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
BlueThe highest of IMO’s three displayed frequency bands. Local earthquakes can be prominent here, but weather and other environmental noise can also raise it.
~2–4 Hz · Mid Frequency
GreenIMO’s middle displayed band. It can respond to several sources and is not a stand-alone magma indicator.
~0.5–2 Hz · Low Frequency
PurpleIMO’s lowest displayed band. Low-frequency tremor can be volcanically important, but weather and other non-volcanic sources can also affect it.
The Three Lines Are Frequency Bands — Not Three Verdicts
IMO plots averaged seismic amplitude (RSAM) in three frequency bands. Those bands help specialists distinguish different kinds of seismic signal, but none of the colours belongs exclusively to one process. A rise in one line — or even all three — is not, by itself, proof that magma is moving or an eruption is approaching.
This matters especially in Iceland because weather can alter recorded seismic amplitude. A broad rise across the chart can therefore look dramatic while having a non-volcanic explanation.
IMO’s colours are frequency bands — not three independent confirmations of volcanic unrest.
All three bands rise
It can look dramatic, but a broad or regional source can affect several frequency bands at once. Three moving lines do not equal three separate pieces of volcanic evidence.
Compare several stations
Look at the timing and pattern across IMO’s network. Similar broad movement at several stations is important context before attributing a change to one volcanic system.
Check IMO weather + monitoring
Weather is a recognised source in IMO tremor interpretation. Compare conditions with earthquakes, deformation and IMO’s specialist assessment before drawing conclusions.
The common trap
“Blue, green and purple are all rising, so magma must be moving.” That conclusion is not supported by the tremor plot alone. IMO explicitly uses these frequency bands to help distinguish weather and teleseismic events, volcanic and flood tremor, and local tectonic earthquakes.
Key rule
Do not use a simple colour rule. Movement in all three bands can still be non-volcanic. Compare nearby IMO stations, check the weather, and look for independent changes in earthquakes, deformation and official IMO observations.
Purple & Green: The Magma Signal
Lower-frequency changes can be important, but frequency alone does not identify the source.
IMO describes volcanic tremor in RSAM as a coherent, emergent signal. Interpretation depends on its frequency content, character and distribution across stations together with the wider monitoring picture. IMO has also explicitly attributed increases in the 0.5–1 Hz band to weather in past monitoring reports.
How a genuine magmatic signal differs from an earthquake spike or wind noise across all three bands
The useful question is not simply “which colour went up?” but where else the change is visible, what the weather is doing, and whether other IMO monitoring data change with it.
All three lines can move and the explanation can still be weather. The tremor plot shows that seismic amplitude changed — the wider monitoring picture is needed to establish why.
Final Summary for Volcoholics
Low-frequency movement
Potentially important, but not uniquely volcanic. Weather and other non-volcanic sources can affect lower frequencies too.
Rock cracking — or surface noise
Spikes mean something broke. But it could be rock, wind, rain, or a lorry. Blue alone proves nothing.
Check context first
Even a broad rise across all three bands is not an eruption diagnosis. Compare stations, weather, seismicity, deformation and IMO updates.
Keep watching the lines
Context matters above all else. Check all three bands, compare nearby stations, check IMO weather observations, earthquakes and deformation, and give IMO’s specialist interpretation more weight than any single tremor plot.
Live Resource
Watch the Live Data Now
Follow the live Iceland earthquake and monitoring data feed while comparing changes across the IMO tremor charts.
Live Iceland earthquake and monitoring data · Use alongside IMO tremor charts and official updates for context
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/