Archive for November, 2010|Monthly archive page

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.