Several folks have asked me questions about the seismic noise that folks like @seismotom and I have been posting, so here's a little primer thread on what seismic noise is, what's changing with it during COVID-19 shutdowns, and what that all means. https://twitter.com/celestelabedz/status/1247192796043161606

First off, what's seismic noise?

We all know that the ground can move and shake pretty dramatically during an earthquake, but the ground is actually moving ALL the time! In the times without earthquakes, the motion is way too small for any person to feel, though.

That continuous hum of tiny motion is what we call "seismic noise". You can think of it like background audio noise. No one is talking or playing music in my house right now, but there's still a little bit of noise like the fridge running, the wind in the trees outside, etc.

These continuous tiny motions are intangible to us, but they can be picked up by seismometers, which are very sensitive motion detectors. A good seismometer can pick up motions of nanometers that no person could ever feel, so they're recording seismic noise all the time!

Ambient ground motion in the earth comes from many sources. One major one is the ocean! You can detect seismic noise from ocean waves even if you're really far away from a coast. You can also get continuous seismic noise from rivers, wind, and other natural sources.

Humans can also make seismic noise, though! Many of our motions (like cars, planes, construction, industry, mining, and even just walking around) can couple with the ground. It's not too surprising if you've ever been near train tracks; you know they can make the ground rumble!

So what's going on with seismic noise and the COVID-19 pandemic?

As many governments, businesses, and citizens are making the smart decision to stay home and isolate except for essential tasks, some of that anthropogenic (human-made) seismic noise is decreasing.

A decrease in transit, industry, and general hustle and bustle means the ground is moving a bit less. The natural sources like the ocean are still there, but the anthropogenic stuff decreases. That's what the graphs we've been posting show!

A pandemic isn't the only time anthropogenic seismic noise lowers. You can see in the graphs that nights are quieter than days, and weekends are quieter than weekdays. The quietest days are major holidays, like Christmas! Those trends all reflect differences in human activity.

Is seismic noise like pollution? Is this drop good?

Normal ambient seismic noise is WAY too small to affect people. Many animals can sense smaller vibrations than humans, but normal seismic noise is still REALLY small (unless you're close to a strong source like a train).

You can draw a parallel to noise pollution, since many anthropogenic seismic noise sources (transit, construction, etc.) also make audio noise. People and animals actually are affected negatively by too much of that kind of noise, so a drop is nice, but not for seismic reasons.

Will this change in seismic noise affect faults or volcanoes?

No. Decreasing anthropogenic seismic noise doesn't affect earthquakes or eruptions. If you live in an area with those hazards, you don't need to be any more or less worried than normal. Just be prepared as always!

Does a drop in seismic noise help with science?

Mostly we seismologists just think it's neat, but it may help with other research in some small ways! It'll take time and investigation to see how much of a difference it'll actually make, but here's two potential impacts:

1. Lower background noise means we can detect smaller seismic waves, like from tiny or distant earthquakes. Just like you can hear your phone better in a library than a rock concert, you can detect earthquakes better when seismic noise is lower.

More earthquakes (even very tiny ones) means more information about what's going on below our feet. Earthquakes too small for any person to feel can still give us more information about potential hazards. When you live in earthquake country, the more information, the better!

2. Lower anthropogenic noise helps us understand the different parts of our seismic noise environment. Some seismologists (like me!) do research with seismic noise, and we want to make sure we're properly matching up sources to signals.

When we see certain parts of the seismic noise decrease with human activity changes, it's a good clue that that's an anthropogenic signal. COVID shutdowns aren't a super game-changer for noise research, though; since we also have nights, weekends, and holidays when it's quiet!

Hopefully this thread cleared up a little about what seismic noise is, why it's changing with COVID shutdowns, and what that means. If you have any questions, you can ask me or other seismologists on Twitter; we're quite friendly!