If you follow activity on Iceland's Reykjanes Peninsula, you've probably got the IMO tremor charts bookmarked. You watch the lines rise, spike, settle. But what are you actually seeing? When they jump — is it magma? An eruption starting? Or just wind hammering the sensor?
The Basics: It's About Frequency
A tremor chart doesn't just show how much the ground shakes — it shows how fast it vibrates.
Seismometers split ground movement into frequency bands. Think of it like sound: high frequency is sharp, fast vibration — treble. Low frequency is deep, slow vibration — bass. The volcano produces both. The chart separates them.
~4–8 Hz · High Frequency
BlueSharp, brittle rock fracturing. Fast, close-to-surface energy — but also the band most vulnerable to surface noise.
~2–4 Hz · Mid Frequency
GreenTransitional energy — a mixture of processes, both mechanical and fluid. Bridges the story between blue and purple.
~0.5–2 Hz · Low Frequency
PurpleDeep, sustained fluid resonance. The voice of magma moving through open pathways — the signal that matters most.
The Blue Line: Rock Breaking… or Just Noise?
When magma forces its way upward, it breaks rock. That produces high-frequency spikes — often thousands of tiny quakes during an intrusion. At eruption onset, blue can spike hard as the final pathway fractures open.
But here's the important bit: high-frequency energy doesn't travel far. If blue is elevated, the source is often near the surface. Which also means blue can respond to things that have nothing to do with magma at all.
High-frequency noise sources — and how to tell them from the real thing
Key rule
If blue is spiky but purple and green stay flat, and there's no ground deformation or earthquake swarm alongside it — you're almost certainly looking at surface noise. Blue alone is not an eruption signal.
Purple & Green: The Magma Signal
Lower frequencies tell a different story entirely.
When magma flows through an open pathway, it generates sustained low-frequency vibration known as harmonic tremor. This is the deep resonance of fluid — molten rock — moving continuously through conduits underground. It doesn't spike and decay like an earthquake. It builds and it holds.
How a genuine magmatic signal differs from an earthquake spike or wind noise across all three bands
This is the pattern to watch for: a steady rise in purple and green, less spiky and more continuous than anything surface noise produces, with elevated levels that remain high rather than decaying back to baseline.
If purple and green rise sharply and stay elevated — magma is moving continuously. That's the strongest eruption indicator on the chart.
Final Summary for Volcoholics
Magma tremor — fluid flow
Sustained rise that holds. The signal you came here to see. Context matters, but this is your strongest indicator.
Rock cracking — or surface noise
Spikes mean something broke. But it could be rock, wind, rain, or a lorry. Blue alone proves nothing.
Strongest eruption signal
The whole chart in one rule: one colour never tells the whole story. Always read all three lines together.
Keep watching the lines
Context matters above all else. One colour never tells the whole story — always watch how all three bands behave relative to each other, and cross-reference with the IMO earthquake map and any live camera feeds before drawing conclusions.
Live Resources
Watch It Happening Now
Put the theory into practice. These live streams let you watch Iceland's volcanic activity and seismicity in real time — the best classroom there is.
Live volcanic camera — Reykjanes Peninsula · Watch for glow, lava, and eruption plumes
Live earthquake map & seismicity data · Cross-reference with the tremor charts for the full picture
Tremor Charts
hraun.vedur.is/ja/oroi/allarsort.html
Earthquake Map
en.vedur.is/earthquakes-and-volcanism/earthquakes/
Official Bulletins
en.vedur.is/earthquakes-and-volcanism/