That is a very cool finding. Though I must say that visualizing the data with a jet colormap and no legend is a little disrespectful to anyone who is sober.
This was my initial reaction as well, but the article mentions that
> Each color cycle represents 4.8 inches (12 centimeters) of ground displacement either toward or away from the satellite.
...so what we're actually interested in is counting the number of cycles through the color map between two points, for which jet actually seems like a perfectly good choice.
this is very standard for interferometry (that represents a height difference modulo some step). There is no useful "legend" that you can associate to this colormap. The only information missing is the step between cycles of the colormap, which is given.
The aftershocks going on here (most small, but higher frequency than usual) is interesting to look at, if not a little disturbing :). A 4.0 in the last hour too (6:48 PM).
The method used here can't measure displacement directly. It measures a phase difference between two radar images. Turning this into a map of displacement is non unique in the presence of noise and limited spatial resolution.
That's why you'll see this type of data displayed in this way. The rainbow palette is mostly convention, but either way, it's the most direct view of what actually measured.
It is certainly pretty. But I'm still not clear what it means.
OK, so the radar is measuring distance between the satellite and reflecting surface. They're compairing data from July 8, 2019 and April 8, 2018. I'm guessing that the two images look pretty much the same. Especially given limited spatial resolution.
But ELI5, what does the interference pattern show? I mean, are there 12 cm amplitude waves of vertical surface displacement? Something like frozen S waves?
An earthquake is rocks sliding past each other. What you're looking at here is a measure of how much they moved. (It doesn't "slide back" afterwards -- the motion is permanent.) For an earthquake of this size, the motion will be on the order of a few meters.
------------
In a bit more detail, the radar can't measure distance precisely enough to detect the movement. The distance measured before and after by radar is the same, within error. However, there's another part of the radar signal beyond just how long it takes to travel. That second part is the phase of the returned signal. Imagine the first time we imaged a small area, we got a return waveform that looked like this (zero phase):
/\ /\ /\
\/ \/ \/
but then the next time we got back a slightly different result (270 degree phase):
\ /\ /\ /
\/ \/ \/
The difference is shape of the returning signal is a phase shift. The radar wave is shifted slightly
We know that it moved at least three quarters wavelength in the ascii art example above. However, we'd get the same result if it moved ten and three quarters, though. We can measure part of the change very precisely, but the bulk of the motion looks the same to us. We're looking at that fined-grained part of the motion (phase difference) not the overall motion itself.
In programming terms, we're looking at the result of a modulo operator.
Since the description says each color cycle represents 4.8 inches, I would say if you trace a line from one red pixel across the blue pixels to another red pixel, you've traversed a cycle, and you've increased or decreased displacement from the reference by 4.8 inches.
