So, apparently WordPress now supports LaTeX. Let’s try that out. This should be a Fourier integral:
Not bad (except for the white background) — looks like it’s using actual LaTeX behind the scenes to do the rendering. Beats trying to make sense of MathML, anyway.
I can sum up where things are at right now with a picture:
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.
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.
I don’t have much to say about the human side of the Haitian earthquake except that it’s terrible, as media reports will tell you. I saw the waves coming in on our seismograph drum, which gave some idea of the distance and size of the earthquake but not the location. The location in itself is about as bad as possible — a large, shallow earthquake very close to a major city, in a country too poor to afford earthquake-resistant construction. All I can do is suggest that you donate to relief organizations and press the Canadian and other governments to get aid in place as quickly as possible.
I can, at least, say something meaningful about the seismology. Read more »
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.
Moved to WordPress, since paying for TypePad when I haven’t updated in ages is kind of ridiculous.
Who knew that volcanoes blow smoke rings? Not me, that’s for sure.
(Yes, I’m thinking about reviving this blog.)
The fall AGU meeting is next week. From their website: "The Fall Meeting is expected to draw a crowd of over 11,000 geophysicists from around the world" — that is, it’s going to be a madhouse, but kind of in a good way.
I’m presenting in this session on Monday afternoon, as is my graduate student. Poster sessions like this are always enjoyable, particularly since there’ll be a number of people I know presenting there, and several of them are also working on the Ontario lithosphere.
I’m also rather pleased with the figure above, which went into a poster I’m bringing. Essentially, it looks like a number of the structures in the Ontario model are correlated with subduction features detected in other studies, as well as with the linear feature previously attributed to the Great Meteor hotspot track; the question marks indicate newly detected features whose significance we’re going to have to figure out.
As only the city gives a meaning to the bleak landscape of the hermit, so the study, with its silence and its order, is simply the place where the oscillations of the seismographs are recorded.
(Italo Calvino, The Castle of Crossed Destinies, translated by William Weaver)
I think this quote’s going on my office door.
I expected to be updating this thing regularly over the summer, and for various reasons that hasn’t been happening. Nonetheless, here’s an update on some current projects:
Superior Province tomography: The Superior Province is an Archean craton — that is, one of the oldest chunks of stable continent in the world — which makes up most of Ontario, along with a decent slice of Quebec and (buried under sediment) about half of Manitoba. The deep roots of these cratons (below the crust) are the subject of much interest, though the Superior has only really been looked at in piecemeal fashion.
Until now, that is. The FEDNOR array has put a coarsely-spaced set of seismic stations out on the Superior; thanks to two Stéphanes, we’ve been able to include data from temporary deployments in the region as well. The result is a 3-D image of seismic velocity beneath most of Ontario:
resulting from recording earthquake traveltimes at stations from FEDNOR/POLARIS, the Canadian National Seismograph Network, the Abitibi and TW~ST experiments, and (thanks to some digital archeology and the IRIS Data Management Centre) the older APT89 experiment. The overall coverage is pretty good (blacked-out areas are not well covered), and in these early results, there appears to be a major difference between the eastern (mostly red, i.e. seismically slow) and western (mostly blue, fast) regions of the Superior Province. A few more FEDNOR stations have just gone in, and will hopefully help close up that resolution hole in the middle; still, even what we have now is a pretty good-sized data set, representing (among other things) a lot of trvel-time picking work by Soo-Kyung Miong, the student who’s been working on this.
Soo also did SKS splitting analysis (a technique which detects directional fabric) at the same stations for her honours thesis; the splits at the western stations are stronger and more consistent in the east. A full presentation of this will go into a paper soonish (once again, it’ll be nice to have those new stations), but in the meantime, in collaboration with Ian and Dave, we’ve been looking at…
…Mantle Fabric in Eastern Ontario: There’s nothing like a journal special issue (with deadline) to make one write a paper in a hurry. This one combines the aforementioned SKS splitting in eastern Ontario with magnetotelluric measurements of geoelectric strike (another measure of fabric, this time in electrical rather than seismic properties) and some receiver function studies of individual stations (which I’ve mentioned before) in order to get an idea of how the fabric varies with depth. There’s no simple punch line, but here’s a map:
showing electrical strike directions (black) along with seismic fast-axis directions (red; lighter red arrows are Soo’s new results). They’re not the same, but they don’t seem to be uncorrelated, either. I still don’t entirely understand the guts of the magnetotelluric method, but watching Ian work on this has been instructive — hanging around smart people is never a bad thing.
The latter paper’s already submitted — I may put up a preprint at some point, if my co-authors don’t mind — and hopefully the other project will be a paper before the end of the year. Then it’ll be time for the next project — and I haven’t even mentioned the project my graduate student, Jinling Zhang, is working on, which is looking extremely promising.
Now I need to decide how much of this I can cram into AGU posters…