Volcanoes and earthquakes
Earthquakes are common at volcanoes and help scientists track unrest, but most never lead to an eruption. Learn how they relate and how to read volcano alert levels and aviation color codes.
Key concepts
- Why earthquakes happen at volcanoes
- When tectonic earthquakes can trigger eruptions
- Volcano Alert Levels
- Aviation Color Codes
Why volcanoes have earthquakes
Many volcanoes sit near plate boundaries, especially around the Pacific, where both earthquakes and volcanoes are common. Earthquakes also happen inside volcanoes themselves, as magma and volcanic fluids move and as rock breaks under the changing pressure. These volcanic earthquakes are usually small.
Monitoring these small earthquakes is one of the main ways observatories track what is happening beneath a volcano. A change in the number, depth or character of earthquakes can be a sign of unrest that scientists investigate alongside other measurements such as ground deformation and gas emissions.
Can earthquakes trigger eruptions?
Sometimes. According to the USGS, a few large regional earthquakes, greater than magnitude 6, are considered related to a later eruption or to some unrest at a nearby volcano. However, a tectonic earthquake can only trigger an eruption if the volcano is already poised to erupt. That requires enough eruptible magma within the volcanic system and significant pressure in the magma storage region.
If those conditions exist, a large earthquake might cause dissolved gases to come out of the magma, a bit like shaking a bottle of soda, raising the pressure. If they do not exist, the earthquake will not cause an eruption. This is why most earthquakes near volcanoes, including many large ones, are not followed by eruptions.
Volcano Alert Levels on the ground
In the United States, USGS volcano observatories describe the activity of each monitored volcano with a ground-based Volcano Alert Level. Normal means typical background, non-eruptive activity. Advisory means elevated unrest above the known background. Watch means heightened or escalating unrest with increased potential for eruption, with the timeframe uncertain, or an eruption underway with limited hazards. Warning means a hazardous eruption is imminent, underway or suspected.
An alert level describes the volcano's current state as assessed by the observatory. It is not a forecast of a specific eruption date.
Aviation Color Codes in the air
Volcanic ash is dangerous to aircraft, so observatories also issue an Aviation Color Code: Green, Yellow, Orange or Red. Green means the volcano is in a typical non-eruptive state, Yellow means elevated unrest, Orange means heightened unrest or an eruption with no or minor ash emission, and Red means an eruption with significant ash emission is imminent or underway.
The two systems usually move together, for example Watch with Orange, but they describe different hazards and can differ. A volcano could have a ground level of Watch for lava flows while its aviation code reflects how much ash it is producing. That is why EQRadar always shows them side by side and never merges them into a single score.
Volcanoes in EQRadar
EQRadar's Volcanoes page shows U.S. volcanoes from the USGS Volcano Hazards Program, with the current alert level and aviation color code published for each monitored volcano, the time of the notice that set them, and the official notice text. A volcano with no published status is shown as unknown, not as normal.
The page does not forecast eruptions, draw ash clouds or interpret earthquake counts as a sign that an eruption is coming. Observatories make those assessments and publish them as official notices.
Three takeaways
- Small earthquakes are common at volcanoes and help observatories monitor unrest, but most do not lead to eruptions.
- A large tectonic earthquake can trigger an eruption only if the volcano is already poised to erupt.
- The ground-based Volcano Alert Level and the Aviation Color Code are separate systems and should be read together, not merged.
Sources
- Can earthquakes trigger volcanic eruptions? — U.S. Geological Survey
- Volcano Watch: What are the Volcano Alert Level and Aviation Color Code? — USGS Hawaiian Volcano Observatory
- Fact Sheet 2006-3139: Volcano alert levels and aviation color codes — U.S. Geological Survey