Archive for the ‘Science’ Category

Getting ready for AGU

I can sum up where things are at right now with a picture:

Cross-section through "pre-SPREE" tomographic model

Plan section through "pre-SPREE" tomographic model, built from Canadian and US data through August, 2009. This slice is at 250 km depth below the surface.

This is a slice through a 3-D tomography model I’ve been working on — basically, I’ve spent odd moments over the past few months measuring relative arrival times of earthquakes at various instruments in central Canada and the US. Red zones are regions where the waves travel more slowly than average, and blue zones are the converse, as determined by a rather lengthy computation that finds an image that matches all of the measurements from different directions as accurately as possible.

So what does it mean? Well, this is a look into the lithosphere — the layer below the Earth’s crust that participates in plate tectonics. So you’d think that the structure of the lithosphere would closely match that of the crust above (the grey lines), but it doesn’t. There are features that match crustal structures (like the little red zone marked “Nipigon”, above), and other features that don’t. Notably, the big blue blob I’m calling the Western Superior Anomaly is a lot smaller than the western portion of the Superior Province, the crustal region it underlies.

SKS split-time contours for the same region.

SKS splitting times for the same map region as above -- contours and colour scale are in seconds, triangles are measurement points.

There’s another, independent line of evidence indicating that the western Superior has something unusual underneath it: the contour map above. More measurements made from earthquakes here, this time looking at the effect of mantle fabric on polarized SKS waves. If the rock in the upper mantle (including the lithosphere) has a strong fabric to it, it will cause the incoming polarized wave to split into two waves that arrive at different times. The map above shows the time difference between the two — and, looking at the contours, it seems clear that the western Superior anomaly stands out in this way as well.

So, what does this all mean? Well, I’m working on that. Hopefully I’ll have something semi-sensible to say at AGU in a few weeks.

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Today I made maps.

Geophysics is a visual science. Nearly every paper I’ve written has started with the figures. The reason for this is that the end result of data analysis or calculation ends up making sense only as an image; the figures in my papers usually contain the results that the text seeks to obtain or interpret.

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A sort of omnibus

It’s been a while since I’ve posted here, so here’s an attempt to catch up (in a rambly, notes-to-self sort of fashion) — four things in chronological order:

1) AGU

This year’s Fall AGU meeting was chockablock with good stuff. As usual, the whole thing was a bit of a sensory-overload blur, but here are the main things that struck me:

  • Noise. Lots of stuff about seismic noise. A number of presentations on the use of seismic noise to calculate an equivalent seismogram between two stations, using cross-correlation — seems weird at first, then starts to make sense when you think about it. The idea is that if the noise you’re recording at your seismic stations reflects a bunch of randomly distributed sources coming from all sides, then some of that noise will actually be picked up more than one station, and will show up as correlated noise between the two — but it’ll be affected by the propagation path from one station to the other. Averaging over all directions, then, the cross-correlated noise spectrum ends up looking very much like a seismogram! Seismology without sources — groovy.
  • More noise — specifically, the Earth’s “hum”. Apparently there’s a part of the seismic noise spectrum that can be localized to the oceans, and has seasonal variability — during stormy times of year, the ocean is actually communicating significant seismic energy to the solid Earth.
  • Another rather startling result is that it seems to be possible to predict how big an earthquake is going to be before it’s finished happening — at least, that’s what’s implied by the ability to predict an earthquake’s magnitude by the first 4-5 seconds of its arrival. I wonder if this breaks down at very high magnitudes?
  • Lots more stuff that was of immediate interest to me — there does seem to be a difference between eastern and western Ontario in more data sets than just ours. The Superior Province isn’t so uniform after all.

As per usual, I left AGU energized and full of ideas — and then promptly took the holidays off and forgot two-thirds of it. Well, such is life.

2) The Sumatra earthquake and tsunami

The biggest earthquake, and perhaps the worst natural disaster, to happen in my lifetime. I found myself alternating between being appalled and being professionally fascinated — that thing was huge, as the calculated rupture on the USGS website shows. So many preventable deaths; with a warning system in place, I’d guess at least half of the dead could’ve been saved. The size and type of the earthquake are about the same as that of the Cascadia megathrust that B.C. and the Pacific Northwest are, with good reason, worried about — this could’ve been us, too, and one day it will be.

3) POLARIS Ontario workshop

Two weeks ago, there was a workshop on the Ontario project I’m participating in, out in Kingston. A small meeting, but a good mix of people, and a friendly venue in which to introduce my grad student to the milieu, I thought. There was some interest in our result that there’s a change between eastern and western Ontario — everything interesting seems to be happening in the middle of a gap in which there aren’t any stations, so hopefully some will be put in to plug the hole. I should really post on that result, incidentally.

4) Term, again

This term, I’m teaching Exploration Seismology, and generating much of it from scratch in the process. It’s a lot of work, but an interesting sort of work, and hopefully it’ll save me a bunch of time and effort next year. Practical courses like this are a balancing act — I want to get the theory right, but I also want the students to learn some immediately useful skills for the working world. A lot of our students are going into the oil industry nowadays — part of our role is to help their job prospects, after all.

It does feel a bit precarious, trying to teach a reasonably complete geophysics program with only two geophysics faculty. But I think we do a creditable job, overall, and my colleague Ian deserves the bulk of the credit for that — he even carried the whole program himself for a brief period. I have no idea how he managed.