A book I picked up at a used bookstore in Halifax




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…


My colleague, Ian Ferguson, just had the somewhat unique experience of reading his own obituary. I’m sure the Leading Edge will be rather embarassed when they figure it out. In the meantime, I can reassure anyone who knows Ian that if he’s dead, he’s doing a remarkable job of not showing it. I think a dryly witty letter to the editor would be called for — I’ll pass on any good suggestions to Ian.

But instead of being swayed by either side, we at UDN, Inc. have found a theory that effectively
merges the strengths of the two theories without the weaknesses.  The intelligent design people
say there are too many holes in the fossil record, and that evolution is only a theory; the
scientists say there’s not enough evidence of intelligent design.  So we say, instead, that
life has indeed been designed, just not very well.

All hail Unintelligent Design!

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.

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.

Here is an elegant demonstration of the cohesiveness of scientific literature.

Or something like that.

I used to read National Geographic as a kid.
Perhaps I should start again. It’s good to see someone be blunt in the presentation of scientific results, for a change.