Make two waves meet — and the seafloor begins to transmit
This is an explanatory model, not the recording of a specific storm. Ground displacement is deliberately exaggerated. Frequency ranges reflect observed primary and secondary microseisms.
On a seismogram, an earthquake begins. The line is relatively quiet before it; then waves arrive and the instrument records an event.
Here there is no event. The line moves for hours, days and years. The background strengthens with ocean storms and changes with the storm seasons. Its trace can be detected far from the sea.
The noise that became a message
Early seismologists treated microseisms as interference because persistent motion obscured earthquakes. But the interference followed the rhythm of the sea. Stable bands appeared in the spectrum, linked to the periods of ocean waves.
Primary microseisms commonly arise when swell presses on shallow water and sloping seafloor. Their period is close to that of the source waves. They are relatively weak and often associated with coasts.
The stronger secondary microseism forms differently. Waves with nearly the same period travel in opposing directions—for example when storm swell meets a coastal reflection or another wave system. Their interaction creates pressure that varies twice as fast as either wave. Some of that energy enters the seafloor and travels through the solid Earth as seismic waves.
Direct pressure from a wave system in shallow water and on slopes. Its period resembles the ocean-wave period.
Opposing waves produce the doubled frequency. Usually the most energetic part of the microseism spectrum.
Too small to feel, yet continuously registered by broadband seismometers.
Dr Elisa Werner · broadband network · watch log 31Archive reconstruction. A clearly identified fictional research layer of Code Gaia.At 02:17 the band rose again. The earthquake catalogue was empty.
The station stood inside the continent. I checked traffic, wind, power and the neighbouring instruments. The same band had appeared on all of them.
The storm was across the ocean, but the laboratory floor reported it first.
We call it background only because it almost never stops.
How a storm crosses land
A microseism source is not a single point beneath a cyclone's centre. It requires the right wave geometry, which may occur near a coast, in deep water, or in several regions at once. The highest ocean waves and the strongest seismic source therefore need not coincide.
The generated motion travels mainly as Rayleigh and Love surface waves; some energy propagates as body waves through Earth's interior. Networks compare timing, direction and spectrum to reconstruct likely source regions.
The noise now probes both the ocean and the Earth
Microseism changes can track storm regions and long-term wave variability. Correlating background noise between stations can also reveal the structure of the crust without an earthquake: the ocean continuously supplies energy for a form of passive scanning.
Raphael Koh · seismic interferometry laboratory · note 8Archive reconstruction. A clearly identified fictional research layer of Code Gaia.We removed earthquakes, explosions and everything we considered an event.
Noise remained.
When months of two station records were combined, a wave emerged as if one station had been a source and the other a receiver. Nobody struck the Earth. The ocean had done it for us millions of times.
The empty seismogram was a map we had not learned to read.
What this does not mean
A seismometer measures ground motion, not sound in air. To “hear” a record, the data must be accelerated or shifted into the audible range. The result is a sonification of measurements—not what an ear pressed to the ground would hear.
Nor can one station reveal the state of an entire ocean. Amplitude is shaped by propagation, local geology, sea ice, bathymetry and overlapping sources. The signal is real; decoding it backwards remains difficult.
What is established — and what remains open
| Status | Finding | Basis |
|---|---|---|
| Established | Ocean waves generate continuous seismic background | The relationship is reproduced by theory, models and global station records. |
| Established | Opposing waves generate a strong signal at doubled frequency | The nonlinear mechanism is predicted theoretically and observed in spectra. |
| Established | Microseisms are recorded far from their ocean sources | Surface and body waves propagate through crust and mantle. |
| Established | Ambient noise can be used to study Earth's structure | Long-record correlations recover the medium's response between stations. |
| Ambiguous | The exact source of every part of an observed record | Coastal reflections, remote storms and multiple paths act at the same time. |
| False | The motion is directly audible without transformation | Its frequency and physical form require instrumentation and sonification. |
The honest conclusion
Microseisms do not mean the planet is approaching a permanent earthquake. They reveal something stranger: the boundary between ocean and stone is not a boundary between independent systems.
A storm gives some of its energy to the seabed. The seabed converts it into waves crossing continents. An inland station records the history of water it has never seen.
After the impact, Earth rang like a bell
CG-064 · GEOPHYSICAL SIGNALSOne pulse every 92 seconds—for nine daysA Greenland fjord landslide trapped a mega-tsunami between rock walls. Moving water became a signal recorded across the planet.
Open dossier →Can water in one fjord make seismometers around the world repeat the same rhythm?