
NASA Maps Surface Changes from California Quakes - infodocket
https://www.jpl.nasa.gov/news/news.php?feature=7448
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blululu
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.

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nemetroid
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.

jofer explains in their comment why the data is shown this way:
[https://news.ycombinator.com/item?id=20398703](https://news.ycombinator.com/item?id=20398703)

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SubiculumCode
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).

[http://scedc.caltech.edu/recent/Maps/118-36.html](http://scedc.caltech.edu/recent/Maps/118-36.html)

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breck
> Each color cycle represents 4.8 inches (12 centimeters) of ground
> displacement in the radar line-of-sight.

Is there a legend somewhere, or at least a max? I'm not sure what is meant by
"color cycle".

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jofer
You're looking at an interferogram.

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's also kinda pretty, i.m.o...

~~~
mirimir
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?

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jofer
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.

~~~
mirimir
Thanks.

I see that the ALOS-2 SAR uses L band, which seems to mean 1-2 GHz (30-15 cm).
So maybe the ALOS-2 actually uses 12 cm?

So does that mean that the pattern shows something like contour lines?

~~~
jofer
Yep! You can think of the bands as contour lines of deformation.

~~~
mirimir
Hey, thanks.

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mturmon
Relatedly, InSAR images of the Kilauea volcano area:

[https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA13910](https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA13910)

And land subsidence due to groundwater pumping in California’s Central Valley,
showing motion of up to 70 cm (!):

[https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA16293](https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA16293)

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golem14
At first glance, this looks very much like some Julia sets.

