Saturday, February 2, 2008

The Tsunami and the Whale

Now that I've pretty much caught up with the load of work on which I procrastinated even more than usual due to last weekend's concerts, I can get back to interacting with the internet rather than just lurking. At least for the time being...

I have two weeks of Journal Club to catch up on here. Both of these weeks involved several people doing overviews of recent journal contents. This broke with the alternating schedule of small papers one week, big paper another week because the professor scheduled to present the big paper had a conflict. This meant I couldn't use reading the seismology paper as an excuse to further procrastinate on the electronic music homework, but it also meant more of an overview of recent research that I'd like to be reading but don't have the time for just yet, no thanks to the impending thesis and comprehensive exams.

The main paper we looked at this week dealt with small earthquakes only, which seems to be a relatively unpopular thing to write about. It makes perfect sense that people would want to focus on the Big Ones, since those are the events that effect society at large and that totally redo the stress balance on a fault system in one fell swoop, but the catalog of hard data (rather than historical description of effects) doesn't really include many of these for any given region. But there are plenty of small ones, and I'm sure there are plenty of studies that could be done on small ones that don't just relate them to big ones. The particular focus of the paper we looked at on Thursday was direction of slip (as in unilateral versus bilateral) and how that effected aftershocks. The largest quake discussed in the paper was a magnitude 4.1, which put most of the aftershocks into the unfeelable range. Looking at a few isolated 4-ish quakes, there did seem to be a correlation between directivity and aftershocks, with unilateral quakes having more aftershocks, which were mostly located within 70 degrees of the rupture plane. Bilateral quakes had fewer aftershocks in a less-confined space. The final example in the paper was a cluster of high-3 low-4 quakes up in the vicinity of the Calaveras Fault and at a pretty deep depth, and that's where the model got a little tricky. Whether each event in the cluster counted as an aftershock of the largest event previous to it or whether the ones close in magnitude counted as separate events was unclear, but those of us in the discussion agreed that the way these events were considered would make a difference in the conclusions. Furthermore, it seemed like the people conducting the study were inconsistent in what they treated as an aftershock versus a separate event, and it looked suspiciously like they might have done this to fit into the 70-degree angle measurement. We all agreed the paper would have been much stronger without the example of the cluster, both for the consistency thing, and also because a different configuration of stress is needed to set off a cluster versus an isolated event and its aftershocks.

Out of all the papers summarized last week, the one that was most interesting to me looked at postseismic slip between a mainshock and a large aftershock. I don't remember a huge number of details about the paper(people go through papers quickly during summary weeks - I've been thinking of asking the person who picked this one where I can find the whole thing), but it focused on a series of events in Japan. The study found a substantial amount of postseismic slip after the mainshock, directed toward the eventual location of the aftershock. These events went down the subduction zone rather than along it. I have to wonder what implications, if any, this observation would have on a creeping strike-slip fault. Could a rupture starting ahead of a creeping section and moving toward it stop where the creep starts, speeding up the rate of creep in the hours following the quake, and eventually set off another event on the other side of the creeping section? I'll definitely be looking to see if there are papers about this.

The other main paper we looked at last week had to do with inferring the size and shape of the area of seafloor displacement in a tsunami-generating earthquake by looking at the tsunami waves and inverting the data. I wasn't quite clear on how this worked based on the brief explanation in the discussion, but it was still a memorable discussion due to an amazing mental image. The professor explaining the paper was describing how DART tsunameters work, and how, unlike buoys, they're stuck to the ocean floor and won't pick up water column changes from surface/storm waves. "Only tsunamis set these things off," he said. "Or if a whale died and fell on one, that might do it, too."
Whale + tsunameter. Is that not fantastic? The mental image kept popping into my head all weekend, even at such inopportune times as the dress rehearsal for the concert (it was all I could do to not crack up). With such persistent visual thoughts, there was really only one thing I could do:

(I'm aware the big wooshy splash thing should be coming off of the whale's whole body. I just became aware too late to undo the coloring. Bleaugh.)
I think I'm going to give the original copy of this to the professor who made the comment, since it's all his fault.


Tuff Cookie said...


Whale + tsunameter. Beautiful. I just spent the last five minutes snickering.

Someone should do a correlation of whale size to "tsunami" intensity.

Julian said...

"Someone should do a correlation of whale size to "tsunami" intensity."

Oh no! Stop giving me ideas when I should be doing actual work!