Put your hand flat on the floor. It feels perfectly still. But to a sensitive seismometer, the ground is never truly quiet. Day and night, all around the world, the Earth is trembling ever so slightly, like a guitar string that never quite stops vibrating.
The short answer: most of that constant background trembling comes from the ocean. As waves roll across the seas, they push and pull on the seafloor, sending faint vibrations through the planet. Seismologists call these vibrations microseisms. The very slowest of them, a barely detectable, never-ending vibration of the whole Earth, is often called the Earth hum.
How Ocean Waves Shake the Ground
The link between waves and ground tremors was recognized early in the history of seismology, and a full theory of how it works was published in 1950 by a researcher named Longuet-Higgins.
There are two main kinds of these ocean-made tremors. The first happens in shallow water, where waves pass over the seafloor and shake it with the same rhythm as the waves themselves, typically repeating about every 16 seconds.
The second kind is stronger. It happens when two sets of waves with nearly the same rhythm travel in nearly opposite directions and run into each other. That collision presses on the seafloor at twice the wave rhythm, producing tremors that repeat roughly twice as fast.
Because ocean waves keep rolling hour after hour, the result isn't a sudden jolt like an earthquake. It's a long, continuous rocking of the ground. Most of it repeats every 4 to 30 seconds, matching the rhythm of ocean swells.
The Slowest Hum of All
Beyond those, the planet as a whole vibrates in very slow, gentle ways, with each cycle taking anywhere from 30 seconds to about 16 minutes. This faint, nonstop vibration is the Earth hum.
Scientists think much of it is driven by very long, low ocean waves interacting with the shape of the seafloor. The main sources are likely along the edges of the continental shelves, where shallow coastal waters drop off into the deep ocean.
Can You Hear It?
Not at all. These vibrations are far too slow for human ears, and too small to feel. The ground motion from microseisms generally doesn't exceed 10 micrometers, a fraction of the width of a human hair.
Don't confuse it with a different mystery also called the Hum. In places such as Taos, New Mexico, and Windsor, Ontario, some people report hearing a low droning noise that others can't. That phenomenon doesn't appear to have a single cause. Suggested explanations include industrial machinery and ear conditions such as tinnitus.
A Noise Scientists Put to Work
This background trembling turns out to be surprisingly useful. Because some of these tremors are tied to distant ocean storms, scientists can use seismic recordings to monitor ocean waves and even track tropical cyclones.
Human activity creates its own faster, higher-pitched seismic noise from traffic and industry. During the COVID-19 pandemic, when travel and industry slowed worldwide, seismometers recorded the longest and most noticeable global drop in human-made seismic noise ever observed.
A Planet That's Never Still
So the solid ground under your feet isn't quite as solid and still as it seems. Thousands of miles away, ocean waves are colliding, and their energy is gently rocking the whole planet.
It's a quiet reminder that everything on Earth is connected, right down to the ground you're standing on.
