearance of satellite-detected thermal anomalies at the summit, a transition from continuous tremor to discrete explosive events, or AVO raising the alert level to ORANGE/WATCH. The observatory’s Hazard Notification System remains the authoritative channel for any changes. Until new data resolve some of the current uncertainties, the most accurate description is a volcano in a prolonged state of unrest, watched closely by scientists, but not yet crossing the threshold into confirmed eruption.
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*This article was researched with the help of AI, with human editors creating the final content.
