misleading headline of the day

From Discovery Channel news feed: Two years later, New Orleans still sinking.

The article itself is quite good - it discusses a recent paper in Geophysical Research Letters by Roy Dokka, Erik Ivins, and Ronald Bloom, who have been modeling observed subsidence data from New Orleans. Their conclusion, apparently (I haven't read the article), is that the subsidence is driven by the weight of the Mississippi River sediments, rather than by things like groundwater extraction.

But the headline seems to imply that Hurricane Katrina caused the sinking of New Orleans... although New Orleans was below sea level long before the levees broke.

I love the culture of New Orleans. I love the music, the food, the literature. I want to see those things preserved, and to see the people of New Orleans get their homes and their lives back. But Hurricane Katrina wasn't just a failure of politics or engineering. The very levees that protect the city contribute to its sinking, and that makes the problems of New Orleans particularly difficult to solve.

Misleading headlines, particularly ones that imply that the problems are new, don't help the situation.

talc and the "weak fault problem"

While I was out with my Advanced Structure class, looking at fault and shear-zone rocks, there was a fascinating article published in Nature about the surprise finding of talc in the San Andreas fault zone.

The San Andreas both makes wonderful sense – it’s a strike-slip plate boundary, THE strike-slip plate boundary that everyone knows about – and it’s mysterious. A year or two ago, the Teaching Structural Geology mailing list put out a call for current enigmas in structure and tectonics, and right there at the top of the list is “the weak fault problem.” The San Andreas fault may be capable of a pretty impressive shake, but it’s not as tough as one might expect. For one thing, the principle stress directions around it are wrong. A strike slip fault should theoretically have its most and least compressive principle stresses at an angle to it, somewhere in the neighborhood of 30 and 60 degrees in an ideal world where all rocks break like in laboratory experiments and nothing ever rotates. But even in a non-ideal world, you shouldn’t get principle stresses parallel and perpendicular to a fault. Those imply that the fault is a free surface, that it has no strength, that it can deform continuously, like water or air. Water could solve the problem, but the evidence for the necessary fluid pressures just doesn’t seem to be there.

Now, a major fault zone like the San Andreas isn’t composed so much of rock as it is of rock that has been broken, fragmented, crushed, cataclasized. But even powdered rock has frictional strength – too much strength for the stress orientations along the San Andreas. Even serpentinite, California’s slippery state rock, found along the eastern side of the fault where it follows the old Coast Range ophiolite, is too strong to explain the fault’s behavior.

So they dug a hole into the San Andreas fault, just north of Parkfield. (As an aside: I think it’s wild that the long-expected Parkfield earthquake happened while they were starting to drill.) And they sent the cuttings to Diane Moore, who studies the metamorphism of the mafic and ultramafic rocks in California. And she found...



Ok, no, not that. But she found talc. Softest mineral known. And, better than that, a mineral whose behavior in deformation seems to fit some of the behavior of the San Andreas. Talc can deform constantly, but only when deformation is slow. Push it too hard, and it becomes stronger. And it’s a common alteration product of ultramafic rocks in continental crust (at least) – just add quartz to serpentine, and you’ve got talc. (In fact, this is why American baby powder makers have switched from talc to corn starch. Talc mines also frequently contain serpentine... and serpentine is one of the minerals that can form asbestos.)

There wasn’t much talc in the cuttings, and it doesn’t solve every problem with the behavior of the San Andreas, but the thought is intriguing. And... well, could there be talc in other faults through ultramafic rock? Are there subduction zones that have managed to incorporate hydrated mantle rock and then added quartz... and become too weak for magnitude 9 earthquakes? I don’t know. I don’t even know how such ideas could be tested, or how anyone could tell if a fault is soft and slippery at depth. (I guess that maybe GPS monitoring would be able to pinpoint faults that deform without major earthquakes. But how to tell if they’ve got talc along them... drilling holes into them all probably isn’t feasible.) But it’s a neat idea.

Reference: Moore, Diane E. and Rymer, Michael J., 2007, Talc-bearing serpentinite and the creeping section of the San Andreas fault: Nature, v. 488, p. 795-797. (It's behind a paywall, so I can't directly link to the paper.)

...'Twas the night before classes...

'Twas the night before classes
and all through the school
the professors were worried
that they would look like a fool.

Are the books in the bookstore?
Is the lab gear all here?
Are all of the syllabi
suffiently clear?

Every year I have the same dream. I walk in on the first day of school, and I don't know what classes I'm teaching, or when they meet, or where, and I sure as heck don't have syllabi or a first lecture prepared.

You think you stop having those dreams when you graduate, do you? Nope. They don't stop. They metamorphose.

And now I should e-mail that Google Earth example to my work address, and hope that I remember to copy it onto a network somewhere before class tomorrow. (I wonder how many students in my intro class will know that there was a big, devastating earthquake in Peru recently?)

so when is the next Big One due, anyway?

Two days ago, I was here, at the mouth of Little Cottonwood Canyon, just south of Salt Lake City. I was waiting for an earthquake. It didn't happen.



Here's the view to the south:



I was standing on the lateral moraine of Little Cottonwood Canyon, looking at the moraine of the next canyon to the south. Those steps in the ridge are the scarps of normal faults. I was standing on the surface trace of the Wasatch Fault, on the eastern edge of the Basin and Range.

G.K. Gilbert actually recognized the fault, back in the 1870's. And he didn't just recognize it. He mapped out the scarps, he concluded that the fault had moved a number of different times... and, in 1883, he issued an earthquake warning for the residents of Salt Lake City.

They kept building.



And the earthquake hasn't happened.

Yet.

