Seismic Waves
December 28, 2012 10:27 PM   Subscribe

Tangentially: Ouch, that colourmap.
posted by narain at 10:42 PM on December 28, 2012 [19 favorites]

Ripples on a pond is what I expected to see and ripples on a pond is what it is, but knowing precisely what the wavelengths are and where nodes form is probably really, really useful information for building codes and structural engineers.
posted by three blind mice at 12:22 AM on December 29, 2012

It says the nodes are 330ft apart - I wonder how they achieve that spacing in what I assume is a very heavily built-up area. Do they just say "Uh, could we put this on your roof?"
posted by GallonOfAlan at 12:27 AM on December 29, 2012

I imagine they just put one on every corner.
posted by Jawn at 12:36 AM on December 29, 2012 [1 favorite]

Pretty neat! So they deployed an array of these, so that they could determine the geological structure below the city by measuring the response to an artificial stimulus, and this earthquake data was just a nice side effect while they had the gear set up.

The actual purpose being to look for oil.

More info.
posted by jeffj at 3:02 AM on December 29, 2012 [2 favorites]

I didn't know what those black lines were until I saw the waves refracting through them. Faults! So cool.
posted by DU at 4:03 AM on December 29, 2012

from the tag line i thought this was going to show real-life movie footage of rippling earth.

i once saw an aftershock from a big quake roll toward me across a field and it looked just like this; slow rolling waves in the earth. the look and the feel were very different. once it hit, the ground seemed to flatten back out and shake rather than roll. i remember being disappointed at the time that it didn't feel like we were on a boat it just felt like an earthquake.
posted by Conrad-Casserole at 7:50 AM on December 29, 2012

I'm curious, what is "ground motion"? Speed? Velocity component? As nariain says, the rainbow intensity mapping can be really problematic. You see these nice red and blue wavefronts... are we to take red and blue to be up and down or vice versa? Are we seeing P waves or S waves? Can you distinguish between them with this setup?

That said, these are neat to look at! I especially like how the waves diffract around the faults.
posted by BrashTech at 9:48 AM on December 29, 2012

Tangentially: Ouch, that colourmap.

I used to work at the USGS, my department did computerized seismic analysis to locate oil and mineral deposits. That was a long time ago. The office was in Golden, CO, and they were searching the Rocky Mountains. There were a lot of mining operations around the state that used large explosive blasts to get at the mineral deposits, most of these were under 3 on the Richter Scale, but measurable if the sensors were within maybe 200 miles of the site. The USGS seismologists measured the blasts from several fixed sites and made estimates of underground resources. This was a pretty standard practice on a smaller scale by oil companies, they'd go out in the plains of Texas or wherever, set up a small seismo array and detonate a small blast, to measure the reflection of waves from underground. But only the USGS had the resources to do it on a large scale (at that time, at least).

Anyway, the point is, at that time, computer graphics were pretty primitive, mostly pen plotter and electrostatic printing in grey scale. When color displays started to become affordable (like under $40k per workstation) they started using the rainbow colormap. It was immensely useful due to the very flaw that article describes. There is a perceptual discontinuity at maybe 75% up the scale. That's exactly what they wanted. Map the scale over the data appropriately, and you got nice red areas with a yellow ring around them, it was easy to spot the peak areas. At that time, most of the actual data analysis was done by visual inspection, sometimes using elaborate stereograph viewers. They weren't interested in a smooth data scale, anything below the peaks was just noise.

One of the more interesting aspects of the NODAL video is before the quake wave hits. There are lots of little high intensity hits on sensors right around the freeway, I think that's the 405. That is the big problem with seismo sensor grids, there is a lot of noise from heavy trucks bashing down the roads and shaking nearby sensors. The grid of sensors helps you determine which data points are noise, since the noise is usually localized to a single sensor.
posted by charlie don't surf at 10:12 AM on December 29, 2012 [3 favorites]

Are we seeing P waves or S waves? Can you distinguish between them with this setup?

Yes, if you filter the data and graph different frequency ranges.
posted by charlie don't surf at 10:20 AM on December 29, 2012

The refracting through faults thing was cool!
posted by ArgentCorvid at 11:49 AM on December 29, 2012

Needs a Hitchcock soundtrack.
posted by Surfurrus at 4:05 PM on December 30, 2012

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