Read on Twitter
It’s here: the lockdown seismology paper is out in @ScienceMagazine! Here’s a thread sharing how this paper came to be, an intro to what we found, and a note on why it’s interesting. https://science.sciencemag.org/content/early/2020/07/22/science.abd2438
Back in March, @seismotom posted this figure to @Seismologie_be of ambient seismic noise on a seismometer in Belgium, showing a decrease in noise power when their local lockdown went into effect: https://twitter.com/Seismologie_be/status/1240952099887865856
Lots of seismologists (myself included) were intrigued when we saw it, so we each started processing data from our local areas, posting the results to Twitter, and discussing it all in the replies. It was social distancing seismic noise, and social media seismology!
Eventually, @seismotom led us all from Twitter to a Slack workspace where we could discuss and share more easily. We figured out what we wanted to do, where we needed more data from, and how we wanted to process it. This paper is the result!
So what did we find? First, I'll describe ambient seismic noise: it’s the continuous motion of the ground below your feet. It’s too subtle for people to feel (not dramatic like an earthquake), but seismometers can record it. It’s happening all the time, everywhere!
Ambient ground motion in the earth comes from many sources. Some are natural, like ocean waves, rivers, wind, and so on. Humans make seismic noise, too, since many of our motions (cars, construction, industry, and even just walking around) can couple with the ground.
So what did we find with seismic noise and COVID-19? As many governments, organizations, and individuals made the smart decision to stay home except for essential tasks, the anthropogenic (human-made) part of the seismic noise decreased, in some places by as much as 50%!
Note that a pandemic isn't the only time anthropogenic seismic noise can drop. Even normally, nights are quieter than days, and weekends and holidays are quieter than business days. Those trends all reflect differences in transit, industry, and general human hustle and bustle.
Here’s a look at the latest data from a seismometer in Los Angeles. You can see the seismic noise levels drop when lockdown starts in March, reach a minimum around Easter, and then rise as things reopen, although it's still below pre-shutdown levels today.
So what does a drop in seismic noise mean for seismologists? First off, we just think it’s neat, but it also may help with other seismology research. It'll take time and investigation to see how exactly we’ll apply what we've learned, 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.
2. Changes in anthropogenic noise help 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 accounting for different sources and signals within the noise.
Before COVID-19 lockdowns, none of us had ever seen a change in seismic noise at such a scale. It’s been absolutely wild to see data from all around the world compiled by seismologists all around the world showing pandemic response all around the world.
The drops in seismic noise levels have been heartening to see, too. In person, I can only see my local area’s lockdown, but with seismometers and collaborators, I can see people all around the world working together to protect their communities, and that makes me hopeful.
If you’d like to learn more cool seismology or ask questions about this paper, check out this list of many of the authors. They’re all awesome and I’m glad to be among them! As shown by this paper’s existence, Seismo Twitter can be pretty great. https://twitter.com/i/lists/1286049102078255104
Click through for @seismo_steve's more in-depth thread on the results in this paper. Also, take a look at the size of that author list! https://twitter.com/seismo_steve/status/1286360433750953984