It's weird, jumping back and forth from geologic to human time scales. As a geologist who has worked on rocks that are 100 million, 400 million, 1700 million years old, I tend to consider the last few thousand years as essentially yesterday. But if you're going to warn people about building a city on an old lake bed beside an active fault, that's not the right perspective to have.

The wonderful field trip guide (Bruhn and others, 2005*) that I was following ended its description of this stop with this carefully worded but rather dire warning:

Evidence from Little Cottonwood Canyon and other nearby sites shows that the elapsed time since the most recent surface faulting on the Salt Lake City segment is equal to or greater than the Working Group's preferred recurrence interval estimate (500-1300-2400 yr) for the segment, indicating that the Salt Lake City segment is a candidate for the next large surface faulting earthquake on the Wasatch fault.

In other words: on average, earthquakes occur on the Wasatch Fault every 1300 years. It’s been 1300 years since the last one. So, therefore...

...except that the error in the recurrence interval is 400 years. And, well, are recurrence intervals actually the best way to predict earthquakes? In this month’s issue of Geology, Robert Yeats published a commentary on paleoseismology and the problem of earthquake “schedules”. Basically, the commentary boils down to this: earthquakes are not periodic phenomena. Earthquakes can occur in clusters. There can be long periods without many earthquakes (for instance, the northern San Andreas fault was quiet for decades after the 1906 earthquake). The slip during an earthquake should change the stresses on adjacent segments of the fault, and on surrounding faults, and some of those changes will increase earthquake risk, and some will decrease it.

So is the idea of an earthquake recurrence interval outdated? Or should it be viewed more like the recurrence interval of floods: like a long-term probability of an event whose real likelihood of occurring is controlled by other events (like weather patterns in the case of flood hazards)?

A thousand-year recurrence interval says something, I think. Building for an earthquake is more important in California (with recurrence intervals in the 100’s of years) than in Utah, and it’s probably more important in Utah than in Boston (which felt shaking from the intraplate Cape Ann earthquake in the 1700’s). But it isn’t anything to set your watch by. Or, for that matter, your calendar.

* Reference:

Bruhn, R. L., DuRoss, C. B., Harris, R. A., and Lund, W. R., 2005, Neotectonics and paleoseismology of the Wasatch fault, Utah: in Pederson, J., and Dehler, C. M., eds., Interior Western United States: Geological Society of America Field Guide 6, p. 231-250.

thinking of Jamaica

I visited Jamaica once, over twenty years ago. It was my first time outside North America, and my first time to the tropics. I remember green mountains, and waterfalls, and a winding road, and tiny run-down houses perched on the mountainsides.

I'm thinking of Jamaica tonight.



Hang in there, Jamaica.

Where on Google Earth #41

I found the Santa Clara River, which feeds sediment into one of Brian's dissertation areas. So it's my turn for WoGE #41.



No rules in effect for this one. Note oblique view, direction of north arrow, etc. Please solve it quickly - I'm leaving early on Sunday morning to go take Advanced Structural Geology on a pre-semester field trip.

(I've also added a "Where on Google Earth" extra credit assignment for my intro class. If anyone has a great (and relatively easy-to-find) example of any geologic feature that you could imagine discussing in an intro class, and if you're willing to share, please e-mail me at shearsensibility at gmail dot com. Blog comments, after all, could be found by students before the answers are supposed to be posted!)

Hurricane Flossy (or: in which correlation is not causation)

This is the current predicted track for Hurricane Flossy:



My first reaction was: "oh, good - it looks like the 5-day cone doesn't include Hawaii any more."

My second reaction was: "wow, the predicted track is almost parallel to the trend of the Hawaiian/Emperor seamount chain! That means it's almost the same as the motion of the Pacific plate!"

Lessons from this:

1) Sometimes people see what they're trained to look for, even when there isn't a good reason to see it.

2) I, for example, teach plate tectonics way too much.

Though I am tempted to make up a reason why a hurricane storm surge would make an earthquake on the Big Island more likely. I don't know if the reason would be valid, though - my explanation would have to do with pore fluid pressure, but I doubt that the little extra weight of water from a storm surge would make a difference for an earthquake that occurs at a depth of 17 km. (I'm also not sure why there's a thrust focal mechanism for the earthquake. It's interesting to speculate about, though.)

summaries for non-scientists: what should they contain?

Chris Rowan had a very thought-provoking post a couple days ago based on a suggestion by Propter Doc: that all scientific journal articles should include (possibly on the web, rather than in a print version) a lay statement written by the authors of the journal article. The basic idea is that the statement could be used by all sorts of people who are interested in the science, but don't have the time or the background (or the access to the journal) to read the entire article. Chris makes the point that these statements would solve some of the problems with press releases that result from not understanding the science in the paper. (And Chris has some great examples of press releases that needed help, too, and great discussion of them.)

I think this is an interesting idea. But, as the person who teaches my department's writing course, I would add that writing for a lay audience is a skill that needs developing. (I make my students write one paper as if they're writing for an audience of non-scientists. It's amazingly difficult to do well.) So, if this is a valuable skill... then students need to practice it.

As luck would have it, right now I'm trying to put together a new Advanced Structural Geology course. I'm going to run it as a semi-seminar (not as individually motivated as grad school seminars generally are - I only teach undergrads), and we'll be discussing journal articles. And I thought: ah-ha! There's a new idea for an assignment! So I'm going to have each student choose one article and write an "executive summary" for it.

But I need to give them some kind of guidance. So, geoblogosphere: what do you think an "executive summary" or a "lay summary" (or a press release, for that matter) would need to do, in order to be successful? How is it different, in your mind, from an abstract? Is its goal to catch the attention of the audience, like a journalist would need to do? Is its goal to accurately summarize the research? Is its goal to explain why the research is interesting and important? All of the above? None of the above?

My students thank you in advance for helping me clarify my assignments before I grade them. :D