Archive for the ‘Seismology’ 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.

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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Geophysipalooza

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.

Interp

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.

On Ontario

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 St├ęphanes, 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:

On_tomo_250resulting 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:

E_on_fabricshowing 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…

A book I picked up at a used bookstore in Halifax

Sf_cover

Sf_flyleaf

Sumatra, again

As you all undoubtably already know, another earthquake hit Indonesia today. Though plenty bad enough, it doesn’t seem to have been the level of catastrophe the previous one — thankfully, there doesn’t seem to have been a widespread tsunami.

What is surprising to me is the range of magnitude estimates. I’ve seen estimates from 8.1 to 8.7 today — even the moment tensor estimates disagree (8.1 and 8.6). I suppose a definitive estimate will have to wait for a rupture model, though. This map is particularly interesting in that regard — could this earthquake have filled in the gap between the slip regions of the 2004 and 1861 events?

I had to pretend to know something about it, since I’m the most conveniently-located seismologist for Winnipeg media outlets. All I did, mostly, was interpret what’s on various web pages, but it seems to have been sufficient — as a result, I spent half of today doing one interview or another. Haven’t looked to see how much of a total idiot I look like on TV, though.

…and another abstract

Today was the abstract deadline for CGU, a smaller (but friendly and low-key) meeting. I put an abstract in on behalf of an undergrad working with me:

Mantle Fabric and Lithospheric Thickness Beneath the Superior Province

S.-K. Miong and A.W. Frederiksen
Department of Geological Sciences, University of Manitoba

The Superior Province, the largest Archean craton in the Canadian Shield, represents an ideal laboratory for understanding the nature and development of cratonic lithosphere. Ontario spans a major portion of the Superior, and, under the auspices of the POLARIS and FEDNOR projects, is in the process of being instrumented with broadband seismometers on a large scale. We present the result of SKS splitting analyses for FEDNOR and CNSN stations spanning the breadth of Ontario, covering the Eastern and Western Superior Province. The Western Superior exhibits very large SKS splits (averaging 1.4 seconds) with a consistent ENE fast direction (averaging 69 degrees azimuth). In the Eastern Superior, the fast directions are much more variable (ranging from east to northeast), with smaller split times averaging 0.8 second. In the Western Superior, the split times align closely with both the current direction of absolute plate motion and the orientation of structural belts in the crust; we therefore interpret the strong splits in this region to represent a combination of lithospheric and asthenospheric fabric sharing a common alignment. In the east, the fast directions show appreciable scatter around the direction of plate motion, though there is general agreement; given the weaker split times in this area, we take the variability to reflect a weaker and more inconsistent lithospheric fabric, since the asthenospheric fabric should vary little across the Superior. Results from other studies, including tomography, heat flow, and elastic plate thickness studies, suggest the possibility that the cratonic lithosphere may have been significantly reworked or thinned beneath the eastern portion of the Superior Province.

This one’s a little more definite because, well, the work’s mostly already done.

Spring AGU abstract

…because these abstract deadlines always manage to sneak up on me:

Inversion of the teleseismic P coda for lithospheric structure: Examples from Ontario and California

A.W. Frederiksen and J. Zhang, University of Manitoba
J. Revenaugh, University of Minnesota

The coda of the teleseismic P wave has become one of the most powerful tools for unravelling fine-scale receiver-side structure, using both single stations and sparse or dense arrays of seismometers. Determining structural information from the coda is an inverse problem that may be treated using either linear or nonlinear methods, depending on what ad hoc assumptions are made about the nature of the coda waves and the structures that generate them. We will review some of the principal methods used in coda imaging and inversion, and examine two methods in greater detail: a non-linear search algorithm applied to single-station data in the presence of anisotropy and dip, and a linearized tomographic inversion of scattered-wave energy in the coda. Examples of applying these methods to detect thinly laminated mantle anisotropy beneath southern Ontario and features correlated with seismicity in California will be given.

It’s kind of vague, because I’m not sure what’s going to make it into the talk at this stage — but, since Partha talked me into co-chairing a session with him, I might as well take the opportunity to try to publicize my scattering-tomography method a bit better. The chairing part of the equation will be a first for me, as well — better not nod off during any talks…

Oddities in journal editing

I’m more lax about reading journals than I really should be, but I do subscribe to the paper version of JGR. I received my copy of the November issue (scientific journals have a tendency to be late) this morning, and on a quick flip through it , I noticed the following articles:

Hearn, Thomas M.; Wang, Suyun; Ni, James F.; Xu, Zhonghuai; Yu, Yanxiang; Zhang, Xiaodong, "Uppermost mantle velocities beneath China and surrounding regions"

Liang, Chuntao; Song, Xiaodong; Huang, Jinli, "Tomographic inversion of Pn travel times in China"

So what’s odd about that? Well, they’re almost the same study — two independent groups of researchers, applying more or less the same method to more or less the same data set. And, of course, getting remarkably similar results. So how’d they end up in the same issue? I have no idea, but my best guess would be that both were in the pipe at the same time, and rather than try to determine priority, the editor decided to slot them in together.

I must say, it’s nice to see a repeatable experiment in geophysics.