The magnitude 6.5-6.8 earthquake to hit New Zealand near Wellington around a week and a half ago brought earthquakes back into the news. Rather than exploring the way the earth rattles and rolls or some other aspect of the science surrounding earthquakes, I thought it might be a great opportunity to explore earthquakes a bit differently. What I want to show off today are some different visualisations (and I use that term loosely here, as will become apparent) of earthquakes that people might find fascinating.
Whenever I hear of a significant earthquake, one of the go-to resources I check out is IRIS, an acronymal consortium for all things earthquakey (IRIS stands for Incorporated Research Institutions for Seismology). As well as getting lots of data on earthquakes, one of the coolest things they have going on are visualisations of earthquake waves moving across the USArray.
As people might be aware, earthquakes can be detected a long way away, especially if they are relatively large (after all, that’s how we detect and locate earthquakes…). This means seismometers on the other side of the world can detect them, even if they cannot be felt by humans. The USArray is a ‘moving array’ of lots of seismometers which has been moving its way across the USA. Data from this can be mapped out, and, because of the coverage, you get a really good illustration of how the waves from distant earthquakes move.
You can build your own movies using a web tool IRIS have created, or, you can pick from some of the pre-computed movies made by IRIS. I have mirrored two pre-computed ones here to show off the method; you can create your own and download them to share with friends or even just direct people to the download links (which do only last 7 days).
I don’t want to explain all of the intricacies you can see in the movies, but just point out how cool they look. Bear in mind they are sped up a reasonable degree, as earthquake waves take a while to travel across the world. In these movies, the circles are individual seismic stations; when they are white, they aren’t moving. Red means the ground is moving up, and blue means the ground is moving down. When you see coherence in the patterns then you’re seeing the motion of earthquake waves: think of them a bit like an ocean wave, with the red being the crests and the blues being the troughs. I also show an example where we look at not just up and down motion, but also side-to-side. This especially highlights some of the later earthquake waves.
Also, down the bottom you get a nice seismic trace with some letters on it (e.g. P, S, PP, R2…). These letters refer to the ‘earthquake phases‘ and are written in a shorthand seismologists use to refer to what type of wave it is and where it has travelled in the earth. The yellow circle is the seismic station that graph has been retrieved from, and finally, there’s the vertical scale bar telling you how much the ground is moving at that period of time. In the first event, we’re looking at vertical ground motions of +/- 1 µm, and in the second, +/- 3 mm. This, by the way, gives you an indication of how strong the second event is.
For the first event, let’s look at the M6.8 from near New Zealand I referred to at the top of this post.
Another event which really shows up the passage of the different waves across the United States is the M8.9 2011 Tōhoku earthquake in Japan which produced the devastating tsunami and consequently the Fukushima nuclear disaster. Here we’re looking at all 3 components of the recorded waves: up and down motion, plus east-west and north-south.
(Both of these videos, again, are produced by IRIS not by us here at oncirc).
Another really cool ‘visualisation’ (I said in the intro I was stretching this term) are earthquakes where the waves recorded at seismographs are sped up and made into sounds that we can hear. The USGS have a page dedicated to this and what some of the differences are, and there’s a good article on The Conversation on this topic as well. Indeed, the examples in that article are of the same event I show the movie for above. I have embedded the example of the 2011 Tōhoku earthquake below.
(Movie from the Georgia Institute of Technology).
To wrap up: do these visualisations help us understand earthquakes any better? Perhaps. Humans are very good at intuitively picking out patterns, both visual and audible, that can be difficult to mine from data using automated techniques. Plus, if we realise those patterns exist, we can look for ways to extract them from the data we collect to learn new things about earthquakes and this can only help us in the future to understand earthquake risk. It is also a great way to engage with the public to bring them into our science and learn more about earthquakes, too.
If nothing else, they’re a great way to experience an earthquake from your desk on a Tuesday morning with a much lower chance of spilt coffee or destruction to the building you’re in.